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Preliminary evidence for differential habitat selection between bird species of contrasting thermal-tolerance levels
By Phillips.
Author’s note: Since coming to college, I have wanted to conduct research on the environmental impacts of agriculture and contribute to efforts to make farming work for both people and nature. In pursuit of this goal, I signed up as an intern with Daniel Karp’s agroecology lab in my freshman year and stayed with them for my entire undergrad. During this internship, I worked alongside several Ph.D. students, such as Katherine Lauck and Cody Pham, who research the cumulative effects of land conversion and climate change on native avifauna at Putah Creek. I was so inspired by their work that I decided to conduct an independent project investigating similar phenomena. Specifically, I was curious about how birds respond to temperature across multiple landscapes, and how this pattern of behavior might influence their choice of habitat. While reading this paper, I would like you to consider the broader implications of the findings as they pertain to species conservation in the context of climate change.
Abstract
Increasing frequency and severity of temperature spikes caused by climate change will disproportionately impact heat-sensitive species. However, certain types of vegetation may protect animals from temperature spikes. Heat-sensitive species can retreat to shaded microhabitats when temperature increases, allowing them to avoid detrimental effects on fitness. Here, we examined habitat selection and behavioral responses to temperature of Western Bluebirds (Siailia mexicana) and Northern Mockingbirds (Mimus polyglottos). We conducted transect surveys and collected behavioral data on bird movement for two months in riparian forest and perennial cropland in the Central Valley of California, where breeding season temperatures are often above 35°C. Bluebirds were observed more frequently in shaded riparian forest, while mockingbirds were observed more frequently in exposed agricultural fields. Correspondingly, bluebirds became less active at higher temperatures, while mockingbirds exhibited no response. Together, our results imply that heat-sensitive species may be more likely to select natural or semi-natural habitats and change their behaviors when temperatures spike. The results of this study imply that the combined effects of anthropogenic land development and climate change may be more destructive for heat-sensitive species than for heat-tolerant species.
Introduction
Climate change is increasing the frequency and intensity of temperature spikes across the world [1]. Many species will likely experience increased mortality due to these extreme conditions [2–4], with heat-sensitive species experiencing especially detrimental effects [5,6]. However, thermally-buffered habitats could mitigate the impact of heat spikes on organisms, as certain habitat features, like vegetative cover, have been shown to cool local temperatures through shading and evapotranspiration [7,8]. Landscapes with high amounts of thermally-buffered habitats, such as closed-canopy forests, have been shown to have less dramatic temperature extremes than open habitats [9,10]. Furthermore, it has been shown that animals in these thermally-buffered habitats are less likely to be impacted by rising global temperatures [11,12]. As such, organisms that are sensitive to temperature extremes may preferentially select for these habitats, and therefore may be able to avoid potentially lethal effects. Birds have been observed to retreat to shaded habitats when temperatures spike [13]. However, it is unclear whether heat-sensitive species specifically select for thermally-buffered habitats, or if heat-tolerant species persist in non-buffered habitats. Therefore, we sought to understand how the habitat selection of bird species may be associated with their behavioral responses to temperature.
Bird populations in North America are in rapid decline [14], and are predicted to continue declining with climate change [15]. As such, determining the habitat requirements of birds in response to increasingly extreme temperatures could be crucial to their conservation. We conducted behavioral surveys of birds in the Central Valley of California to address two questions: 1) does habitat selection differ between Western Bluebirds and Northern Mockingbirds, and 2) are behavioral responses to temperature different between these species? We hypothesized that birds species which exhibit significantly different behavior during high temperature will preferentially select habitats with more vegetation cover.
Methods
Experimental design
We selected two sites along Putah Creek in the Central Valley of California. In this system, temperatures often reach 35°C during the hottest months of the year. These sites contain a combination of riparian (forest existing along a river bank) and agricultural land and are approximately five miles apart from each other. At each site, the two focal land cover types–riparian forest and perennial agriculture–were present within one half-mile of each other (Figure 1). We obtained observations along four 100 meter (m) transects. In the riparian areas, we placed transects along regions of the sites where vegetation was sparse enough that birds could be observed, as dense vegetation made it difficult to track the individual birds. In the agricultural areas, we placed transects along areas that were close enough to the crops that birds could be spotted. Transects were placed approximately 50 meters apart from each other.
We focused on Western Bluebirds (Siailia mexicana) and Northern Mockingbirds (Mimus polyglottos) due to their high abundance at Putah Creek. Additionally, we chose these species because they forage on the ground rather than in the air, and therefore were easier to observe with the naked eye.
Figure 1. Our two study sites were in close proximity to both riparian and agricultural habitats along Putah Creek. At each site, we observed birds along a total of 16 transects (depicted in red).
Data collection
We conducted our surveys from late April to early July 2022, the height of the breeding season for our study species. We visited each site at least once a week, in either the morning or early afternoon. During each visit, I would walk along the transects. Once a bird of either target species was spotted, I would track the bird for two minutes and record all behaviors displayed, along with the amount of time spent engaging in each behavior. These behaviors included “foraging” (searching for, chasing, or eating an insect), “moving” (locomotion with wings or legs), “resting” (standing or sitting motionless), “singing” (repetitive vocalization for more than three seconds), “preening” (use of beak to position feathers), and “disputing” (fighting between birds that occurs due to territorial disputes). I recorded temperature and wind speed each hour using a Kestrel 2000 Weather Meter.
