Generality of associations between biological richness and the rates of metabolic processes across microbial communities

Biological richness is positively associated with the rates of some metabolic processes performed by microbial communities. It remains unclear, however, whether these positive associations are a general feature of the metabolic processes performed by microbial communities or whether they are specific to certain types of metabolic processes. For example, it was hypothesized that the strength of any particular positive association depends on how many different genotypes within a microbial community perform the metabolic process of interest (i.e. the 'rarity hypothesis'). We tested the generality of these positive associations by measuring the taxonomic richness, functional gene richness and rate constants for 71 different metabolic processes across 30 independent microbial communities. We found that both taxonomic and functional gene richness do indeed tend to positively associate with the rates of metabolic processes. In addition, we found that positive associations occur across a wide range of different environmental conditions. Counter to the 'rarity hypothesis', however, we did not detect a relationship between the strengths of the positive associations and the rarity of each metabolic process. Together, our data provide empirical evidence that positive associations with biological richness may indeed be a general feature of the metabolic processes performed by microbial communities.