THE MOLECULAR PHYSIOLOGY OF MIXOTROPHIC PHYTOPLANKTON UNDER IRON-LIMITED UPWELLING CONDITIONS

Mixotrophs, defined as phytoplankton capable of both autotrophy and heterotrophy, are an emerging topic of interest due to their multiple nutrient acquisition strategies. We investigated the molecular physiology of mixotrophs within an upwelling zone as a function of iron status. Our study occurred during the diatom-dominated upwelling season of the California Current System. Subsurface waters from an active upwelling site were incubated for 14 days under control (Ctrl), iron-replete (Fe), and iron-limited (DFB) treatments. Day 7 revealed distinct phytoplankton communities; Ctrl/Fe transcripts were dominated by diatoms, whereas DFB transcripts mapped to higher proportions of mixotrophs. Iron-limited mixotrophs reduced expression of photosynthetic processes and increased expression of processes linked to iron stress, signal transduction, and phagocytosis. This contrasted with autotrophic diatoms, which showed minimal change in photosynthetic machinery or signal transduction under iron limitation. Our results suggest mixotrophy as a viable strategy used by phytoplankton to survive under variable iron conditions.