Chemodiversity controls microbial assimilation of soil organic carbon: A theoretical model

Modeling soil organic carbon is crucial for predicting ecosystem feedbacks to global change. Soil C models typically divide organic C into pools with different turnover rates, even though pools contain complex mixtures of organic molecules. Because microbes assimilate different organic molecules via different metabolic pathways, these models may not capture effects of organic substrate chemodiversity on soil C cycling. In the presence of many substrates, microbes must either allocate resources toward metabolizing specific substrates while ignoring others, or split resources among enzymes for different available substrates. To investigate the influence of chemodiversity on microbial assimilation, we developed a theoretical model that represents microbial assimilation of distinct substrates, corresponding enzymes, and the costs and benefits associated with building enzymes. With this model we show two mechanisms by which chemodiversity could suppress microbial assimilation of available organic substrates. In the first, microbes ignore substrates that aren't abundant enough to justify investing in corresponding enzymes, effectively shrinking the effective size of the decomposing pool. In the second mechanism, assimilating a diverse substrate pool requires microbes to make multiple enzyme types, and dividing resources among different enzyme types suppresses microbial assimilation. Using this model, we show how chemodiversity of organic substrates could suppress the overall microbial C assimilation rate, particularly when overall substrate availability is low. With our results, we also suggest a way to incorporate chemodiversity as a parameter to modify microbial assimilation in other SOC models.