Battling Trophic Modes: How Do Mixotrophs Respond to Iron Limitation in Upwelling Zones?

Mixotrophs, defined as phytoplankton capable of both phototrophy and heterotrophy, are an emerging topic of interest due to their role in ocean food webs and carbon cycling. As climate change is expected to reduce iron bioavailability within upwelling zones, mixotrophs may have an advantage due to an array of possible iron acquisition strategies. 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.  Results 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 mixotroph reads. DFB mixotrophs downregulated photosynthetic processes while upregulating those linked to iron stress, signal transduction, and the phagolysosome compared to Fe mixotrophs. This response contrasted with phototrophic diatoms, which showed less 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.

Presentation Preference: Oral

Primary Presenter: Emily Speciale, University of North Carolina at Chapel Hill (speciale@unc.edu)

Authors:

Emily Speciale, University of North Carolina at Chapel Hill  (speciale@unc.edu)

Prisca Lim, University of North Carolina at Chapel Hill  (prisca@live.unc.edu)

Yubeen Jeong, University of North Carolina at Chapel Hill  (ybjeong@unc.edu)

Adrian Marchetti, University of North Carolina at Chapel Hill  (amarchet@email.unc.edu)