Phytoplankton communities have been the model system of choice to tackle the problem of species coexistence and diversity since Hutchinson’s Paradox of the Plankton and Tilman’s resource competition theory. Phytoplankton are also ideally suited for advancing the trait-based framework of community ecology through the synergy of theoretical and empirical approaches. Here we focus on how functional traits of phytoplankton that define their ecological niche, such as resource utilization and temperature response traits, and size, vary across and within species and how this variation promotes diversity under different environmental conditions. We compiled experimentally measured trait data for a wide range of marine and freshwater phytoplankton, including some data on intraspecific variation and used them to assess the degree of variation and identify major trade-offs. We also show experimentally how trait variation arises from rapid evolution. We then use models of resource competition to explore how realistic trait variation interacts with the environment to produce contrasting community structure and diversity patterns. We also show that trait-based framework can be useful to understanding other microbes, such as host-associated microbiota.
Our trait compilations show that phytoplankton functional groups differ in their major functional traits and that the degree of inter- and intraspecific variation along different trait axes may depend on the temporal and spatial variation in environmental factors, as well as on the group’s evolutionary history. We also found significant trait dependence on environmental factors that is rarely considered but affects community assembly. Similarly, rapid trait evolution and the associated change in traits not under selection may alter our predictions for community structure. A simultaneous consideration of traits relevant to different environmental dimensions, such as temperature and resources, may reveal additional coexistence mechanisms. For example, under warming, seasonal variation in temperature may allow resource monopolization by a species less adapted to warming and its subsequent persistence, thus contributing to diversity. In host-associated microbiota, trade-offs among resource utilization traits and life history traits may mediate succession and response to disturbances. Multi-trait approaches taking into account environmental variation provide a mechanistic framework to explain and predict species coexistence.