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

Abstract

Abstract. 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 the Kerguelen Islands (KEOPS2). Total Fe uptake in surface waters was on average 34 ± 6 pmol Fe L −1 d −1 , and microplankton (> 25 µm size fraction; 40-69 %) and pico-nanoplankton (0.8-25 µm size fraction; 29-59 %) were the main contributors. The contribution 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 in incubations with bacteria alone, and dropped in incubations containing other components of the microbial community. Interestingly, the decrease in bacterial Fe uptake rate (up to 26-fold) was most pronounced in incubations containing piconanoplankton and bacteria, while the bacterial Fe uptake was only reduced by 2-to 8-fold in incubations containing the whole community (bacteria + pico-nanoplankton + microplankton). In Fe-fertilized waters, the bacterial Fe uptake rates normalized to carbon biomass 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 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.

Fe and C co-limitation of heterotrophic bacteria in the naturally fertilized region off the Kerguelen Islands

“…Despite the good performance of chemical analytical methods, the bulk concentrations of DFe and DOC do not provide information on the biologically available fractions, and therefore it is not possible to determine the limiting concentration of these resources based on in situ concentrations. A simple comparison between the bacterial Fe quota in Fe-limited cultures (9 µmol Fe mol C −1 ; Tortell et al, 1996) and the KEOPS study region (4-8 µmol Fe mol C −1 ; Fourquez et al, 2015) and the DFe : DOC ratio (range 3-7 µmol DFe mol DOC −1 ; Table 1) indicates similar cellular and in situ molar ratios, Figure 2. Extent of stimulation of bacterial heterotrophic production by Fe (+ Fe) or C (+ C) addition and in situ dissolved iron (DFe) concentrations.…”

“…This could thereby relieve, in part, the limitation by this micronutrient for the summer bacterial community. The idea of seasonal changes in resource limitation is further supported by the higher bacterial Fe quota and cell-specific Fe uptake rates in spring than in summer, which point to enhanced bacterial Fe requirements early in the season (Fourquez et al, 2015). Thus, resource supply and biological interactions determine both the extent of Fe limitation of heterotrophic bacteria, with possible important feedbacks on the Fe and C cycles in the HNLC Southern Ocean.…”

“…The concurrent requirement of DFe by bacteria and phytoplankton could lead to competition for this nutrient between heterotrophic and autotrophic members of the microbial community. Incubation experiments performed during KEOPS2 have indeed shown that the bacterial iron uptake was negatively affected by the presence of phytoplankton (Fourquez et al, 2015). Even though DOC is present in the micro-molar range, the fraction of this bulk DOC that is biologically available is much smaller, in particular under nonbloom conditions in the Southern Ocean.…”

“…We consider two possible underlying mechanisms to explain this seasonal pattern. First, heterotrophic bacteria and phytoplankton are competing for Fe acquisition (Fourquez et al, 2015). In spring, the phytoplankton community is dominated by small, fast-growing cells that outcompete heterotrophic bacteria for Fe acquisition, whereas in summer, the lower primary production by less competitive large diatoms could result in a reduction of Fe limitation for heterotrophic bacteria (Quéguiner, 2013;Fourquez et al, 2015).…”

“…First, heterotrophic bacteria and phytoplankton are competing for Fe acquisition (Fourquez et al, 2015). In spring, the phytoplankton community is dominated by small, fast-growing cells that outcompete heterotrophic bacteria for Fe acquisition, whereas in summer, the lower primary production by less competitive large diatoms could result in a reduction of Fe limitation for heterotrophic bacteria (Quéguiner, 2013;Fourquez et al, 2015). Alternatively, or concomitantly, Fe limitation could be reduced in summer compared to spring due to an overall increased Fe availability resulting from enhanced Fe regeneration mediated by biological activity (Bowie et al, 2014).…”