Differences in Siderophore Production During the Growth of Phytoplankton in Southern Ocean Incubation Experiments Under High- and Low-Iron Conditions.

Iron is an important micronutrient required by marine microorganisms. The Southern Ocean is an ecosystem where primary production is limited by the availability of iron, but how microorganisms in this region adapt to low-iron conditions or respond to episodic iron inputs is understudied. We tested how external iron additions would impact bacterial production of siderophores during phytoplankton growth using shipboard incubation experiments under a range of iron conditions. Similar heterotrophic bacteria communities and phytoplankton biomass were observed whether iron was added from the continental margin or as inorganic iron. Under both high- and low-iron conditions, the heterotrophic bacteria community remained the same but produced distinct iron-binding siderophores. Siderophores were detected in the highest concentrations when phytoplankton and heterotrophic bacteria were iron-limited, and unique compounds were produced in low-iron conditions compared to when iron was added. Hydrophobic siderophores dominated when iron concentrations were low, and hydrophilic compounds when iron concentrations were higher, and this was likely due to environmental factors rather than changes in the microbial siderophore producers. Our results also highlight that phytoplankton growth likely played an important role in stimulating siderophore production across both low- and high-iron conditions, with important implications for phytoplankton and bacteria interactions.