Literature Review

Importance in the Gut Microbiome in Buffering Amphibians from heat Stress

Climate change is largely acknowledged by scientists as one of the biggest threats posed to animals. One result of climate change is global warming. Research shows that our earth is warming. Due to this scientists have studied the ways in which warmer climates will and have affected wildlife. Animals, like humans, have gut microbiomes composed of millions of different bacteria with a multitude of purposes from digestion to producing nutrients. Because of this, scientists have studied the ways in which the gut microbiome will respond to the many threats posed by climate change. One of these is the increasing temperatures caused by climate change. 

Since gut microbiomes mediate many physiological aspects of animals, it is important to understand how climate change and, specifically, warming temperatures will affect animal gut microbiomes. By answering this question, we can more accurately make predictions about climate change's imminent impact on animals, as well as work to develop ways in which to further our conservation efforts.

Animal gut microbiome being impacted by numerous factors means studies done on lab animals are insufficient in fully and accurately understanding wild animals gut microbiome and its response to heat stress, because of this researchers have argued that more studies done on wild populations must be done to fully understand how species will be impacted by heat stress(Greyson Gaito 2020). 

Factors impacting the animal Gut Microbiome

Gut microbiomes are heavily influenced by numerous external and internal factors. Researchers have found that gut microbial communities of related species are closely related. Diet, environment, and host phylogeny are all factors proven to contribute to host microbiome makeup. A study examining the gut microbiomes of 15 species of amphibians found that geography as well as the evolutionary history of frogs influenced their gut microbiome makeup(Andres et al, 2023). More broadly, a study comparing mammal gut microbiomes found that despite surveying the same species in different environments. For example, red pandas surveyed at two different sites displayed extremely similar gut microbiomes. In addition, they found that generally diets clustered together (herbivores, omnivores, and carnivores) (Ley et al 2008). This is important as when examining the effects of heat stress on gut microbiome, results will vary drastically depending on the factors impacting gut microbiome. 

Heat Stress’s Effect on Gut Microbiome Composition

First, it is essential to establish the gut microbiome's response to heat stress in order to understand ways in which animals respond defensively to heat stress. A study done on damselfly larvae found that when exposed to heat spikes, Pannonibacter and Rheinheimera increased and there was a decrease in (Acidovorax, Pseudomonas, Comamonadaceae and Rickettsiaceae). The decrease in these bacteria may be linked to the lower growth rates in the heat spiked field which is consistent with a study done on Daphnia (Akbar et al., 2020). This change demonstrates that in some species, heat stress presents a threat to growth rate, which is often detrimental to development and survival. However, gut microbiome response to heat stress varies greatly across species. For example a study done on cattle revealed that Firmicute concentration increased as they examined cattle from sequentially warmer environments. In addition, they found that as they moved from colder northern temperatures to warmer southern temperatures, gammaproteobacteria and pseudomonadaceae decreased as temperature increased (Zhang et al 2022). This result suggests that these bacteria, previously identified to be present during necrotizing enterocolitis, sepsis and developmental delay, are not effective for regulating heat stress and in fact may have a negative outcome on cattles ability to tolerate heat stress. As explored earlier, gut composition is highly dependent on host phylogeny, diet, and environment. Because of this it is difficult to generalize responses to heat stress across species lines. Therefore, studies should aim to examine specific trends in heat stress responses within specific species.

Cross-species gut microbial transplants

One way in which researchers have examined the gut microbiome's role in response to heat stress is through cross-species gut microbial transplants. Cross-species gut microbial transplants involve transplanting the microbiome of a usually more heat-resistant species into a similar species with a lower CTmax. Studies have shown that gut microbial transplants are effective in some species. For example, this study done on Gilthead Seabream and Atlantic Salmon examined the effect of transplanting the gut microbiome of the Atlantic Salmon into the Gilthead Seabream. They also examined the effects of diet after gut microbial transplant, feeding the two groups of Gilthead Bream either an Atlantic Salmon diet or a standard Gilthead Bream diet. They found that gut microbiome transplants can be effective at altering the gut microbiome, however, diet played an important role. Gilthead Seabream consuming their natural diet showed little long term change in gut microbial community, highlighting the role of diet in maintaining the effects of cross species microbial transplants (Ruiz Et al, 2024)Having established that gut microbiota transplants have the ability to alter the composition and abundance of the gut microbiome, scientists have taken to studying the ways in which gut microbial transplants can benefit animals and buffer them from the effects of heat stress. One study was done in which wood frogs were inoculated with the microbiome of the green frog, an invasive species with a higher CTMax. Results showed that after the wood frogs were inoculated, their CTMax increased by about 0.6c. (Dallas 2024). While small, this change still demonstrates the gut microbiome’s ability to alter the animal's physiological response to heat.  In addition to this the study also identified an increase in bacteria associated with producing short chain fatty acids. This increase in SCFA producers implies that SCFA are in some way beneficial to host thermal tolerance. Because cross-species gut microbial transplants have been shown to be beneficial, scientists should continue to explore this area and attain a better understanding of how cross-species microbial transplants could benefit animals living in the wild.

