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

Iron is a critical micronutrient that limits primary production in the Southern Ocean. While it is known that marine microorganisms adapt to low-iron conditions, the specific dynamics of how heterotrophic bacteria produce iron-binding ligands—known as siderophores—in response to varying iron levels and phytoplankton growth remain poorly understood. This study investigated how different iron sources and concentrations influence siderophore production and the resulting iron bioavailability.

Researchers conducted two shipboard incubation experiments using seawater collected near the Antarctic Peninsula. These experiments tested the response of natural phytoplankton and bacterial communities to different iron conditions: one experiment added inorganic iron, while the other used natural iron-rich water from the continental margin. The team monitored nutrient drawdown, chlorophyll a (as a proxy for phytoplankton biomass), bacterial community composition, and the chemical identity of dissolved siderophores over approximately two weeks.

• Siderophore Production: Siderophore concentrations were highest when both phytoplankton and heterotrophic bacteria were iron-limited. Production was not solely driven by low iron; it was significantly stimulated by phytoplankton growth, likely due to the release of labile organic carbon that supports bacterial metabolism.
• Chemical Diversity: The study identified 70 unique siderophores. The chemical nature of these compounds shifted with iron availability: hydrophobic and amphiphilic siderophores were dominant in low-iron conditions, whereas hydrophilic siderophores were more prevalent when iron concentrations were higher.
• Community Dynamics: Despite the different iron treatments and resulting phytoplankton biomass, the heterotrophic bacterial community composition remained remarkably similar across treatments, suggesting that the observed changes in siderophore production were a physiological response to environmental conditions rather than a shift in the microbial population.