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:XŸA_C:XŸUE_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.