Siderophore production in the mesopelagic ocean is an indicator of iron stress in heterotrophic bacteria

Iron has long been recognized as a limiting micronutrient for phytoplankton growth in regions of the ocean where the iron supply is low relative to macronutrients. Despite the widespread recognition of the impact of iron on phytoplankton growth, comparatively little attention has been directed towards the influence of iron on heterotrophic bacteria and the corresponding controls by heterotrophic bacteria on the oceanic iron cycle. Evidence suggests that heterotrophic bacteria in the mesopelagic ocean are experiencing iron stress in the water column where respiration rates are high, likely due to the high iron demands of electron transport systems essential for this process. In-situ measurements of the production of siderophores, high affinity organic compounds produced by heterotrophic bacteria in order to facilitate iron uptake, is rapid in the mesopelagic and is associated with heterotrophs regenerating particulate iron from sinking particles. Siderophore production at these depths solubilizes this large particulate iron pool and transfers it into a more bioavailable form for bacteria uptake. In addition, gene expression of the uptake and production of siderophores is also elevated in the mesopelagic ocean relative to the surface, despite the lower biomass at these depths. Preliminary evidence on the iron quotas of mesopelagic heterotrophic bacteria suggest that heterotrophs require a large range of iron concentrations depending on carbon availability and iron nutritional status. Iron additions to natural communities in the North Pacific transition zone promoted bacteria growth and enhanced respiration, suggesting that heterotrophs may be serially or co-limited by iron in these regions. Together this evidence suggests that heterotrophic bacteria may be starved for iron in the mesopelagic ocean, thus altering the biogeochemical cycling of iron at these depths.