Intraspecific diversity in thermal performance determines phytoplankton ecological niche

Temperature has a primary influence on phytoplankton physiology and affects biodiversity and ecology. To examine how intraspecific diversity and temperature shape plankton populations, we grew 12 strains of the ecologically-important coccolithophore Gephyrocapsa huxleyi isolated from regions of different temperature for [~]45 generations (2 months), each at 6-8 temperatures, and characterized the acclimated thermal response curve of each strain. Even with virtually identical temperature optima and overlapping cell size, strain growth rates varied between 0.45 and 1 day-1. While some thermal curves were effectively symmetrical, others had more slowly declining growth rates above the "thermal optimum," and thermal niche widths varied between 16.7 and 24.8 {degrees}C. This suggests that different strains use distinct thermal response mechanisms. We investigated the ecological implications of such intraspecific diversity on thermal response using an ocean ecosystem simulation resolving distinct phytoplankton thermal phenotypes. Resolving model analogs of thermal "generalists" and "specialists" (similar to those observed in G. huxleyi) resulted in a distinctive global biogeography of preferred thermal niche widths with a nonlinear latitudinal pattern. We leveraged the model output to predict the ranges of the 12 strains we studied in the laboratory and demonstrated how this approach could refine predictions of phytoplankton thermal geographic range in situ. Our combination of observed thermal traits and modeled biogeography highlights the capacity of diverse groups to persist through temperature shifts. Significance StatementIntraspecific diversity in the phytoplankton may underpin their distribution. We show that within a single coccolithophore species, thermal response curves have diverse trait parameters. For example, many strains had a variable range of temperatures at which they could survive (thermal niche width). Adding this thermal niche width diversity to an ecosystem model simulation impacted phytoplankton coexistence and overall biomass. These observations show that thermal niche width is a gap in phytoplankton representation in ecosystem models that impacts modeled phytoplankton biogeography and concomitant carbon cycle dynamics. Including thermal tolerance is crucial to predictive modeling as ocean temperature dynamics change.