Theoretical modeling C- and N- acquiring exoenzyme activities to balance microbial demands during decomposition

Background/Question/Methods

We seek to estimate the allocation of extracellular enzymes by decomposer microorganisms between three general classes of enzymes: one that oxidizes polyphenolic compounds (C₃≈lignin), one that hydrolyzes polysaccharides (C₂≈holocellulose) and one that hydrolyzes N-containing substrates (C₁, e.g., proteins). Our central assumptions are that allocation (1) maximizes total C-yield from C₂-C₃ complexes (lignocellulose) while (2) simultaneously matching C and N requirements for microbial growth. We then estimate allocation over a range of litter qualities, represented by lignocellulose index (LCI = C₃/[C₂+C₃]) values from 0 to a theoretical maximum of LCI_max, and for different quantities of total nitrogen content (i.e., proportional to C₁). Reverse Michaelis-Menten (RMM) equations estimate the proportional allocation of the total extracellular enzymes (E_T) to oxidative (E₃) and hydrolytic (E₂ and E₁) pools (E_T=E₁+E₂+E₃) necessary to achieve these decay rates, i.e., dC_i/dt = V_maxi·E_i/(K_Ei+E_i). This is done by first setting E₂=ß·(E₂+E₃) and E₃=(1-ß)·(E₂+E₃), and solving for ß to maximize C-yield from dC₂/dt+dC₃/dt. Then we set E₁=ø·(E₁+ß·(E₂+E₃)) and E₂=(1-ø)·(E₁+ß·(E₂+E₃)), and solve for ø to match C-yield from dC₁/dt+dC₂/dt+dC₃/dt to N-yield from dC₁/dt, given the C:N content of C₁. 

Results/Conclusions

The resulting model is consistent with generally reported patterns of enzyme activity concomitant with litter quality. For example, E₃ > 0 only when the net yield of assimilable C from dC₂/dt+dC₃/dt > the yield from dC₂/dt, alone. It also demonstrates that relative N-availability associated with C₁ (and presumably as mineral N) can reduce both E₂ and E₃ with respect to the balance of realized C-yield from lignocellulose decay. In other words, available N can inhibit lignin decay by shifting enzyme allocation to maximize overall carbon use efficiency, potentially mineralizing N from C₁ even if C₂ is available in C₂-C₃ complexes. However, this mineralization threshold varies with the sizes and relative assimlation efficiencies of the substrate pools, E_T, and CN ratios of substrates and microorganisms.