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ADVANCING METHODOLOGY FOR SUB-ZERO TEMPERATURE APPLICATION OF DGT TECHNIQUE ON SEA ICE SAMPLES FOR TWO-DIMENSIONAL IMAGING OF BIOGENIC METALS

THURSDAY SEPTEMBER 26 - 4 PM

ARNE BRATKIČ
UNIVERSITY OF LIEGE

ABSTRACT: Ice-associated communities colonize the brine-filled spaces and form biofilms in response to major biogeochemical and physical changes: temperature fluctuations, salinity, dissolved oxygen, light, pH, the surrounding organic matrix, and nutrient stress. Biofilms play a major role in macro- and micro-nutrient storage, transformation and mobilization, particularly Fe. 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. Using Peltier element to precisely control heat flux, 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.
Simultaneously, we have also developed a non-invasive technique to measure Fe redox speciation in a DGT resin gel. The new method couples photothermal Beam Deflection Spectroscopy (BDS) with DGT technique. BDS detects light intensity change due to thermal field around absorbing sample and correlates this change to concentration. Thus, light-absorbing complexes are detected after reaction of Fe2+ with 1,10-phenanthroline. Fe3+ is detected as Fe2+ after reduction with L-ascorbic acid before reaction with phenanthroline.
Our result are the first two-dimensional images of biogenic metal micronutrients in the sea ice (and Fe redox species in the sediments), 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 procedures have immense potential 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 OM growth/remineralization on a fine spatial scale, thus increasing our understanding of processes occurring in polar oceans and its feedback on the ongoing global change.

BIO: Dr. Arne Bratkič is a marine biogeochemist, whose research focuses on microbial transformations of trace and biogenic metals. He is particularly interested in the controls of metal bioavailability – in the age of anthropogenic environmental changes these have immense impact on the interaction with the biota worldwide and can have either detrimental or beneficial consequences for ecosystem health. Arne currently holds a FNRS postdoctoral fellowship at the University of Liege.

Graphical Abstract