Main Page |
The major findings from this award include the repeated and highly successful demonstrations that significant discoveries can be made by examining unaltered (unmelted) ice directly, in three dimensions, at a scale relevant to microorganisms, using the novel techniques developed during this award. These techniques have revealed the presence of active Bacteria, especially members of the Cytophaga-Flavobacteria-Bacteroides group, but also Archaea at temperatures as low as &endash;20°C and correspondingly high salt concentrations (~20%). They have also revealed the geophysical features that help to explain their continuing activity at such extreme conditions. In particular, we discovered that the spatial orientation of a microorganism within the fluid inclusions of wintertime sea ice, specifically the organism's association with sediment grains, detrital particles, ice walls, or possibly salt precipitates, plays an essential role in its ability to be metabolically active at subzero temperatures. Through development and adaption of microscopic imaging and magnetic resonance imaging techniques at very low temperatures, we were able to show that even at the lowest temperatures, intergranular pore space provided for substantial connectivity at scales relevant to the microorganism. This connectivity helped to explain the clustered distribution of both particulates and bacteria in the pore matrix and furthermore substantiated and extended earlier findings by Freitag and Eicken of finite permeabilities of sea ice even at temperatures well below the percolation threshold of -5ûC. Through development of suitable image-processing techniques, we were also able to document and quantify the distribution of particulate inclusions in sea ice from a full meter- to the sub-millimeter scale for the first time. This work resulted in the formulation of a simple microgeometric model of in-situ particle distributions in sea ice, an important prerequisite to understand and model the interaction between microorganisms and particle surfaces and impacts on substrate uptake. At the same time, we demonstrated the applicability of a sediment resuspension/ice-entrainment model that helped explain spatial and interannual variability in shallow water environments and were thus able to further understanding of sediment entrainment and potential implications for bacterial distribution and activity in a circum-Arctic context. Comparative studies of Arctic lake and sea ice forming in the same temperature environment, demonstrated that the microstructural and physico-chemical properties of sea ice provide for more space and apparent opportunity for active bacteria than lake ice. We also documented rapid motility (~50 µm/sec) of a model cold-adapted bacterium, Colwellia psychrerythraea strain 34H, at temperatures as low as &endash;10°C. The genome sequence of this organism (obtained with other resources) confirms its possession of chemotaxis as well as motility genes. As for the comparative lake-ice studies on spatial limitations, however, a full exploration of bacterial motility at subzero temperatures, including the effects of salts, remains for the future. With regards to sea-ice algae observed directly in wintertime ice, we discovered that diatom cells (the predominant ice algae) remain remarkably intact through the winter season, almost always occupying spatial habitats larger than the longest dimension of the cell. Studies of extracellular polysaccharide substances (EPS) in sea ice, collected during different seasons of the year, indicated high levels of EPS throughout the wintertime. Experimental manipulations yielded evidence for EPS production under subzero temperatures and the likely use of EPS by ice algae as a cryoprotectant and means to alter (enhance) inhabitable space.
|