Data analysis
We ran Fisher’s exact tests to determine if mockingbirds and bluebirds preferentially selected different landscape types across sites. The variables in this model included ‘species,’ and ‘landscape type,’ which was defined as either “Agriculture” or “Riparian.” We ran the model across both sites and did not distinguish between the two separate sites depicted in Figure 1.
Then, we implemented multiple linear regression models examining the relationship between the time spent engaging in various behaviors and temperature for each species. We considered the time spent engaged in a particular behavior to be the percentage of time during the two-minute observation period in which the individual bird exhibited that behavior (i.e., time spent moving, foraging, resting, preening, singing, or disputing).
To account for the effects of spatial autocorrelation (or the tendency of areas which are close together to provide similar data values), we first included a site covariate in our models. We additionally attempted to control for the effects of a natural circadian rhythm on behavior by including a time-of-day covariate. As temperature and time were highly correlated (r = 0.696 for bluebird observations and 0.548 for mockingbird observations), we included these covariates using a temperature residual approach. Specifically, we regressed time against temperature and obtained residual values, representing whether temperatures were hotter or cooler than the average expected temperature at any given time of day. We then ran a multiple linear regression including the effects of temperature residuals, time of day, and site on bird behavior.
Results
Landscape preference
Bluebirds and mockingbirds exhibited significantly different habitat preferences. Bluebirds preferentially resided in riparian areas, whereas mockingbirds preferentially resided in agricultural landscapes across both sites (Fisher’s exact test, p = 4.583E-15; Figure 2).
Figure 2. Mockingbirds (n=34) are observed to reside in agricultural landscapes more frequently than riparian landscapes. Bluebirds are observed to reside in riparian landscapes more frequently than agricultural landscapes (n=35).
Changes in patterns of behavior
We found that temperature negatively affected the amount of time that bluebirds spent moving (Linear regression, p = 0.0077, F = 8.069, df = 1, 33; Figure 3; Supp. 1). However, temperature did not significantly affect mockingbird movement (Linear regression, p = 0.297, F = 1.125, df = 1, 32; Figure 3; Supp. 1).
Results were broadly similar after including ‘site’ as another effect in the model to account for multiple observations at the same location. Specifically, temperature still did not affect mockingbird movement (Multiple regression, p = 0.635, F = 0.577, df = 3, 30; Supp. 3) and marginally affected bluebird movement (Multiple regression, p = 0.0682, F = 2.622, df = 3, 31; Supp. 3). However, one of the sites had very few bluebird observations (n=4); when this site was removed from the model, temperature again negatively affected bluebird movement (Linear regression, p = 0.0123, F = 7.138, df = 1, 29; Supp. 2).
The last model we ran tested the effects of both temperature residuals and time of day on bird behavior. Using these models, temperature again did not have a significant effect on the behavior of mockingbirds but did have a marginal effect on bluebird movement (p = 0.07; Supp. 4).
For all of the models, resting, foraging, disputing, singing, and preening of bluebirds and mockingbirds exhibited no significant association with any environmental variable (Supp. 1, Supp. 2, Supp. 3, Supp. 4).
Figure 3. Bluebirds (left) are observed to reduce the percentage of time they spend moving as temperature increases. Mockingbird movement (right) did not significantly decline with rising temperature. The black points represent individual bird observations, the solid lines represent the linear model predictions, and the gray bands represent the 95% confidence intervals.
Discussion
Our results suggest that bluebirds select for shaded riparian habitats, while mockingbirds select for exposed agricultural habitats. Correspondingly, the temperature-altered patterns of movement in bluebirds suggest that they are sensitive to heat and may potentially select for thermally-buffered habitats as a result. In contrast, a lack of observed heat sensitivity in mockingbirds suggests that persistence in open habitats could in part be driven by thermal tolerance. While more data are required to make definitive conclusions, considering only patterns at our site with sufficient data, we found significant evidence for temperature-altered patterns of movement. Together, these results suggest that temperature sensitivity could drive patterns of habitat selection.
Previous research also suggests that habitats with low vegetative cover (i.e., without thermally-buffered microclimates) are likely to contain heat-tolerant species [16,17]. For example, Wilson et al. 2007 demonstrated that populations of leaf-cutter ants (Atta sexdens) residing in cities took 20% longer to succumb to high temperatures than ants dwelling in rural areas. In Brans et al. 2017, it was observed that water fleas (Daphnia magna) from urban areas were more tolerant to high temperatures than rural populations, partially because they had smaller body sizes. Both studies imply that organisms must have high heat tolerance to live in habitats with low vegetative cover. This is similar to our finding that mockingbirds, a heat-tolerant species, were more likely to reside in unvegetated agricultural landscapes than were bluebirds, a heat-sensitive species. However, while the previous studies provide evidence that organisms become heat-tolerant in these landscapes due to natural selection, our findings suggest that behavioral differences between heat-tolerant species and heat-sensitive species may also cause unvegetated landscapes to become dominated by heat-tolerant species.
