Iron availability modulates bacteria carbon cycling in the mesopelagic

The mesopelagic ocean is an important boundary between the surface and deep sea and is a key region for carbon cycling. Heterotrophic bacteria in this region dominate organic carbon (Corg) degradation and its conversion back to CO2. While iron (Fe) is known to limit primary production in the surface ocean, its impact on mesopelagic heterotrophic bacteria growth and Corg consumption is less understood. This study demonstrates serial Fe-limitation and Fe-Corg colimitation of bacteria within mesopelagic waters across oligotrophic and productive oceanic regimes. Without added Fe, bacteria increased siderophore production, reducing growth efficiencies. Mesopelagic bacteria exhibited Fe inventories much higher than those previously characterized in surface waters, which indicates luxury uptake and storage of Fe. In the presence of added Corg these cellular Fe quotas demonstrated high plasticity with bacterial growth and provide insights into how bacteria may cope with low Fe availability and transitory Corg availability in the mesopelagic ocean.