Microbial diversity and competition for resources in a model seasonal ecosystem

Abstract: We share our world with communities of microbes. These microbes have co-evolved to live in every environment on our planet, comprising an essential part of every ecosystem, from the human body, to rainforest soil and coral reef. When inhabiting a host organism, they provide services essential for host health. A defining feature of microbial life is that wherever they are found, microbes compete fiercely for limited resources, a competition possibly as old as life itself. A second defining feature of microbial life is its astonishing diversity. Natural ecosystems typically display a hugely diverse array of coexisting microbial species.

How is microbial diversity maintained, in light of the aforementioned resource competition? Here, we focus on consumer-resource models subject to serial dilutions. Using a theoretical framework we developed, we probe the effects of resource competition, cross-feeding, mutation, and adaptation on diversity in microbial ecosystems. We find that diversity is influenced by these mechanisms in very different ways, suggesting that real ecosystems may not obey a universal nutrient-diversity relationship. Our results can be explained by an early-bird effect whereby species that grow quickly because their preferred nutrients are supplied, then leverage that early advantage - even after those preferred nutrients are depleted and the remaining nutrients are more efficiently metabolized by competitors.