141 New Advances in Geobiology

This session will focus on new research at the intersection between geologic and biologic processes with special emphasis on novel materials and methods, new field sites, and advances at the intersections of scientific fields.

Monday, 20 October 2025
1:30 PM - 5:30 PM
HBGCC, 305

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

Paper number: 141-4
Program: Technical Sessions
Time: 2:25 PM – 2:40 PM

Authors

Abstract

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 (AAESS) 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 AAESS 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 AAESS δ13C to quantify the degree to which host tissues are built from dietary versus microbially derived AAESS 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 AAESS, 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 Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412603

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