A Comparison of the Combined Impact of CO2, Temperature, and Light on Growth and Productivity of Two Marine Phytoplankton – the Cyanobacteria Synechococcus elongatus, and the Diatom Thalassiosira pseudonana

Warming associated with the predicted increase in atmospheric CO₂ will result in increased sea-surface temperatures and shoaling of the mixed layer, thereby exposing phytoplankton to a combination of high CO₂, high temperature, and increasing fluxes of irradiance. While studies have looked at the effects of individual variables on specific phytoplankton species, their response to simultaneous multiple environmental stressors remains unclear. Furthermore, different phytoplankton would likely respond differently to the same environments, and a clear understanding of these differences is essential to accurate predictive ecological modeling. In this study, we first describe the growth, photophysiology, and cellular resource allocation of the marine cyanobacteria Synechococcus elongatus, under three CO₂ concentrations (410ppm, 750ppm, and 1000ppm), four temperatures (spanning their thermal tolerance levels), and three light regimes (sub-optimal, optimum, and inhibitory) in a multi-factorial design. Changes in abundance and size were monitored by flow cytometry, while the maximum quantum yield, non-photochemical quenching, and photosynthesis rates were measured using a handheld PAM fluorometer. Cellular C:N ratios, and nutrient uptake across treatments were also monitored. We then juxtapose these findings against results from our previous work that describes the responses of the marine diatom Thalassiosira pseudonana to similar environments – thereby highlighting differences between the two phytoplankters. Our results revealed non-linear and non-uniform trends. For example, higher light regimes favored higher abundances and growth rates in both diatoms and cyanobacteria. In contrast, a decrease in thermal tolerance of diatom photosystem II (PSII) at elevated CO₂ concentrations was more pronounced than the decrease observed in cyanobacterial PSII. Our results thus highlight (a) a complex interplay of three environmental variables, (b) differential responses across traits within taxa, and (c) taxon-specific responses across phytoplankton - thereby emphasizing the necessity to consider these components collectively to assess the impacts of climate change on phytoplankton.