Increased temperature influences iron growth kinetics in the key polar diatom Fragilariopsis cylindrus

Temperature and iron availability are important variables influencing the growth and physiology of marine diatoms - temperature influences enzymatic turnover rates and iron is used as a cofactor in many vital cellular proteins. Current climate change models predict a simultaneous change in both temperature and iron in much of the world’s oceans, yet, pertinent studies on phytoplankton responses to these variables have mainly focused on their individual effects. As a result, we have limited phenomenological and mechanistic understanding of how concurrent changes in temperature and iron influence diatom growth. We designed a multifactorial study where we grew a dominant polar diatom (Fragilariopsis cylindrus) under nine different iron concentrations at 1 °C and 3 °C and 6 °C and measured its growth rate, photophysiology and cell size. We show that temperature and iron interactively --not independently-- influence growth. Warming decreased the demand for iron, allowing cells to maintain half maximal growth rate at lower iron concentrations. The apparent increase in iron use efficiency with warming was accompanied by an increase in light harvesting cross section, which could allow cells to maintain photosynthesis while lowering photosystem iron demand. Additionally, warming caused a more pronounced reduction in cell size under low iron compared to high iron availability. This may allow smaller cells to uptake iron more efficiently and reduce iron demand per cell. These results suggest that future changes in temperature and iron availability could have important ramifications to marine primary productivity rates and size-dependent ecosystem processes including grazing susceptibility and carbon export.