The dark side of the biological carbon pump: iron availability modulates bacteria growth in the mesopelagic ocean

The biological carbon pump is a process that transfers carbon fixed from phytoplankton biomass from the surface ocean to the deep ocean and plays an important role in global carbon cycling. The efficiency of this pump depends on the surplus organic carbon produced in photosynthesis relative to that consumed by respiration in the upper ocean. Iron (Fe) is a key micronutrient controlling primary production in large areas of the surface ocean, but its impacts on the growth of heterotrophic bacteria consuming organic carbon below the photic zone has been less studied. Here we present results from field experiments showing the strong coupling between Fe and organic carbon availability, influencing the growth of these bacteria in the mesopelagic ocean. The addition of Fe together with glucose yielded significantly higher bacterial biomass (1.4–60 fold), growth rates (1.2–6 fold), and total carbon consumption compared to glucose alone. When carbon was abundant but Fe was limited, bacteria invested cellular resources in Fe acquisition through the production of siderophores, resulting in lower growth efficiencies. Our estimates of mesopelagic bacteria Fe requirements show significantly higher quotas than those previously observed in surface waters, and its plasticity in the presence of new carbon may provide a mechanism for understanding the microbial response to carbon export in the mesopelagic ocean.