Wild Studies:

 As mentioned above, many researchers have argued that studies done on wild animals are essential to understanding how heat stress will impact animal gut microbiomes. However, studies done on wild animals present many challenges. Sample collection and preservation is one of the most difficult aspects of wild studies as samples must be preserved in order to be valuable in analyzing the microbes present in the gut microbiome. In addition to sample preservation, wild studies present a whole new set of variables that will impact the gut microbiome of animals. Diet, lifestyle, and body condition can be hard to measure in wild animals, and therefore Wild studies have a much harder time controlling these variables. A study done on captive animals as well as wild animals highlights the similarities as well as the differences between the gut microbiome of Captive and wild animals of the same species. They found that wild to captive gut microbiomes differed based on species. Goats, horses and Rabbits exhibited low changes in beta diversity from Wild to Captive animals, while giraffes, Zebras, and elephants showed a larger difference in the Captive and wild populations' gut microbiome. Specifically for Elephants the genera of Collinsella was more abundant in Wild animals, whereas captive animals displayed an increase in the genera of Lactobacillus. Lactobacillus is known to be associated with dairy products, and therefore, the researchers hypothesized that diet was one of the main factors driving the difference in the Gut microbiome of wild and captive animals of the same species.(Jonge, 2022) By understanding these differences, captive studies may be able to mimic or change their methods in order to more closely mimic the gut microbiome of the same species in the wild.

Future directions:

Since it is clear that gut Microbiome plays a role in animals' physiological response to heat, it demonstrates an important topic of research. As explored above, scientists have researched numerous species' gut microbial responses to heat stress; however many of the mechanisms behind this relationship remain largely unknown. More research is needed to fully investigate and understand the impacts. One other area that represents an interesting direction for future research is the role that SCFA plays in increasing host thermal tolerance. However it must first be confirmed that SCFA are directly linked to an increase in host thermal tolerance to further explore the mechanisms behind this increase in host thermal tolerance. These areas provide opportunities to further investigate the role gut microbiome plays in regulating animal physiological responses, specifically to temperature. 

Works Cited

Akbar S, Gu L, Sun Y, Zhou Q, Zhang L, Lyu K, et al. Changes in the life history traits of Daphnia magna are associated with the gut microbiota composition shaped by diet and antibiotics. Science of The Total Environment [Internet]. 2020 Feb;705:135827: https://www.sciencedirect.com/science/article/pii/S004896971935822X?via%3Dihub

Andrés E. Brunetti, Mariana L. Lyra, Juliane P. C. Monteiro, Juan P. Zurano, Diego Baldo, Celio F. B. Haddad and Andrew H. Moeller. Convergence of gut microbiota in myrmecophagous amphibians. The Royal Society. 2023; 290(2011). Doi: https://doi.org/10.1098/rspb.2023.2223

de Jonge N, Carlsen B, Christensen MH, Pertoldi C and Nielsen JL (2022) The Gut Microbiome of 54 Mammalian Species. Front. Microbiol. 13:886252. doi: 10.3389/fmicb.2022.886252

Greyson-Gaito CJ, Bartley TJ, Cottenie K, Will M. C. Jarvis, Newman AE, Stothart MR. Into the wild: microbiome transplant studies need broader ecological reality. The Royal Society. 2020 Feb; 287(1921). Doi: https://doi.org/10.1098/rspb.2019.2834

Jason W. Dallas, Anna Kazarina, Sonny T. M. Lee, Robin W. Warne; Cross-species gut microbiota transplantation predictably affects host heat tolerance. J Exp Biol 1 January 2024; 227 (1): jeb246735. doi: https://doi.org/10.1242/jeb.246735

Ley RE, Hamady M, Lozupone C, Turnbaugh PJ, Ramey RR, Bircher JS, et al. Evolution of Mammals and Their Gut Microbes. Science [Internet]. 2008 May 23;320(5883):1647–51: https://www.science.org/doi/10.1126/science.1155725

Ruiz, A., Gisbert, E. & Andree, K.B. Impact of the diet in the gut microbiota after an inter-species microbial transplantation in fish. Sci Rep 14, 4007 (2024). https://doi.org/10.1038/s41598-024-54519-6

Theys C, Verheyen J, Janssens L, Tüzün N, Stoks R. Effects of heat and pesticide stress on life history, physiology and the gut microbiome of two congeneric damselflies that differ in stressor tolerance. Science of The Total Environment [Internet]. 2023 Jun;875:162617. https://www.sciencedirect.com/science/article/pii/S0048969723012330#f0025

Zhang X, Cui K, Wen X, Li L, Yu X, Li B, et al. The Association between Gut Microbiome Diversity and Composition and Heat Tolerance in Cattle. Microorganisms [Internet]. 2022 Aug 19 ;10(8):1672–2: https://www.mdpi.com/2076-2607/10/8/1672

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