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Abstract Herbivory is a dominant feeding strategy among animals, yet herbivores are often protein limited. The gut microbiome is hypothesized to help maintain host protein balance by provisioning essential macromolecules, but this has never been tested in wild consumers. Using amino acid carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analysis, we estimated the proportional contributions of essential amino acids (AA_ESS) synthesized by gut microbes to five co‐occurring desert rodents representing herbivorous, omnivorous and insectivorous functional groups. We found that herbivorous rodents occupying lower trophic positions (Dipodomysspp.) routed a substantial proportion (~40%–50%) of their AA_ESSfrom gut microbes, while higher trophic level omnivores (Peromyscusspp.) and insectivores (Onychomys arenicola) obtained most of their AA_ESS(~58%) from plant‐based energy channels but still received ~20% of their AA_ESSfrom gut microbes. These findings empirically demonstrate that gut microbes play a key functional role in host protein metabolism in wild animals. 

Primary production is the fundamental source of energy to foodwebs and ecosystems, and is thus an important constraint on soil communities. This coupling is particularly evident in polar terrestrial ecosystems where biological diversity and activity is tightly constrained by edaphic gradients of productivity (e.g., soil moisture, organic carbon availability) and geochemical severity (e.g., pH, electrical conductivity). In the McMurdo Dry Valleys of Antarctica, environmental gradients determine numerous properties of soil communities and yet relatively few estimates of gross or net primary productivity (GPP, NPP) exist for this region. Here we describe a survey utilizing pulse amplitude modulation (PAM) fluorometry to estimate rates of GPP across a broad environmental gradient along with belowground microbial diversity and decomposition. PAM estimates of GPP ranged from an average of 0.27 μmol O 2 /m 2 /s in the most arid soils to an average of 6.97 μmol O 2 /m 2 /s in the most productive soils, the latter equivalent to 217 g C/m 2 /y in annual NPP assuming a 60 day growing season. A diversity index of four carbon-acquiring enzyme activities also increased with soil productivity, suggesting that the diversity of organic substrates in mesic environments may be an additional driver of microbial diversity. Overall, soil productivity was a stronger predictor of microbial diversity and enzymatic activity than any estimate of geochemical severity. These results highlight the fundamental role of environmental gradients to control community diversity and the dynamics of ecosystem-scale carbon pools in arid systems.