Macroscale analysis of microbial community homeostasis
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 (LC:X) and microbial biomass (BC: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 (EEAC:X), assimilation efficiency (AC:X) and use efficiency (UEC:X): BC:X = (EEAC:XAC:XUEC:E)LC: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.
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 Vmax) and substrate availability (apparent Km) 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 B0.25. 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 (2Km/Vmax) 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.