Dataset: Transient exposure to novel high temperatures reshapes coastal phytoplankton communities

Experimental setup: Because of the oligotrophic conditions and low biomass at SPOT, nutrients were added to stimulate photoautotrophic growth and so enable measurements of the effects of temperature on microbial communities. Each incubation experiment used triplicate one-liter flasks enriched with nitrate, silicate and phosphate added to final concentrations of 30, 30 and 2  micro molar, respectively. Iron, other trace metals, and vitamins were added at replete concentrations equivalent to Aquil medium to avoid micronutrient limitation. Enriched SPOT water was split into temperature treatments intended to simulate present and predicted-future surface-water temperatures at both constant and fluctuating temperatures. Present temperatures were set to match the temperature at SPOT at the time of collection. The present-constant treatments acted as our experimental control. Future temperatures were increased 4 °C (spring and fall) or 5 °C (summer) from the present temperature. The two fluctuating temperature treatments had the same means as the constant treatments, but they alternated between a warm phase above (+4 °C) and a cold phase below (−4 °C) the mean value sequentially every 24 h, yielding a 48 h complete thermal cycle.

Elemental analysis: To measure particulate organic carbon (POC) and nitrogen (PON),samples were filtered onto precombusted GF/F filters (2h at 450 °C) and analyzed using a Costech Elemental Combustion system (Valencia, CA, USA). POC was used  to  estimate  bulk assemblage  growth  rates  by recording the change in particulate organic carbon over 2 days, capturing growth during both cool and warm periods. To measure biogenic silica (BSi), samples were filtered onto 3 micron polycarbonate filters and measured to estimate diatom biomass. Similar to POC-derived growth rates, changes in BSi over two days were used to derive diatom-specific growth rates in our treatments. Cells were filtered onto precombusted GF/F filters for particulate organic phosphorus (POP) measurements. In addition to indirectly measuring Chlorophyll using in vivo fluorescence, we measured total chlorophyll by filtering onto GF/F filters and extracting in 90% acetone for 24h. Extracted Chlorophylla and in vivo fluorescence were measured on a Turner AU-10 (Turner Designs Inc., Sunnyvale, CA). During spring and fall experiments, carbon fixation rates were measured by spiking 30 ml of each enrichment with 50 microliter of 14C labeled sodium bicarbonate, then incubated for 3h, filtered onto GF/F filters, and placed in 4.5 ml of scintillation solution.Total radioactivity (TA) was measured using triplicate solutions of combined isotope and scintillation solution spiked with 100 μl of phenyethlamine. We accounted for filter  absorption  using  10ml  from  each  replicate-enrichment spiked with identical amounts of isotopes and filtered immediately. Samples were incubated in the dark overnight and radioactivity was measured with a Tri-Carb  2500TR  liquid  scintillation  counter  after  24h (Beckman Coulter Inc., Brea, CA).