Multitrait diversification in marine diatoms in constant and warmed environments

Phytoplankton are photosynthetic marine microbes that affect food webs, nutrient cycles, and climate regulation. Their roles are determined by a correlated set of phytoplankton functional traits including cell size, chlorophyll content, and cellular composition. Here, we explore how interrelated trait values and correlations evolve. Because both chance events and natural selection contribute to phytoplankton trait evolution, we used population bottlenecks to diversify six genotypes of Thalassiosirid diatoms. We then evolved them in two environments where natural selection could act on this diversity. Interspecific variation and within-species evolution were visualized for nine traits and their correlations using reduced axes (a trait-scape). Shifts in both trait values and correlations, resulting in movement of evolving populations on the trait-scape, occurred in both environments, and were more frequent under environmental change. Which trait correlations evolved was strain-specific, but greater departures from ancestral trait correlations were associated with lower population growth rates. There was no single master trait that could be used to understand multitrait evolution. Instead, repeatable multitrait evolution occurred along a major axis of variation defined by several diatom functional traits and trait relationships. Because trait-scapes capture changes in trait correlations and values together, they offer an insightful way to study multitrait variation.