UC Santa Barbara

As human induced climate change continues to alter the world’s oceans, it becomes

increasingly important to hone the predictive power of models to understand the ecosystem

level changes and challenges that the coming decades will bring. However, models are only

as robust as the data upon which they are formulated, and the experimentation required to

inform them must be based on an interconnected and concomitantly changing set of

conditions. Phytoplankton, specifically diatoms, are a worthy focus group as they are

particularly ecologically successful—and responsible for approximately 40% of marine

primary production. By scrutinizing the potential effects of climate change on

phytoplankton, the base of the marine food web, researchers can obtain crucial information

upon which to build predictions for entire ecosystems.

This series of experiments was designed to investigate the combined effects of

temperature and light on the growth and photophysiology of two strains (one coastal and one

open-ocean) of the diatom Thalassiosira pseudonana. The goal in producing this data-set is

to add to the growing body of multi-stressor research which will aid in understanding the

response of phytoplankton to future ocean conditions. This set of experiments takes

advantage of advances in culturing techniques and utilizes a bioreactor (multicultivator

Z160-OD) containing individual treatment vessels, thus allowing for the easy cultivation of

diatoms under eight different light regimes at the same temperature. Through the use of

higher treatment numbers across a gradient of conditions, we exploit the opportunity to

detect and quantify potential non-linear response patterns.

Our results show that the response of T. pseudonana to simultaneous changes in

temperature and irradiance is dependent on the measured response trait, which suggests that

interpretation of performance curves requires clear identification of all conditions under

which they were generated. Our data also suggest subtle differences between the two strains

in the response of growth rate at suboptimal irradiances. Over the range of temperatures

tested in these experiments where growth was possible, temperature proved unimportant to

the growth rate of the open ocean strain (CCMP 1014) at suboptimal light levels. Whereas

the coastal strain (CCMP 1335) demonstrated an interactive relationship between light and

temperature at suboptimal irradiances. As temperatures were pushed above the optimal, the

cellular characteristics of carbon content and size of the open ocean strain exhibit a clear

split based upon irradiance; with high light leading to large carbon-poor cells and low light

resulting in small, carbon-dense cells. Our findings also support the idea that the relationship

between growth rate and cellular carbon content, while complex and non-linear, is likely

predictable. The “choices” and energy trade-offs employed by this species of diatom under

the simplified set of experimental conditions in this study, highlight the importance of

having clear understandings of the mechanisms driving these changes before they are

incorporated into models, as hypothetical outcomes could be missed if only values obtained

under specific ranges are used for prediction.