A tale of two drivers: exploring the response of the marine diatom, Thalassiosira pseudonana to changes in temperature and irradiance

As human induced climate change continues to alter the world’s oceans, it becomesincreasingly important to hone the predictive power of models to understand the ecosystemlevel changes and challenges that the coming decades will bring. However, models are onlyas robust as the data upon which they are formulated, and the experimentation required toinform them must be based on an interconnected and concomitantly changing set ofconditions. Phytoplankton, specifically diatoms, are a worthy focus group as they areparticularly ecologically successful—and responsible for approximately 40% of marineprimary production. By scrutinizing the potential effects of climate change onphytoplankton, the base of the marine food web, researchers can obtain crucial informationupon which to build predictions for entire ecosystems.This series of experiments was designed to investigate the combined effects oftemperature and light on the growth and photophysiology of two strains (one coastal and oneopen-ocean) of the diatom Thalassiosira pseudonana. The goal in producing this data-set isto add to the growing body of multi-stressor research which will aid in understanding theresponse of phytoplankton to future ocean conditions. This set of experiments takesadvantage of advances in culturing techniques and utilizes a bioreactor (multicultivatorZ160-OD) containing individual treatment vessels, thus allowing for the easy cultivation ofdiatoms under eight different light regimes at the same temperature. Through the use ofhigher treatment numbers across a gradient of conditions, we exploit the opportunity todetect and quantify potential non-linear response patterns.Our results show that the response of T. pseudonana to simultaneous changes intemperature and irradiance is dependent on the measured response trait, which suggests thatinterpretation of performance curves requires clear identification of all conditions underwhich they were generated. Our data also suggest subtle differences between the two strainsin the response of growth rate at suboptimal irradiances. Over the range of temperaturestested in these experiments where growth was possible, temperature proved unimportant tothe growth rate of the open ocean strain (CCMP 1014) at suboptimal light levels. Whereasthe coastal strain (CCMP 1335) demonstrated an interactive relationship between light andtemperature at suboptimal irradiances. As temperatures were pushed above the optimal, thecellular characteristics of carbon content and size of the open ocean strain exhibit a clearsplit based upon irradiance; with high light leading to large carbon-poor cells and low lightresulting in small, carbon-dense cells. Our findings also support the idea that the relationshipbetween growth rate and cellular carbon content, while complex and non-linear, is likelypredictable. The “choices” and energy trade-offs employed by this species of diatom underthe simplified set of experimental conditions in this study, highlight the importance ofhaving clear understandings of the mechanisms driving these changes before they areincorporated into models, as hypothetical outcomes could be missed if only values obtainedunder specific ranges are used for prediction