Effects of Iron Limitation on the Elemental Stoichiometry and Molecular Physiology of Four Southern Ocean Diatoms

Diatoms are an ecologically important group of phytoplankton in the Southern Ocean (SO), forming the base of the polar marine food web and significantly contributing to carbon (C) export and nutrient fluxes. Changes in their physiological status can alter their growth, productivity and elemental stoichiometry, with consequences for their ecology, food web dynamics and ocean biogeochemistry. Iron limitation is a chronic feature of the SO and can affect the elemental stoichiometry of some diatoms. Specifically, iron-limited diatoms may have elevated Si relative to N or C; however, the molecular mechanisms that underpin these changes in ratios are still not completely understood. By using steady state culturing experiments under varying iron conditions, we performed a comparative transcriptomic analyses to gain a mechanistic understanding of how four SO diatom species alter their elemental stoichiometry in relation to iron limitation. Our results show that Pseudo-nitzschia subcurvata, Chaetoceros sociales, Thalassiosira antarctica and Fragilariopsis striatula grew at comparable rates in iron replete conditions, but there was a differential effect of iron limitation on each diatom’s growth and photophysiology. Using a combination of next-generation sequencing technologies, we obtained thousands of differentially expressed genes representing the full complement of Si metabolism and transport, as well as photosynthesis, iron homeostasis, and N assimilation. By coupling elemental analysis with transcriptomic techniques, this research provides new insights on the physiological response of ecologically relevant SO diatom species to variations in iron availability and their potential effects on SO ecosystem processes and carbon export.