IRON BIOAVAILABILITY FOR MARINE BACTERIA: A SINGLE CELL APPROACH

participantHeterotrophic bacteria are key players in the biogeochemical cycle of iron in the marine environment. They modify the chemical speciation and therefore the bioavailability of iron, and they compete for the access to iron with other microorganisms. The capability of different bacterial groups to access various chemical forms of iron is, however, ignored thus far. Over the past few years microautoradiography was successfully applied to link bacterial diversity to the cycling of elements such as C, P and S. The aim of this study is to develop a similar approach based on the radioisotope 55Fe. I will present the first results of this technique applied to bacterial strains (Alteromonas macleodeii) grown under iron-controlled conditions, and to a natural community of heterotrophic bacteria collected in the NW Mediterranean Sea. I will discuss the potential, the difficulties and the limits of microautoradiography using 55Fe in the marine environment

Rôle des bactéries hétérotrophes dans le couplage des cycles du fer et du carbone dans l'océan

Les réponses métaboliques à la limitation de deux souches bactériennes d Alteromonas macleodii issues d environnements contrastés (côtier et océanique) ont été étudiées. Chez A. macleodii, la limitation en fer entraine une réduction de la respiration et induit des variations significatives dans l expression d enzymes clé du catabolisme du carbone, en particuliers celles impliquées dans le cycle de Krebs et la glycolyse. Par ailleurs, la souche océanique est moins affectée dans sa croissance que la souche côtière, suggérant une adaptation de ces microorganismes à leur environnement. La combinaison des techniques de microautoradiographie et de CARD-FISH a été développée avec le radioisotope 55Fe, et a démontré le potentiel de cette méthode pour tracer le devenir du fer au sein d une communauté bactérienne. Les premières applications ont révélé une contribution importante des Gammaprotéobactéries, incluant le genre Alteromonas, dans l acquisition du fer en mer Méditerranée et en océan Austral. En océan Austral, la demande en fer des bactéries hétérotrophes a aussi été évaluée pendant la campagne KEOPS2. En début de bloom printanier, les bactéries sont en concurrence avec le pico-nanoplancton pour l accès au fer. L incorporation du fer par les bactéries est près de 20 fois supérieure lorsque le pico-nanoplancton est retiré des expériences. Une forte corrélation entre incorporation du fer par les bactéries et production primaire a aussi été trouvée. Les résultats suggèrent fortement que la disponibilité en carbone organique dissous conditionne la demande en fer des bactéries hétérotrophes et influe indirectement sur les relations trophiques.The metabolic response of two strains of Alteromonas macleodii, isolated from contrasting marine environments (coastal and oceanic), was investigated. Iron limitation leads to a decrease in respiration and significant changes in expression of several key enzymes associated with carbon catabolism, specifically those involved the citric acid cycle and glycolysis. The study shows strain-specific responses to iron limitation. Growth rate of the oceanic strain was less sensitive to low iron concentrations compared to those of the coastal strain. The study provides new insights into how heterotrophic bacteria acclimatize to low iron concentrations. Thus, by altering carbon metabolism and energy acquisition of heterotrophic bacteria, Fe may affect cycling of carbon in certain oceanic regions. In order identify marine bacterial assemblages actively incorporating iron, microautoradiography combined with CARD-FISH was developed using 55Fe as the radioisotope. Initial application of this technique highlights its potential and shows the major contribution of Gammaproteobacteria, including Alteromonas genus, to iron incorporation in both NW Mediterranean Sea and Southern Ocean. In the Southern Ocean bacterial iron demand was measured during the KEOPS2 cruise. At the beginning of the spring bloom, bacteria competed strongly with pico-nanoplankton for iron. Iron incorporation by bacteria was 20 times higher when pico-nanoplankton was removed. The results suggest that the availability of dissolved organic carbon is a crucial parameter for bacterial iron demand and could indirectly influence trophic relationships.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

A method for the use of the radiotracer 55Fe for microautoradiography and CARD-FISH of natural bacterial communities

