Communication à un colloque
Résumé : Ice-associated communities colonize the brine-filled spaces and are exposed to major biogeochemical and physical changes: temperature fluctuations, salinity, dissolved oxygen, light, pH, the surrounding organic matrix, and nutrient stress. A key adaptive response is the formation of biofilms, which play a major role in macro- and micro-nutrient storage, transformation and mobilization. Considerable enrichment of Fe and other trace metals has been recorded in sea ice, supposedly being adsorbed onto organic matter. Current methods for collecting pristine ice samples mostly involve melting an ice core, erasing any spatial information and discrimination between solid, liquid and gaseous phases. As a result, sea ice analytical methods have an insufficient spatial resolution to detect or describe microbial processes at submillimetre scale (in biofilms within the brine network), without any existing alternative. We have developed a new Diffusive Gradients in Thin-films (DGT) procedure for sea ice application, based on DGT capacity for imaging 2-dimensional distribution of total labile metal concentrations in soil/sediment. During the optimization process, we considered atypical conditions for DGT application at sub-zero temperatures; hydrogel freezing, slow diffusion, high brine salinity. We defined diffusive coefficients at water freezing temperatures and assured contact with hydrogel and thus diffusion. Using Peltier element to precisely control ambient temperature, slow equilibration to in situ temperature of -1.8°C successfully maintained the brine liquid, ice remained solid, and the hydrogel did not freeze. This allowed diffusion to occur, and importantly, allowed sea ice to de-gas. Without gradual equilibration, gases from sea ice were trapped between hydrogel and ice, separating the two and preventing diffusion. Our result are the first two-dimensional images of biogenic metal micronutrients in the sea ice, revealing a clear spatially diverse signal. Fe, Zn and Mn were associated with organic matter-rich micro-locations where the biofilm communities were clearly visible. The new procedure has muchpotential to advance our understanding of the sea ice biogeochemistry. It could provide missing empirical evidence to connect hypothesized reductive conditions in biofilm with trace element and organic matter growth/remineralization on a fine spatial scale, thus increasing understanding of processes occurring in polar oceans and its feedback on the ongoing global change.