Context : Biogeochemical processes in the Southern Ocean are key for the global ocean and atmosphere. Due to the continuous supply of high concentrations of major inorganic nutrients, surface waters of the Southern Ocean hold a large potential for biological activity, in particular primary production and carbon dioxide (CO2) drawdown. Biological processes are, however, limited by the essential nutrient iron (Fe). Several mesoscale fertilization experiments (reviewed in Boyd et al. 2007) and two investigations in naturally fertilized environments (Blain et al. 2007; Pollard et al. 2009) were undertaken to better understand the biogeochemical consequences of Fe addition to the Southern Ocean. Independent of the mode of fertilization, all these studies have clearly demonstrated that the addition of Fe to Southern Ocean surface waters stimulates phytoplankton primary production. We have investigated the mechanisms of natural Fe fertilization and its impact on ecosystem structure and biogeochemical cycles in the region southeast of Kerguelen Island (Indian Sector of the Southern Ocean) in several previous multidisciplinary projects (KEOPS1&2, SOCLIM, MOBYDICK). An important finding was the marked metabolic response of heterotrophic prokaryotes (Bacteria and Archaea) to the phytoplankton bloom induced by natural Fe fertilization above the Kerguelen plateau (Obernosterer et al. 2008). We further observed major differences in the prokaryotic diversity between the communities associated with the Kerguelen bloom and those in high nutrient low chlorophyll (HNLC) waters (West et al. 2008, Obernosterer et al. 2011).
Objective: What are the sources of Fe in the Kerguelen region? We are presently addressing this question in the context of the project BINGO (Bioavailability of Iron contained in Nanoparticles of Glacial Origin). Our main objective is to better understand the role of glaciers as Fe source and its impact on microbial activity. The determination of the bioavailability of Fe is a challenging issue, because not all the chemical forms of Fe are equally available (Shaked and Liss 2012). Fe bioavailability can be studied from a geochemical perspective where the chemical composition, the redox state or the types of minerals are assumed to be descriptors of bioavailability. A biological approach is based on microbial activity in the presence of different forms of Fe, reflecting the diversity of metabolic capabilities to acquire and utilize given Fe forms (e.g. dissolved organic complexes or particulate iron) and to adapt to low Fe concentrations. In the context of the project BINGO we will combine these geochemical and biological perspectives, and thereby better constrain the nature and the magnitude of bioavailable Fe from glacier melting and its impact on marine plankton.
The aim of the Professional Practice is to investigate the bioavailability of different forms of colloidal and particulate Fe by a biological approach. The Master student will analyse microbial samples collected during incubation experiments carried out during the project BINGO in January 2020 on Kerguelen Island. A first objective is to determine the abundance of free-living and particle-attached heterotrophic prokaryotes during the incubation experiments. These analyses will be done by flow cytometry and microscopic observations. A second objective is to determine the fraction of active prokaryotes in the presence of different forms of Fe, using the recently developed method BONCAT (Bioorthogonal non-canonical amino acid tagging)(Leizeaga et al. 2017). This technique is based on click-chemistry and allows to visualize active cells by microscopic observations.
The Master 1 internship will take place at the LOMIC in Banyuls sur mer, where all the facilities for flow cytometry and microscopic observations are available.
References
Blain, S., AND OTHERS. 2007. Effect of natural iron fertilization on carbon sequestration in the Southern Ocean. Nature 446: 1070-1075.
Boyd, P. W., AND OTHERS 2007. Mesoscale Iron Enrichment Experiments 1993-2005: Synthesis and future directions. Science 315: 612-617.
Leizeaga A., M. Estrany, I. Forn and M. Sebastian (2017) Using click-chemistry for visualizing in situ changes of translational activity in planktonic marine bacteria. Front. Microbiol. 8: 2360
Obernosterer, I., U. Christaki, D. Lefèvre, P. Catala, F. Van Wambeke, and P. Le Baron (2008), Rapid bacterial remineralization of organic carbon produced during a phytoplankton bloom induced by natural iron fertilization in the Southern Ocean, Deep-Sea Res. II, 55: 777-789
Obernosterer, I., P. Catala, P. Lebaron, and N. J. West. 2011. Distinct bacterial groups contribute to carbon cycling during a naturally iron fertilized phytoplankton bloom in the Southern Ocean. Limnol Oceanogr 56: 2391–2401.
Pollard, R., AND OTHERS. 2009. Southern Ocean deep-water carbon export enhanced by natural iron fertilization. Nature 457: 577-581.
Shaked, Y., and H. Lis (2012), Disassembling Iron Availability to Phytoplankton, Front. Microbiol., 3, doi:10.3389/fmicb.2012.00123.
West, N., I. Obernosterer, O. Zemb, and P. Lebaron. 2008. Major differences of bacterial diversity and activity inside and outside of a natural iron-fertilized phytoplankton bloom in the Southern Ocean.Environ. Microbiol. 10: 738-756.
