Natural seawater was enriched for photoautotrophs and split into multiple temperatures for two weeks. After the enrichment period, Synechococcus was isolated from each temperature. Each isolate's thermal niche was measured through a series of lab experiments and sequenced.

Methods: Thermal niches were calculated by measuring each strain’s thermal performance curve. This was done by acclimating aliquots of each culture for two weeks to a broad range of temperatures between 9° and 33°. Temperatures >33° were added as needed for strains able to grow at these levels.  This temperature range was chosen because it exceeds the current summer high and low temperatures in Narragansett Bay, and encompasses the projected warmer temperatures expected in coming decades. Strains were grown at each temperature in triplicate 8 ml borosilicate vials containing 5ml of F/2 medium. Biomass was recorded every two days using in vivo chlorophyll a fluorescence measured on a Turner AU-10 fluorometer (Turner Designs Inc., Sunnyvale, CA, USA), and growth rates and Eppley-Norberg thermal performance curves (Norberg 2004) were calculated in R (Team 2019) using the package growthTools (DOI:10.5281/zenodo.3634918). Rare cultures containing contaminants (verified using fluorescence microscopy) were excluded from the dataset. In two strains for low temperature treatments, LA20 and LA27, after two weeks of acclimation no Synechococcus cells were observed in the culture, so the growth rate was set to zero for these cultures. The results of these thermal performance curves are hereafter referred to as a strain’s phenotype.