Heterotrophic bacteria are key players in the biogeochemical cycle of iron (Fe) in the ocean, but the capability of different bacterial groups to access this micronutrient is ignored thus far. The aim of our study was to develop a protocol for the combined application of microautoradiography (MICRO) and catalyzed reporter depositionfluorescence in situ hybridization (CARD-FISH) using the radioisotope 55Fe. Among the different washing solutions tested, Ti-citrate-EDTA was the most efficient for the removal of extracellular 55Fe providing sufficiently low background values. We further demonstrate that the washing of cells with Ti-citrate-EDTA and the fixation with paraformaldehyde or formaldehyde do not induce leakage of intracellular 55Fe. Incubating natural bacterial communities collected from contrasting environments, the NW Mediterranean Sea and the Southern Ocean, with 55Fe revealed that 329% of bacterial cells were associated with silver grains. Combining microautoradiography with CARD-FISH, we demonstrate that the contribution of different bacterial groups to total 55Fe-incorporating cells was overall reflected by their relative contribution to abundance. An exception to this pattern was the proportionally higher contribution of Gammaproteobacteria, SAR86 and Alteromonas. Our study demonstrates the feasibility of MICRO-CARD-FISH using the radiotracer 55Fe and provides the first description of marine bacterial assemblages actively incorporating Fe

First report on biological iron uptake in the Antarctic sea-ice environment

International audienceMelting sea ice is a seasonal source of iron (Fe) to the Southern Ocean (SO), where Fe levels in surface waters are otherwise generally too low to support phytoplankton growth. However, the effectiveness of sea-ice Fe fertilization in stimulating SO primary production is unknown since no data exist on Fe uptake by microorganisms in the sea-ice environment. This study reports a unique dataset on Fe uptake rates, Fe-to-carbon (C) uptake ratio (Fe uptake normalized to C uptake) and Fe:C uptake rate (Fe uptake normalized to biomass) by in situ microbial communities inhabiting sea ice and the underlying seawater. Radioisotopes 55 Fe and 14 C were used in short-term uptake experiments during the 32-day Ice Station POLarstern (ISPOL) time series to evaluate the contributions of small (0.8–10 µm) and large (> 10 µm) microbes to Fe uptake. Overall, results show that over 90% of Fe was bound to the outside of the cells. Intracellular Fe (Fe intra ) uptake rates reached up to 68, 194, and 203 pmol Fe L −1 d − 1 in under-ice seawater, bottom ice, and top ice, respectively. Inorganic carbon uptake ranged between 0.03 and 3.2 µmol C L −1 d −1 , with the lowest rate observed in under-ice seawater. Importantly, between the start and end of ISPOL, we observed a 30-fold increase in Fe intra normalized to carbon biomass in bottom sea ice. This trend was likely due to changes in the microbial community from a dominance of large diatoms at the start of the survey to small diatoms later in the season. As the Antarctic icescape and associated ecosystems are changing, this dataset will help inform the parameterisation of sea-ice biogeochemical and ecological models in ice-covered regions

Resource Colimitation Drives Competition Between Phytoplankton and Bacteria in the Southern Ocean

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Microbial Competition in the Subpolar Southern Ocean: An Fe–C Co-limitation Experiment

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Effects of iron limitation on growth and carbon metabolism in oceanic and coastal heterotrophic bacteria

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Chasing iron bioavailability in the Southern Ocean: Insights from Phaeocystis antarctica and iron speciation: Bioassays, iron speciation, and isotopic analysis uncover a significant range of variability in dissolved iron bioavailability in the Southern Ocean

International audienceDissolved iron (dFe) availability limits the uptake of atmospheric CO 2 by the Southern Ocean (SO) biological pump. Hence, any change in bioavailable dFe in this region can directly influence climate. On the basis of Fe uptake experiments with Phaeocystis antarctica , we show that the range of dFe bioavailability in natural samples is wider (<1 to ~200% compared to free inorganic Fe′) than previously thought, with higher bioavailability found near glacial sources. The degree of bioavailability varied regardless of in situ dFe concentration and depth, challenging the consensus that sole dFe concentrations can be used to predict Fe uptake in modeling studies. Further, our data suggest a disproportionately major role of biologically mediated ligands and encourage revisiting the role of humic substances in influencing marine Fe biogeochemical cycling in the SO. Last, we describe a linkage between in situ dFe bioavailability and isotopic signatures that, we anticipate, will stimulate future research