Mixotrophy as an alternative strategy under iron-limited upwelling conditions

Abstract Mixotrophs, phytoplankton capable of both phototrophy and heterotrophy, serve crucial yet complex roles 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 under control (Ctrl), iron-addition (+Fe), and desferrioxamine B (DFB, a strong iron chelator) treatments. Results revealed differing phytoplankton communities; Ctrl/+Fe communities had high biomass and RNA transcripts dominated by diatoms, whereas the DFB community displayed symptoms of iron limitation, lower biomass, and a higher proportion of putative mixotrophs. Iron-limited mixotrophs reduced expression of photosynthetic processes while highly expressing those linked to iron stress, signal transduction, and the phagolysosome compared to iron-replete mixotrophs. This response contrasted with diatoms, which displayed less change in photosynthetic machinery or signal transduction under iron limitation. Our results suggest mixotrophy as an alternative strategy used by some phytoplankton to cope with iron limitation, and although diatoms dominate under high-iron scenarios, low-iron scenarios may pose a different outcome.