Divergent physiological and molecular responses of light- and iron-limited Southern Ocean phytoplankton

<p>It has recently been shown that Southern Ocean phytoplankton species have evolved to optimize their light-harvesting potential without increasing the high iron-requiring proteins used for photosynthesis. We measured molecular and physiological responses of phytoplankton cultures under a combination of iron and light conditions. While iron-replete cultures mostly increased biovolume, photochemical efficiency (<i>F</i>_<i>v</i>/<i>F</i>_<i>m</i>) and the relative abundance of photosystem II (PSII) and Cytochrome <i>b</i>₆<i>f</i> protein compared to iron-limited cultures, light also regulated cellular chlorophyll <i>a</i> content and played a role in controlling PSII protein abundance. Investment of protein resources into the carbon fixing enzyme Ribulose 1,5-bisphosphate carboxylase oxygenase (Rubisco) was species-specific, but increased growth rates correlated with increased investment into Rubisco for all species. Our results suggest that <i>Proboscia inermis</i> uses a divergent molecular strategy to compete for nutrients, light, and CO₂ in the Southern Ocean.</p>