Microbial iron uptake in the naturally fertilized waters in the vicinity of the Kerguelen Islands: phytoplankton-bacteria interactions

• Published in: Biogeosciences
• Main Authors: Fourquez, M, Obernosterer, I, Davies, DM, Trull, TW, Blain, S
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus GmbH 2015
• Subjects: Earth Sciences; Oceanography; Biological Oceanography; Kerguelen; Kerguelen Islands
• Online Access: https://doi.org/10.5194/bg-12-1893-2015; http://ecite.utas.edu.au/99902

Iron (Fe) uptake by the microbial community and the contribution of threedifferent size fractions was determined during spring phytoplankton blooms inthe naturally Fe-fertilized area off the Kerguelen Islands (KEOPS2). Total Feuptake in surface waters was on average 34 6 pmol Fe L -1 d -1 , and microplankton (> 25 μmsize fraction; 4069%) and pico-nanoplankton (0.825 μmsize fraction; 2959%) were the main contributors. The contribution ofheterotrophic bacteria (0.20.8 μm size fraction) to total Feuptake was low at all stations (12%). Iron uptake rates normalized tocarbon biomass were highest for pico-nanoplankton above the Kerguelen Plateauand for microplankton in the downstream plume. We also investigated thepotential competition between heterotrophic bacteria and phytoplankton forthe access to Fe. Bacterial Fe uptake rates normalized to carbon biomass werehighest in incubations with bacteria alone, and dropped in incubationscontaining other components of the microbial community. Interestingly, thedecrease in bacterial Fe uptake rate (up to 26-fold) was most pronounced inincubations containing pico-nanoplankton and bacteria, while the bacterial Feuptake was only reduced by 2- to 8-fold in incubations containing the wholecommunity (bacteria + pico-nanoplankton + microplankton). InFe-fertilized waters, the bacterial Fe uptake rates normalized to carbonbiomass were positively correlated with primary production. Taken together,these results suggest that heterotrophic bacteria are outcompeted by small-sized phytoplankton cells for the access to Fe during the spring bloomdevelopment, most likely due to the limitation by organic matter. We concludethat the Fe and carbon cycles are tightly coupled and driven by a complexinterplay of competition and synergy between different members of themicrobial community.