Additionally, we demonstrate that riparian and other thermally-buffered habitats could be crucial to the persistence of heat-sensitive species. Other studies have shown that vertebrates are more likely to exhibit heat-related mortality in habitats with low vegetation cover [12,18]. For example, Zuckerberg et al. 2018 demonstrated that avian survival in small grassland patches was negatively associated with temperature, while survival in large grassland patches was not. Additionally, Lauck et al. 2023 showed that temperature spikes are associated with a decline in bird reproduction across the continental United States for organisms living in agricultural areas, but not for organisms living in forests. These results suggest that vegetation protects vertebrates from heat stress. Although the mechanisms of this protection are not clear, one potential explanation is that vegetation provides shaded areas that animals can use as refuges to avoid lethal temperatures [7]. Additionally, it has been shown that plants regulate local temperatures through evapotranspirative cooling [8], potentially playing a role in protecting vertebrates from heat spikes.
One caveat of our study is that bluebird responses were only marginally significant under multiple regression models that included time of day as a covariate. Associations between bird behavior and time could either be due to circadian rhythms or temperature shifts; it is difficult to statistically disentangle the effects of temperature and time of day. However, the significant results from the models including only temperature imply that bluebirds do indeed alter their behavior in response to environmental factors that likely include temperature.
Conclusion
Our findings provide preliminary evidence that Western Bluebirds are temperature-sensitive and preferentially select vegetated habitats, while Northern Mockingbirds do not preferentially select vegetated habitats. To obtain enough data to provide definitive evidence of these patterns, the methods could be repeated for several more years and across more sites. Nonetheless, the results from this study suggest that anthropogenic land development will be more destructive for heat-sensitive species than for heat-resistant species. As such, we suggest incorporating thermally-buffered habitats such as groups of trees or hedgerows in working landscapes to mitigate the negative impacts of anthropogenic land development on heat-sensitive organisms.
Lazarus Dies, Lazarus Lives Again
By Jesse Kireyev, History ‘21
Each of these photos captures a landscape in slow degradation. Berryessa, for all the wintergreen beauty that it holds, has experienced horrifying fires numerous times over the past few years. The natural bridge that dominates the landscape of its namesake park in Santa Cruz now remains alone, at risk of collapsing like its sibling did, forever leaving the shoreline empty of its beauty. This risk only grows as sea levels rise and as human interaction puts it at greater risk. The salt flats of the Dead Sea used to be covered in water — now nature struggles to fill the few remaining pools as the sea rapidly shrinks. Captured in these three horizons are the struggles of nature to sustain itself despite the present beauty. For all the tranquility of the Ansel Adams-esque lines jutting forth from the foreground, a great and slow war is playing itself out in the back, often hidden to the gazing eye of the unaware viewer. The horizons both serve as a reminder of the danger that lurks in our future, as well as the distant (and perhaps unreachable) hope of resurrection in the face of annihilation.
1. Berryessa Foothills, Solano, California.
Storm clouds move over the fields and lush wetlands, both morphing into the mountains hugging Lake Berryessa. Just a few months prior, the mountains had been scorched by the dizzying flames of the LNU Lightning Complex Fire, a fire whose smoke blotted out the sun for weeks in two of the largest metropolitan areas in America. The ebb-and-flow of the surroundings give us a stark reminder of just how fast a place can be destroyed and can flourish once again from the ashes. Canon EOS 5D Mark III. April, 2021.
2. West Cliff, Natural Bridges State Park, Santa Cruz, California.
Pelicans and seagulls huddle together as they hunt for fish and fight the buffeting winds. The remainders of the natural bridges, which once dominated the state beach, still serve as a helpful vantage point for the seabirds. Locals hope that this vantage point can survive, even as climate change puts the bridge at greater risk every year. Canon EOS 630, Kodak Tri-X 400TX 35mm film. June, 2017.
3. Dead Sea Salt Flats, Masada, Israel.
The salt flats are all that is left of the once sea-filled expanse below Masada. A combination of climate change and human changes to the environment are driving the evaporation of the Dead Sea, which at current rates is expected to be gone in the next three decades. Sony a700. December, 2018.
Not All Heroes Wear Capes: How Algae Could Help Us Fight Climate Change
By Robert Polon, Biological Sciences Major, ’21
Author’s Note: In my UWP 102B class, we were assigned the task of constructing a literary review on any biology-related topic of our choice. A year ago, in my EVE 101 class, my professor briefly mentioned the idea that algae could be used to sequester atmospheric carbon dioxide in an attempt to slow the rate of climate change. I found this theory very interesting, and it resonated with me longer than most of the other subject matter in that class. I really enjoyed doing the research for this paper, and I hope it gives people some hope for the future. I’d like to thank my UWP professor, Kathie Gossett, for pointing me in the right direction throughout the process of writing this paper.
Abstract
With climate change growing ever more relevant in our daily lives, scientists are working hard to find solutions to slow and reverse the damage that humans are doing to the planet. Algae-based carbon sequestration methods are a viable solution to this problem. Photosynthesis allows algae to remove carbon dioxide from the atmosphere and turn it into biomass and oxygen. It has even been proposed that raw algal biomass can be harvested and used as a biofuel, which can provide a greener alternative to fossil fuel usage. Though technology is not yet developed enough to make this change in our primary fuel source, incremental progress can be taken to slowly integrate algal biofuel into daily life. Further research and innovation on the subject could allow full-scale replacement of fossil fuels with algal biofuel to be a feasible option. Methods of algal cultivation include open-ocean algal blooms, photo bioreactors, algal turf scrubbing, and BICCAPS (bicarbonate-based integrated carbon capture and algae production system). There are many pros and cons to each method, but open-ocean algal blooms tend to be the most popular because they are the most economical and produce the most algae, even though they are the most harmful to the environment.
