Advancing methodology for sub-zero temperature application of DGT technique on sea ice samples for two-dimensional imaging of biogenic metals

Description

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 much
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
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.

Period	18 Sep 2019
Event title	DGT Conference 2019
Event type	Conference
Location	Vienna, Austria
Degree of Recognition	International