M.S. Thesis Defense Seminar of Emily Speciale

The gene expression of mixotrophic phytoplankton under iron-limited upwelling conditions

Mixotrophs, defined as phytoplankton capable of both phototrophy and heterotrophy, are an emerging topic of interest due to their possible impacts on ocean food webs and carbon cycling. As climate change is expected to reduce iron bioavailability in many regions of the ocean, including upwelling zones, mixotrophs may have an advantage due to an array of possible iron acquisition strategies. However, there are currently a limited number of studies that provide evidence of this advantage. We paired physiological measurements with metatranscriptomics to investigate the molecular mechanisms of mixotrophs within an upwelling zone as a function of iron status. Our study occurred during a biologically productive upwelling season within the California Current System, typically dominated by large phototrophic diatoms. Subsurface waters from an active upwelling site were incubated for 14 days under control (Ctrl), iron-replete (Fe), and induced iron-deplete (DFB) treatments through the addition of desferrioxamine B. Physiological and bioinformatic analyses at day 7 revealed differing phytoplankton communities; Ctrl/Fe communities had high biomass and RNA reads dominated by diatoms, whereas the DFB community had low biomass and a higher proportion of reads mapping to known mixotrophic organisms. Iron-deplete mixotrophs downregulated photosynthesis-related processes while upregulating those linked to iron stress, signal transduction, and the phagolysosome compared to iron-replete mixotrophs. This response contrasted with phototrophic 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 cope with iron limitation, and although diatoms dominate under high-iron scenarios, low-iron scenarios may pose a different outcome.