Keywords
Algae | Biofuel | Climate Change | Carbon Sequestration
Introduction
As we get further into the 21st century, climate change becomes less of a theory and more of a reality. Astronomically high post-Industrial Revolution rates of greenhouse gas emissions have started to catch up with humans, as the initial consequences of these actions are now coming to light with fear that worse is on the way. Many solutions have been proposed to decrease greenhouse gas emissions, but very few involve fixing the damage that has already been done. It has been proposed that growing algae in large quantities could help solve this climate crisis.
According to the Environmental Protection Agency, 76% of greenhouse gas emissions come in the form of carbon dioxide. As algae grows, it removes carbon dioxide from the atmosphere by converting it to biomass and oxygen via photosynthesis. Algae convert carbon dioxide to biomass at relatively fast rates. On average, one kilogram of algae utilizes 1.87 kilograms of CO2 daily, which means that one acre of algae utilizes approximately 2.7 tons of CO2 per day [1]. For comparison, one acre of a 25-year-old maple beech-birch forest only utilizes 2.18 kilograms of CO2 per day [2]. This amount of carbon dioxide sequestration can be done by only 1.17 kilograms of algae. After its photosynthetic purpose has come to an end, the raw algal biomass can be harvested and used as an environmentally-friendly biofuel. This literary review will serve as a comprehensive overview of the literature on this proposal to use algae as a primary combatant against global warming.
Carbon Dioxide
For centuries, heavy usage of fossil fuels has tarnished Earth’s atmosphere with the addition of greenhouse gases [3]. These gases trap heat by absorbing infrared radiation that would otherwise leave Earth’s atmosphere. This increases the overall temperature of the earth, which leads to the melting of polar ice caps, rising sea levels, and strengthening of tropical storm systems, among many other devastating environmental effects [4]. The most commonly emitted greenhouse gas, carbon dioxide, tends to be the primary focus of global warming treatments.
These algal treatment methods are no different. Any algal treatment option is dependent upon the fact that algae sequester atmospheric carbon dioxide through photosynthesis. It converts carbon dioxide into biomass and releases oxygen into the atmosphere as a product of the photosynthetic process [5].
Algal Cultivation
There are four proposed methods of algal cultivation: open-ocean algal blooms, photobioreactors, algal turf scrubbing, and BICCAPS. These techniques all differ greatly, with various benefits and drawbacks to each.
Open-Ocean Algal Blooms
Algae is most abundant on the surface of the open ocean. With the addition of its limiting nutrient, iron, in the form of iron(ii) sulfate (FeSO4), massive algal blooms can be easily sparked anywhere in the ocean [3]. This seems to be the way that most scientists envision sequestration because, of all proposed cultivation techniques, this one produces the most algae in the least amount of time. Intuitively, this method removes the most carbon dioxide from the atmosphere, as the amount of CO2 removed is directly proportional to the quantity of algae undergoing photosynthesis.
There are many benefits to open-ocean algal blooms. There is no shortage of space on the surface of the ocean, so, hypothetically, there is a seemingly infinite amount of algal mass that can be cultivated this way. This technique is also very cost-efficient, as all you need to employ it is some iron(ii) sulfate and nature will do the rest [3].
Once the algal bloom has grown to its maximum size, there is an overabundance of algal biomass on the surface of the ocean. Some researchers have proposed that this mass be collected and used as a biofuel [5,6,7]. Others have proposed that we let nature play its course and allow the dead algae to sink to the bottom of the ocean. This ensures that the carbon dioxide it has taken out of the atmosphere is stored safely at the bottom of the ocean [8]. Here, the algal biomass is easily accessible for consumption by shellfish, who store the carbon in their calcium carbonate shells [3].
This solution is not an easy one to deploy, however, because algal blooms bring many problems to the local ecosystems. Often referred to as harmful algal blooms (HABs), these rapidly growing algae clusters are devastating to the oceanic communities they touch. They increase acidity, lower temperature, and severely deplete oxygen levels in waters they grow in [9]. Most lifeforms aren’t prepared to handle environmental changes that push them out of their niches, so it’s easy to see why HABs kill significant portions of marine life.
HABs can affect humans as well. Many species of alga are toxic to us, and ingestion of contaminated fish or water from areas affected by these blooms can lead to extreme sickness and even death. Some examples of these diseases are ciguatera fish poisoning, paralytic shellfish poisoning, neurotoxic shellfish poisoning, amnesic shellfish poisoning, and diarrheic shellfish poisoning [10]. The effects of harmful algal blooms have only been studied in the short-term, but from what we have seen, they are definitely a barrier in using this form of algae cultivation [11].
Photobioreactors
Photobioreactors are another frequently-proposed tool for cultivating algae. These artificial growth chambers have controlled temperature, pH, and nutrient levels that make for optimal growth rates of algae [12]. They can also run off of wastewater that is not suitable for human consumption. Photobioreactors minimize evaporation and, with the addition of iron, magnesium, and vitamins, increase rates of carbon dioxide capture are increased [1]. Due to the high concentration of algae in a relatively small space, photobioreactors have the highest rates of photosynthesis (and subsequently carbon dioxide intake) out of all of the cultivation methods mentioned in this paper.
