141-4 Built by Microbes: How Herbivores Thrive on Low-Protein Diets

Session: New Advances in Geobiology

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Herbivory is the most common foraging strategy among terrestrial mammals, however these plant diets, rich in complex carbohydrates and dietary fiber, typically contain low concentrations of nitrogen. Extensive research on wild mammalian herbivores has revealed how their specialized gut microbiomes ferment dietary fiber into short chained fatty acids that the host uses for energy. Less is known about the role of gut microbes in synthesizing amino acids that the host can use for constructing proteins needed to build and maintain their bodies. Recent field and laboratory studies on small mammals show that symbiotic gut microbes can convert carbon from dietary carbohydrates into essential amino acids (AA_ESS) that are assimilated by the host to synthesize proteins in red blood cells. Here we assess microbiome contributions to protein-rich tissues of terrestrial herbivores by analyzing carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of AA_ESS in bone or dentin collagen from several species of large mammals (zebra, wildebeest, impala, elephant, black rhinoceros, giraffes) from Amboseli National Park, Kenya. This is a savanna-woodland ecosystem where herbivores have different diets (C₄ grazers, C₃ browsers, and mixed feeders) and digestive physiologies (hindgut versus foregut fermenters) for processing low quality forage. Our goal is to use AA_ESS δ¹³C to quantify the degree to which host tissues are built from dietary versus microbially derived AA_ESS as well as δ¹⁵N analysis to understand if amino acid synthesis by gut microbes increases apparent trophic position of the host. δ¹³C analysis indicates that grazers consuming C₄ grasses characterized by low-protein but high-fiber concentrations have more microbially synthesized AA_ESS, particularly in foregut fermenters, due to their specialized adaptations to promote fermentation and nitrogen recycling. Additionally, δ¹⁵N analysis reveals that higher microbial contributions are associated with elevated apparent trophic position in foregut relative to hindgut fermenters, effectively adding an intermediate bacterial trophic level that inhabits their digestive tracts. Together, these findings demonstrate a novel application of compound-specific stable isotope analysis to disentangle dietary versus microbial contributions to host tissue synthesis, highlighting the importance of microbially synthesized protein to nitrogen balance in the large herbivores in diverse terrestrial ecosystems. This integrative approach advances our understanding of how large mammals physiologically meet the ecological challenges of building and maintaining their bodies while subsisting on low-quality (protein-poor) resources.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7184

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