Iron (Fe) uptake by the microbial community and the contribution of three different size-fractions was determined during spring phytoplankton blooms in the naturally Fe fertilized area off Kerguelen Islands (KEOPS2). Total Fe uptake in surface waters was on average 34 ± 6 pmol Fe L<sup>−1</sup> d<sup>−1</sup>, and microplankton (>25 μm size-fraction; 40–69%) and pico-nanoplankton (0.8–25 μm size-fraction; 29–59%) were the main contributors. The share of heterotrophic bacteria (0.2–0.8 μm size-fraction) to total Fe uptake was low at all stations (1–2%). Iron uptake rates normalized to carbon biomass were highest for pico-nanoplankton above the Kerguelen plateau and for microplankton in the downstream plume. We also investigated the potential competition between heterotrophic bacteria and phytoplankton for the access to Fe. Bacterial Fe uptake rates normalized to carbon biomass were highest when bacteria were incubated in the absence of both micro- and pico-nanoplankton. The absence of microplankton resulted in a decrease in bacterial Fe uptake rates by up to 20-fold, while in incubations with the whole microbial community bacterial uptake rates were reduced by 2- to 8-fold. In Fe-fertilized waters, the bacterial Fe uptake rates normalized to carbon biomass were positively correlated with primary production. Taken together, these results demonstrate that heterotrophic bacteria are outcompeted by small sized phytoplankton cells for the access to Fe during the spring bloom development, most likely due to the limitation by organic matter. We conclude that the Fe and carbon cycles are tightly coupled and driven by a~complex interplay of competition and synergy between different members of the microbial community.
Fourquez, M., Obernosterer, I., Davies, D. M., Trull, T. W., & Blain, S. (2015). Microbial iron uptake in the naturally fertilized waters in the vicinity of the Kerguelen Islands: Phytoplankton-bacteria interactions. Biogeosciences, 12(6), 1893–1906. https://doi.org/10.5194/bg-12-1893-2015