This innovative technology was driven primarily by the need to come up with an alternative to triggering open-ocean algal blooms. Photobioreactors eliminate pollution and water contamination risks that are prevalent in harmful algal blooms. Furthermore, they make raw algal biomass easily accessible for collection and use as a biofuel, which open-ocean algal blooms do not [12].
The main drawback to this method is that the cost of building and maintaining photobioreactors is simply too high to be economically feasible right now [12]. Technological developments need to be made to lower the short-term cost of operation and allow for mass production if we want to use them as a primary source of carbon sequestration. Their long-term economic feasibility still remains unknown, as most of the cost is endured during the production of the photobioreactors. Money is made back through the algae cultivated, but the technology hasn’t been around long enough to show concrete long-term cost-benefit analyses without speculation [14].
Algal Turf Scrubbing (ATS)
Proposed in 2018 by botanist Walter Adey, algal turf scrubbing (ATS) is a new technique created to efficiently cultivate algae for use in the agriculture and biofuel industries. The process involves using miniature wave generators to slightly disturb the flat surface of a floway and stimulate the growth of numerous algal species in the water. Biodiversity in these floways increases over time, and a typical ATS floway will eventually have over 100 different algae species [11].
Heavy metals and other toxic pollutants occasionally make their way into the floways; however, they are promptly removed, to ensure that the product is as nontoxic as possible. The algal biomass is harvested bi-weekly and has a variety of uses. Less toxic harvests can be used as fertilizers in the agricultural industry, which research claims is the most economically efficient use for the harvest. It can also go towards biofuel use, although the creators of the ATS system believe the majority of their product will go towards agricultural use because they will not be able to produce enough algae to keep up with the demand (if our society moves towards using it as a biofuel) [11].
The problems with ATS are not technological, but sociopolitical, as the research team behind it fears that they will not get the funding and resources needed to perform their cultivation at an effective level [11].
BICCAPS
The bicarbonate-based integrated carbon capture and algae production system (BICCAPS) was proposed to reduce the high costs of commercial algal biomass production by recycling bicarbonate that is created when algae capture carbon dioxide from the atmosphere and using it to culture alkalihalophilic microalgae (algae that thrive in a very basic pH above 8.5). Through this ability to culture more algae, the system should, in theory, cut costs of carbon capture and microalgal culture. It is also very sustainable, as it recycles nutrients and minimizes water usage. The algae cultivated can also be turned into biofuel to lower fossil fuel usage [13].
The main drawback to this closed-loop system is that it does not cultivate as much algae as the other systems, though work is currently being done to improve this. It has been proven that the efficiency of BICCAPS significantly improves with the addition of sodium to the water, which stimulates the growth of alkalihalophilic microalgae [13]. This means that, with a little bit of improvement to the efficiency of the system, BICCAPS could become a primary algal biomass production strategy because of its low cost and sustainability.
Use of Algae as a Biofuel
While algae may not have the energetic efficiency of fossil fuels, it is not far behind. It can be burned as a biofuel to power transportation, which would allow us to lower our use of fossil fuels and, subsequently, our greenhouse gas emissions. When dry algal biomass is burned, it releases more oxygen and less carbon dioxide than our current fuel sources. The increase in oxygen released into the atmosphere not only helps to lower CO2 emissions but increases the overall atmospheric ratio of oxygen to carbon dioxide. More research still has to be done to find the best possible blend of algal species for fuel consumption [12]. Solely using algae as a biofuel would not meet the world’s energy demand, but the technology for photobioreactors continues to improve, giving hope to one day use algae more than fossil fuels [6].
A common counterargument to proposals for algal biofuel usage is that burning dry algae only provides half the caloric value of a similarly-sized piece of coal. While this is true, it should be taken into consideration that that coal has an extraordinarily high caloric value and that the caloric value of algae is still considered high relative to alternative options [3].
It is often suggested that bioethanol, which essentially uses crops as fuel, should be used over algal biofuel. The main problem with this proposal is that farmers would spend more time cultivating inedible crops because they make for better fuel. This would lead to food shortages on top of the current hunger problem in our world. Farming crops also take up land, while growing algae does not [7].
Drawbacks
The main problems associated with using algae as a biofuel are technological and economical. We simply do not have the technology in place right now to produce enough algae to completely replace fossil fuels with it. In order to do this, we would have to establish full-scale production plants, which is not as economically viable as simply continuing to use the fossil fuels that degrade our planet [12]. Receiving funding for the commercialization of algae is the biggest obstacle this plan faces. It’s difficult to get money allocated to environmental conservation efforts because, unfortunately, it doesn’t rank very highly in our government’s priorities. Algal carbon sequestration has also never been observed at a commercial scale, so there is hesitation to fully commit resources to something that seems like a gamble.
Alternative Uses
It has also been proposed that algal biomass grown to sequester carbon dioxide should be used in the agricultural industry. As previously mentioned, the creators of ATS have suggested using it as a fertilizer [11]. Others say that it can be used to feed livestock or humans, as some cultures actually consume algae already [12]. The seemingly infinite supply of microbes can also be harvested and used heavily in the medical industry in the form of antimicrobial, antiviral, anti-inflammatory, anti-cancer, and antioxidant treatments [7].
