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OOS 72-7
Macroscale analysis of microbial community homeostasis

**Background/Question/Methods**

The capacity of microbial communities to maintain steady state biomass composition and metabolic rates regulates biogeochemical processes. The elemental C,N and P stoichiometries of organic matter (L_{C:X}) and microbial biomass (B_{C:X}, where X=N or P) are connected through a meta-metabolomic sequence of extracellular digestion, assimilation and cellular metabolism linked by ratios of ecoenzymatic activity (EEA_{C:X}), assimilation efficiency (A_{C:X}) and use efficiency (UE_{C:X}): B_{C:X} = (EEA_{C:X}ŸA_{C:X}ŸUE_{C:E})ŸL_{C:X}. The dependent relationships among these variables can be described by stoichiometric (*S*) and elasticity coefficients (ε) estimated from ln-ln regressions. Using meta-analysis, we calculated S and ε coefficients and frequency distributions for these variables to identify the functional constraints on microbial community organization.

**Results/Conclusions **

The abundance of C, N and P substrates in the environment are generally correlated with *S* ~ 1.0. The most widely assayed C, N and P-acquiring ecoenzyme activities (EEA) show a similar pattern (*S* ~ 1.0), as do the relationships between EEA and organic matter and between EEA and microbial production (μ). Across a broad range of enzymes and ecosystems, catalytic capacity (apparent V_{max}) and substrate availability (apparent K_{m}) covary with ε ~ 0.5, a condition optimal for maintenance of steady state. The coefficients for production vs. biomass (B) are similar for fungi and bacteria (ε ~ 0.75), as are the coefficients for respiration (R) and production (*S* ~ 0.5). Because R lags μ, carbon use efficiency (CUE) increases with B^{0.25}. Mean CUE for both bacteria and fungi is 0.3. Median biomass turnover times (B/μ) for bacteria and fungi (112 and 1128 h, respectively), overlap substantially with substrate turnover times (2K_{m}/V_{max}) in proportion to the ratio of assimilation efficiency to carbon use efficiency (A/CUE). Collectively, these relationships describe the homeostatic constraints on microbial community organization and merge ecological theory with the microbiome paradigm.