Importance of polysaccharides for the cycling of minerals and trace elements in the ocean

Abstract

Transparent exopolymer particles (TEP, Alldredge et al., 1993) are marine gel particles that form abiotically from dissolved organic precursors produced mainly by phyto- and bacterioplankton (Passow, 2000). TEP are made up of acidic polysaccharides since they are identified by staining with Alcian Blue (Alldredge et al., 1993). TEP exhibit high stickiness values, promote aggregation (Kiørboe et al., 1994; Damand and Drapeau,1995; Logan et al.,1995; Engel, 2000) and influence the density of marine aggregates in the ocean (Mari, 2008; Azetsu-Scott and Passow, 2004). Thus, TEP control particles fluxes. This PhD dissertation is an attempt to gather further information about the role of TEP and their specific properties in the cycling, the export and the flux estimations of the organic carbon in the ocean. First, the sticking properties of TEP are reduced by elevated concentrations of metals found in situ in the southwestern lagoon of New-Caledonia. This contributes to maintain the organic matter in the water column and drives the ecosystem towards enhanced remineralization. Second, TEP are formed simultaneoulsy with particulate organic carbon through aggregation between minerals and dissolved organic carbon during laboratory experiments. TEP are important to structure the aggregates formed with clay and calcium carbonate minerals, whereas TEP only coat diatom frustules fragments without promoting the formation of macroaggregates (> 500 µm). In the case of clay minerals, the process described could account for the export of a significant fraction of particulate organic carbon under specific trophic conditions. Third, elevated concentrations of TEP reduce indirectly the dissolution efficiency of frustules of aggregated diatom, probably enhancing the export of organic carbon contained within diatoms aggregates. Finally, naturally occuring radiotracers (210Po, 210Pb and 234Th) that are applied to estimate the export of organic carbon present high affinities for TEP. The variations of TEP concentrations in the environment likely influence the respective efficiencies of these radiotracers to estimate organic carbon export.