We incubated serially transferred cultures of Synechococcus (CC9311, BL107, ROS8604, CC9902, representing two clades) in triplicate 1 L flasks at 16°C, 18°C, 20°C, 22°C, 25°C, and 27°C. We supplied ambient light (60 µmol quanta m^-2 s^-1) using white fluorescent lamps on a 12:12 light-dark cycle. We grew cultures in modified artificial seawater media. To ascertain that we did not impose growth rates upon cultures and the observed growth rates are a product of strain and environmental conditions, we used nutrient replete media and sampled prior to cultures encountering nutrient limitation; we supplied nitrate (NO₃^-) and phosphate (PO₄³⁻) in concentrations of 125 µM and 10 µM, respectively. Cells utilized a mean of 17% of the supplied nitrogen, and 22% of the supplied phosphorus, indicating cultures did not reach nutrient limitation. We transferred media and diluted cultures using an open flame in a hood in order to avoid contamination. We diluted weekly by approximately an order of magnitude to avoid nutrient limitation and maintain a stable growth rate.

We measured particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate organic phosphorus (POP), as well as cell enumeration by flow cytometry, following the methods outlined in Garcia et al. (2016). We sampled after seven doublings or one month to acclimate cells to the temperature conditions. We vacuum filtered POC, PON (150 mL), and POP (50 mL) samples onto pre-combusted GF/F Whatman glass filters (450°C) at 10 psi. We dried particulate organic carbon and particulate organic nitrogen samples at 50-80°C for a minimum of 48 hours and pelletized prior to analysis using a Flash EA 1112 NC Soil Analyzer (Thermo-Scientific, MA). We rinsed filtered particulate organic phosphorus samples with 0.17 M NaSO₄, immersed the filter in 2 mL of MgSO₄, dried at 80°C overnight, and combusted at 450°C for 2 hours. We then added 5 mL of 0.2 M HCl and baked the samples at 80-90°C. We measured particulate organic phosphorus samples via colorimetric assay following the Bermuda Atlantic Time-series methodology using a Genesys 10S UV-vis spectrophotometer (Thermo-Scientific) at 885 nm.

We measured culture cell density every two-three days and immediately prior to sampling using a NovoCyte 1000 flow cytometer (excitation laser 488 nm, emission peak 575 nm) and forward scatter. We assessed growth rate using flow cytometry based on increases in biomass measured across time points using the following equation: µ = ln(CD2)-ln(CD1))/(T2-T1), in which CD2 and CD1 are cell counts in cells/mL^-1 on the most recent count and the previous count, respectively, and T2 and T1 represent time points. We accounted for recent dilutions in accounting for growth rate; we counted cells prior to and after dilutions to ensure growth rate was accurately assessed. We counted the cells at a flow rate of 35 µL/min. To assess heterotrophic populations, we stained the cultures with SYBR Green (Thermo-Fisher) for 15 minutes at room temperature, vortexed them, and counted using the (excitation laser 488 nm, emission peak 520 nm) channel. We recorded duplicate cell counts at each sampling.

We measured cell diameter by microscopy under oil immersion at 1000x magnification using the Axioplan2 and AxioView 1.4.5 sizing software (Carl Zeiss, Goettingen, Germany) with reference to a staged micrometer (Ted Pella Inc., Redding, CA). To estimate cell diameter, we created a conversion factor determined by plotting the mean observed cell diameters and mean forward scatter values for several strains of Synechococcus.

Locations: Laboratory batch cultures in incubators from 16-25°C. The original isolation locations for the Synechococcus strains used were:
31.91, -124.17, Pacific Ocean, California Current
32.87, -117.26, Pacific Ocean, California Current
41.72, 3.55, Mediterranean Sea, Balearic Sea
48.72, -3.98, Atlantic Ocean, Britanny coast

Organism Identifier (LifeSciences Identifier, LSID):
Synechococcus, urn:lsid:marinespecies.org:taxname:160572