Conclusion
Algae can be used to fight climate change because it removes carbon dioxide from our atmosphere, stores it as biomass, and replaces it with oxygen. Arguments have been made in many directions over the best method of algal cultivation. Triggering open-ocean algal blooms is certainly the most cost-efficient of these methods, and it produces the most algal biomass. The problem with using this technique is that these algal blooms have devastating ecological effects on the biological communities they come in contact with. Photobioreactors are another popular method among those who favor this strategy because of their ability to efficiently produce large quantities of algae; however, the main inhibition to their usage is the extremely high cost of construction and operation. With more focus on developing lower cost photobioreactors, they can potentially become the primary source of algal growth. Algal turf scrubbing is another strategy of algae cultivation that struggles with the problem of acquiring adequate funding for the operation. BICCAPS is a relatively inexpensive and eco-friendly way to grow algae in a closed system, but it yields low quantities of algal biomass compared to the other systems.
The raw algal biomass from these growth methods can potentially be used as a biofuel. Dry alga has a high caloric value, which makes it great for burning to power equipment. It does not burn as well as fossil fuels, but it does release more oxygen and less carbon dioxide than fossil fuels when burned. Of course, funding will be needed for increased algae production to make this a possibility, but with more research and advances in the field, algal growth would be a great way to remove large amounts of carbon dioxide that is stuck in Earth’s atmosphere and become our primary fuel source down the line.
References
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A Chemical Report on Heptachlor (Heptachlor Epoxide)
By Kaiming Tan
Author’s Note: This report analyzes and explains the biological, chemical, and environmental importance of heptachlor. More and more in today’s society, we are utilizing synthetic compounds as agricultural insecticides, which makes understanding what these chemicals do to our bodies and the environment of utmost importance. Farming strategies may seem far-removed from our daily lives, but these chemicals do not stay on the farm. They travel to our cities, to our grocery stores and markets, then make their way onto our dinner plates and into our children’s stomachs. I am constantly amazed at the power of scientific research to transform and demystify the detrimental nature of environmental and biological toxicants; this, combined with my passion in toxicology, has inspired me to research this topic and write this manuscript.
Keywords
Heptachlor, heptachlor epoxide, insecticide, environmental toxicology, biological toxin
Introduction
Heptachlor was introduced into the United States in the 1940s and 1950s, among the other chlorinated hydrocarbon insecticides, such as DDT (4). In 1971, 500,000 kilograms of heptachlor were applied to agricultural fields in soils and seeds to prevent termite infection in woods (4). However, after it became evident that heptachlor’s toxicity was a serious health concern, the Environmental Protection Agency (EPA) banned all registered use of heptachlor because of its carcinogenic properties and bioaccumulation in food and water. With the exception of treatment of fire ants in underground power transformers, there is no use of heptachlor in the United States now (2). In contrast, heptachlor is still used as an insecticide in some areas in Asia, Africa, and Eastern Europe (5). The form of greater toxicological concern is heptachlor epoxide, which is a metabolite of heptachlor in soil and the human body. Heptachlor epoxide is more toxic because it degrades slower than heptachlor, thus, is more persistent in the environment. Both heptachlor and heptachlor epoxide can be found in freshwater, estuarine, and marine systems (6). This review will cover the current literature on heptachlor toxicity and investigate the impact of heptachlor on biological systems.
Biological Fate (Absorption, Distribution, Metabolism, Elimination)
Humans and animals can be exposed to heptachlor and heptachlor epoxide through soil, air and water; however, the main route of exposure is oral, through consuming contaminated food or drinking contaminated water. Children and infants are especially vulnerable to heptachlor exposure if they consume large amounts of breast milk from their mother. For instance, if the mother’s breast milk is contaminated by heptachlor, heptachlor can be easily consumed by the infant through breast milk feeding. In human breast milk, heptachlor epoxide was detected ranging from 0.13 to 128 ppb (parts per billion) (8). Another route of exposure is through inhalation. When heptachlor is deposited into the soil, it becomes heptachlor epoxide, which then spreads into the air. As a result, workers who use heptachlor to kill termites are potentially susceptible to inhaling heptachlor epoxide from the air (2). In addition, touching contaminated soils causes exposure to heptachlor dermally (1).
Heptachlor persists in the environment after agricultural application. Even though there was no information on the background levels of heptachlor and heptachlor epoxide in the air or soil, there are 20 to 800 ppt (parts per trillion) heptachlor in drinking water and groundwater in the United States (2). Contaminated fish and shellfish were detected with levels of 2 to 750 ppb of heptachlor and 0.1 to 480 ppb of heptachlor epoxide. To date, there is no available data regarding the biological fate of heptachlor on humans. However, in rats, at least 50% of the orally-administered dose of heptachlor is absorbed in the gastrointestinal tract.The absorption is indicated by the presence of heptachlor and/or heptachlor epoxide in the rats’ liver (9). When heptachlor epoxide enters the bloodstream after absorption, it becomes the predominant metabolite in the body of laboratory female rats. Heptachlor epoxide in the bloodstream is positively correlated to the heptachlor dose administered (10). In other words, the higher heptachlor dose one consumed, the more heptachlor epoxide would appear in the bloodstream.
