Empirical determination of the interaction landscape for temperature, CO2 and nitrate for a model diatom

Phytoplankton are the small but mighty single-celled photosynthetic organisms that form the base of most aquatic food webs, affect nutrient cycles, and are responsible for approximately half of the carbon fixation on Earth. Among the phytoplankton, diatoms are responsible for about 40% of marine primary production, which is up to 20% of global production, and are believed to be responsible for approximately 40% of global carbon export, some of which is sequestered in the deep ocean. At mid and high latitudes, such as around the UK, they increase the trophic efficiency of marine foodwebs, providing a direct link between primary production, grazers, and larger marine animals.

The ocean environment where diatoms live is changing rapidly through warming, increases in CO2, and decreases in the nutrient levels (particularly nitrogen) in the upper part of the ocean where photosynthesis occurs. We have a reasonably good understanding of how phytoplankton react to these environmental changes in isolation. At high latitudes at least, increases in temperature and CO2 increase phytoplankton growth, whereas decreases in nitrogen availability decreases growth. Surprisingly, though, there is little experimental data on how phytoplankton react to all of these things changing simultaneously. Recent studies show that it is important to consider environmental changes together, as the organismal responses to several changes at once is not predictable from the organismal responses to each of the changes alone. For example, not only does lower nitrogen decrease the overall population size that diatoms can achieve, but it also makes them more sensitive to high temperatures, so that the effects of ocean warming may be exacerbated by nutrient limitation. In addition, we have data from other phytoplankton that suggest that elevated CO2 can partially mitigate negative effects of some other environmental changes, but we don't know if this applies to diatoms. Despite the importance of diatoms in marine ecosystems, there are very few experiments that enable us to understand, in a generalisable way, how diatoms respond to temperature, nitrate, and CO2 all changing at once. This, when combined with uncertainty in predictions about future ocean conditions themselves, means that currently, we are not even able to project whether global primary production is expected to increase or decrease over the coming decades. This uncertainty makes is harder to accurately project ecosystem services including the potential for carbon export in oceans.

Our research uses laboratory experiments to learn how simultaneous changes in temperature, nitrate, and CO2 levels affect diatom growth and the physiology that underlies it. Our experiments are designed to test and choose between general equations describing how fundamental, conserved growth processes are affected under multiple simultaneous environmental changes. We will then use our empirical findings to explore how interactions between environmental changes affect future projections of diatom spatial distributions and growth in a widely-used global marine ecosystem model. The project as a whole will improve our ability to project changes to phytoplankton growth in the oceans, which can then improve projections of climate and ecosystem services, as well as our understanding of how the largest ecosystem on earth functions.