Dr Ashish Malik¹, Dr Tami Swenson², Claudia Weihe¹, Dr Eric Morrison¹, Prof Jennifer  Martiny¹, Prof Eoin Brodie^2,3, Prof Adam Martiny¹, Prof Kathleen Treseder¹, Prof Trent Northen², Prof Steven Allison¹

¹University of California Irvine, Irvine, United States, ²Lawrence Berkeley National Laboratory, Berkeley, United States, ³University of California Berkeley, Berkeley, United States

Microbial physiology may be critical for projecting changes in soil carbon. Still, predicting the ecosystem implications of microbial processes remains a challenge. Here we argue that this challenge can be met by identifying microbial life history strategies based on their phenotypic characteristics, or traits, and representing these strategies in models simulating different environmental conditions. By adapting several theories from macroecology, we define microbial high yield (Y), resource acquisition (A), and stress tolerance (S) strategies. We empirically validated our Y-A-S framework by studying variations in community traits along gradients of resource availability and abiotic conditions. Metatranscriptomics and metabolomics were used to infer variations in traits of in situ microbial communities on plant leaf litter in response to long-term drought in Californian grass and shrub ecosystems. We hypothesised that drought causes greater microbial allocation to stress tolerance relative to growth pathways and that shrub litter of poorer chemical quality further constrains growth through increased investment in resource acquisition traits. The most discernable physiological adaptations to drought in grass litter communities were production or uptake of compatible solutes like trehalose and ectoine as well as inorganic ions to maintain cellular osmotic balance. Grass communities also increased expression of genes for synthesis of capsular and extracellular polymeric substances possibly as a mechanism to retain water. These results show a clear functional response to drought in grass litter communities with greater allocation to survival relative to growth that could affect decomposition under drought. In contrast, communities on chemically complex shrub litter had smaller differences in gene expression and metabolite profiles in response to drought, suggesting that the drought stress response is constrained by litter chemistry. Overall, our findings suggest trade-offs between drought stress tolerance, resource acquisition and growth yield, thereby providing a framework to link microbial physiology with ecosystem function.