Because of heptachlor epoxide’s high lipophilicity (likelihood in dissolving fats), its residue is found highest in adipose tissue in human infants (0.32±0.10ppm), while undetectable in the brain. These data also suggest that heptachlor epoxide can be transferred between pregnant women and their babies through the placenta (11). Furthermore, heptachlor and heptachlor epoxide can be stored in human breast milk. Jonsson et al.’s study in 1977 sampled the breast milk of 51 women who had no previously known exposure to heptachlor and detected an average concentration of heptachlor epoxide between 0.0027ppm and 0.019ppm (12). To the researchers, the results proved worrisome as the women’s breast milk contained high levels of heptachlor, which could damage the infant. The results were even more of a concern because there was no known interaction between the women studied and heptachlor.
After heptachlor is absorbed in the body, the primary metabolite of heptachlor in humans and rats is heptachlor epoxide. Heptachlor and heptachlor epoxide are usually metabolized by Cytochrome P450 (CYP450) enzymes. CYP450 enzymes are the body’s major class of enzymes responsible for detoxification. Moreover, heptachlor alters liver function by increasing gluconeogenic enzymes’ activities, which upregulates glucose synthesis from glycogen. Since most of heptachlor is metabolized in the liver, humans with liver diseases may have increased bioaccumulation of heptachlor epoxide because of decreased CYP450 enzyme activity (13). Despite the metabolism of heptachlor in the liver, most heptachlor epoxide can be stored in the adipose tissue because of its high lipophilicity and long half-life (1).
In terms of excretion, orally-administered heptachlor in rats was excreted in the form of heptachlor epoxide and also as heptachlor. Radiolabeled heptachlor epoxide is excreted ten times more in the feces compared to urine after ten days of oral administration in rats, due to the high lipophilicity of heptachlor metabolites (8).
Mechanism of Action
The primary organ and organ systems that heptachlor targets are the liver, central nervous system and reproductive system, while secondary target organs include kidneys and lungs. Heptachlor is primarily metabolized in the liver into heptachlor epoxide, which has the same toxic potential. The reproductive system is a sensitive target for heptachlor toxicity because this system lacks a comprehensive detoxification system like the CYP450 enzymes in liver. CYP450 enzymes facilitate chemical reactions that chemically convert the toxicant into a more hydrophilic metabolite, thereby enhancing toxicant elimination by urine. Oral exposure of 1.8 mg/kg/day to female rats over 14 days caused decreased fertility due to decreased estradiol and progesterone levels. Estradiol and progesterone are important hormones produced by the ovaries, placenta, and adrenal glands; decreased levels may result in endocrine disruption and delayed sexual development. Oral exposure of 0.65 mg/kg/day over 70 days caused decreased sperm count in male rats. The mechanism of heptachlor’s reproductive toxicity remains unknown (1-2).
GABA, the molecule that normally binds the GABA receptor, is an amino acid which can also serve as an inhibitory neurotransmitter in the mammalian brain. Heptachlor primarily acts as a noncompetitive antagonist of the chloride channels of the GABAA receptors in mammals. In other words, heptachlor blocks the Chlorine channel in the cells thus preventing GABAA activation (1-2, 14). By inhibiting the inhibitory neuron, heptachlor causes hyperexcitability of the cells. Although the heptachlor toxicity mechanism in liver is unknown, it is observed that in rats that presence of heptachlor elevates intracellular calcium levels and induces protein kinase C (PKC), which in turn activates the activator protein-1 DNA binding thereby causing tumorigenesis (2, 14).
When animals are exposed to heptachlor above the toxic threshold, symptoms occur including tremors, convulsions, ataxia, and changes in EEG patterns (1). Also, rats who were orally exposed to >320 ppm of heptachlor had consistently lower body weight than rats that were unexposed (15). In humans, clinical symptoms of heptachlor toxicity include seizure, vomiting, and convulsions (2).
Although heptachlor epoxide is theoretically an effective mutagen because of its high reactivity as an electrophile, research studies on heptachlor mutagenicity yielded mostly negative results. For example, heptachlor was not mutagenic in the auxotrophic strains of E. coli by reversion bioassays (14,15). In addition, heptachlor is proven to increase liver tumor incidence as an epigenetic promoter in previously initiated B6C3F1 mice. Heptachlor is a carcinogen in mice by inhibiting intercellular communications to promote tumor growth (14,16). On the other hand, research studies that investigate heptachlor’s teratogenicity, or tendency to disturb embryonic development, yielded mostly negative results. For instance, although feeding rats with diets containing heptachlor produces pups with higher mortality, no congenital malformation was found (2, 14, 17).
In male rats, oral LD50 (lethal dose for 50% of the study population) of heptachlor is 40 to 100 mg/kg body weight based on data from two studies. In contrast, the oral LD50 of heptachlor in male chickens is 62 mg/kg body weight (1, 17). Symptoms displayed by heptachlor acute toxicity in animal subjects include hyperexcitability, tremors, convulsions and paralysis (18).
Long-term exposure to heptachlor can cause liver damage in animals. In rats, prolonged exposure of sub-lethal doses of heptachlor is associated with increased CYP450 enzyme induction and other hepatic microsomal enzymes, in addition to liver hypertrophy (18). In humans, chronic exposure to heptachlor results in storage in adipose tissue and breast milk, because heptachlor is very lipophilic. Infants are at risk of being exposed to large doses of heptachlor if they consume contaminated breast milk (17). Despite the lack of human studies on long-term exposure, a multi-generation study conducted on rats concluded that oral exposure of 6 mg/kg/day of heptachlor is associated with decreased litter size, increased mortality and lens cataract (2). Because of its high lipophilicity, heptachlor residues can remain in the body over time. Heptachlor’s long-term toxicity damages the body gradually, further underlying the need for strict regulations of heptachlor use worldwide.
