Iron sources alter the response of Southern Ocean phytoplankton to ocean acidification

The projected rise in anthropogenic CO₂ and associated ocean acidification (OA) will change trace metal solubility and speciation, potentially altering Southern Ocean (SO) phytoplankton productivity and species composition. As iron (Fe) sources are important determinants of Fe bioavailability, we assessed the effect of Fe-laden dust versus inorganic Fe (FeCl₃) enrichment under ambient and high p CO₂ levels (390 and 900 μatm) in a naturally Fe-limited SO phytoplankton community. Despite similar Fe chemical speciation and net particulate organic carbon (POC) production rates, CO₂-dependent species shifts were controlled by Fe sources. Final phytoplankton communities of both control and dust treatments were dominated by the same species, with an OA-dependent shift from the diatom Pseudo-nitzschia prolongatoides towards the prymnesiophyte Phaeocystis antarctica. Addition of FeCl₃ resulted in high abundances of Nitzschia lecointei and Chaetoceros neogracilis under ambient and high p CO₂, respectively. These findings reveal that both the characterization of the phytoplankton community at the species level and the use of natural Fe sources are essential for a realistic projection of the biological carbon pump in the Felimited pelagic SO under OA. As dust deposition represents a more realistic scenario for the Felimited pelagic SO under OA, unaffected net POC production and dominance of P. antarctica can potentially weaken the export of carbon and silica in the future.