Overview of Latest Research
Recent research on heptachlor focuses on its toxicity, specifically on the oxidative stress caused by heptachlor and its metabolism in aquatic animals. For example, Vineela et al.’s study investigates the impact of sub-lethal concentrations of heptachlor on carp fish Catla catla by measuring enzymatic activities of mostly Phase II enzymes (19). Phase II enzymes, a part of the CYP450 enzyme class, primarily perform conjugation reactions to convert the chemical into more water-soluble form to enhance urine elimination. Oral exposure of heptachlor at 1.46mg/L (20% of LC50 concentration) for 45 days causes a significant increase in lipid peroxidation, superoxide dismutase, glutathione-S-transferase and catalase activity in Catla catla. The results suggest that carp fish have a sensitive biological defense system against heptachlor, because low concentration activates the detoxification by increased biomarkers of primarily Phase II activity to prevent heptachlor toxicity and organ damage (19).
In addition to heptachlor toxicity, current research also focuses on how to remove existing heptachlor from the environment via microbial degradation, as this method is more environmentally mindful and cost-effective than current physicochemical methods. Qiu et al.’s study discovered a novel strain of bacteria, named strain H, that can metabolize heptachlor efficiently. Strain H is a Gram-negative, short rod-shaped, single-cell bacterial strain that can degrade heptachlor at a rate of 88.2% degradation in 130 hours when exposed to 300 μg/L of heptachlor at 30oC. The main metabolites of heptachlor by strain H include heptachlor epoxide, chlordane epoxide, and 1-hydroxychorodene. This innovation allows possible bioremediation by microorganisms like strain H in heptachlor-contaminated soil and water to reduce heptachlor toxicity and threat to the environment and animals (20).
Recent studies on heptachlor took a creative approach in gaining a deeper understanding in reducing heptachlor toxicity by enzymatic activity in aquatic model organisms and inventing novel microorganisms to metabolize heptachlor into less toxic metabolites. These new techniques will benefit public health by developing cost-effective ways for toxicant removal and controlling the environmental/biological fate of toxicants without causing additional harm.
Conclusions
When initially used as an insecticide, heptachlor’s toxicity became a threatening health concern. The ban of commercial heptachlor use in the United States was a step in the right direction, as misuse of heptachlor can cause severe environmental consequences such as prolonged residue in soil and water habitats along with toxicities in humans and animals. Based on this research, other countries should pursue a substitute pesticide that has less potential for environmental and biological damage than heptachlor.
Most studies conducted regarding heptachlor toxicity were conducted between the 1950s and the late 1980s. Future research can focus on the dose-response relationship of heptachlor exposure within large populations and whether genetic polymorphism contributes to the metabolism of heptachlor. Environmental toxicologists can also study the synergistic toxicity of heptachlor on environmental damage with other pesticides, since common pesticides often contain more than one chemical (1,2).
Acknowledgment
The author would like to thank Dr. Matthew Wood and Mr. Thomas Sears for providing feedback on early versions of this manuscript.
Works Cited (in order of appearance)
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- Hodgson, E. (2004). A textbook of modern toxicology (3rd ed.). Hoboken, N.J.: Wiley-Interscience.
- United States. Environmental Protection Agency. Office of Research Development. (2002). The Foundation for Global Action on Persistent Organic Pollutants a United States Perspective.
- Schimmel, S., Patrick, J., & Forester, J. (1976). Heptachlor: Toxicity to and uptake by several estuarine organisms. Journal of Toxicology and Environmental Health, 1(6), 955-965.
- Mnif, W., Hassine, A. I. H., Bouaziz, A., Bartegi, A., Thomas, O., & Roig, B. (2011). Effect of Endocrine Disruptor Pesticides: A Review. International Journal of Environmental Research and Public Health, 8(6), 2265–2303. http://doi.org/10.3390/ijerph8062265
- Savage, E., Keefe, T., Tessari, J., Wheeler, H., Applehans, F., Goes, E., & Ford, S. (1981). National study of chlorinated hydrocarbon insecticide residues in human milk, USA. I. Geographic distribution of dieldrin, heptachlor, heptachlor epoxide, chlordane, oxychlordane, and mirex. American Journal of Epidemiology, 113(4), 413-22.
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- United Nations Environment Programme, World Health Organization, International Labour Organisation, & Commission of the European Communities. (1976). Environmental health criteria. Geneva: World Health Organization.
- World Health Organization, & International Program on Chemical Safety. (1988). Heptachlor health and safety guide. Geneva : Albany, NY: World Health Organization ; WHO Publications Center USA [distributor].
- Vineela, D., Janardana Reddy, S., & Kiran Kumar, B. (2017). Impact of Heptachlor on Antioxidant Enzyme Markers of Fish Catla Catla. World Journal of Pharmaceutical Research, 6(12), 759-771. doi:10.20959/wjpr201712-9679
- Qiu, Liping, Wang, Hu, & Wang, Xuntao. (2018). Conversion mechanism of heptachlor by a novel bacterial strain. RSC Advances, 8(11), 5828-5839.
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