Sea ice microbial communities

Sea Ice Microbial Communities (SIMCO) refer to groups of microorganisms living within and at the interfaces of sea ice at the poles. The ice matrix they inhabit has strong vertical gradients of salinity, light, temperature and nutrients. Sea ice chemistry is most influenced by the salinity of the brine which affects the pH and the concentration of dissolved nutrients and gases. The brine formed during the melting sea ice creates pores and channels in the sea ice in which these microbes can live. As a result of these gradients and dynamic conditions, a higher abundance of microbes are found in the lower layer of the ice, although some are found in the middle and upper layers. Despite this extreme variability in environmental conditions, the taxonomical community composition tends to remain consistent throughout the year, until the ice melts.

Much of the knowledge concerning the community diversity of the sea ice is known through genetic analyses and next-generation sequencing. In both the Arctic and Antarctic, Alphaproteobacteria, Gammaproteobacteria and Flavobacteriia are the common bacterial classes found. Most sea ice Archaea belong to the phylum Nitrososphaerota while most of the protists belong to one of 3 supergroups: Alveolata, Stramenopile and Rhizaria. The abundance of living cells within and on sea ice ranges from 10⁴-10⁸ cells/mL. These microbial communities play a significant role in the microbial loop as well as in global biogeochemical cycles. Sea ice communities are important because they provide an energy source for higher trophic levels, they contribute to primary production and they provide a net influx of Carbon in the oceans at the poles.

The autumnal decrease in atmospheric temperatures in the Arctic and Antarctic leads to the formation of a surface layer of ice crystals called frazil ice. A mixture of salts, nutrients and dissolved organic matter (DOM) known as brine is expelled when frazil ice solidifies to form sea ice. Brine permeates through the ice cover and creates a network of channels and pores. This process forms an initial semisolid matrix of approximately 1 meter in thickness with strong temperature, salinity, light and nutrient gradients.

Since thickening of the sea ice during winter months results in more salts being expelled from the frazil ice, atmospheric temperatures are strongly and negatively correlated to brine salinity. The sea ice-seawater interface temperature is maintained at the freezing point of seawater (~1.8 °C) while the sea ice-air interface reflects more the current atmospheric temperature. Brine salinity can increase to as much as 100 PSU when sea ice temperature reaches ~3 °C below the freezing point of seawater. Brine temperature typically ranges from -1.9 to -6.7 °C in the winter. Sea ice temperatures fluctuate in response to irradiance and atmospheric temperatures, but also change in response to the volume of snowfall. Accumulating snow on the ice cover combined with harsh atmospheric conditions can lead to the formation of a snowpack layer that absorbs UV radiations and provides insulation to the bottom ice layer. The fraction of irradiance reaching the sea ice matrix is thus also controlled by the amount of snowfall and varies from <0.01% to 5% depending on the thickness and density of the snowpack.

The surface of sea ice also allows the formation of frost flowers, which have their own unique microbial communities.

The fluctuation of brine salinity, which is controlled by atmospheric temperatures, is the single-most influential factor on the chemistry of the sea ice matrix. The solubility of carbon dioxide and oxygen, two biologically essential gases, decreases in higher salinity solutions. This can result in hypoxia within high heterotrophic activity regions of the sea ice matrix. Regions of high photosynthetic activity often exhibit internal depletion of inorganic carbon compound and hyperoxia. These conditions have the potential to elevate brine pH and to further contribute to the creation of an extreme environment. In these conditions, high concentrations of DOM and ammonia and low concentrations of nutrients often characterize the ice matrix.

High brine salinity combined with an elevated pH reduces the rate at which gases and inorganic nutrients diffuse into the ice matrix. The concentration of nutrients such as nitrate, phosphate and silicate inside the sea ice matrix relies largely on the diffusive influx from the sea ice-water interface and to some extent on the atmospheric deposits on the sea ice-air interface. Iron concentrations in the Southern Ocean ice cover are thought to be regulated by the amount of new iron supply at the time of ice formation and were shown to be reduced during late winter.

The chemical properties of the sea ice matrix are highly complex and depend on the interaction between the internal sea ice biological assemblage as well as external physical factors. Winters are typically characterized by moderate oxygen levels that are accompanied by nutrient and inorganic carbon concentrations that are not growth limiting to phytoplankton. Summers are typically characterized by high oxygen levels that are accompanied by a depletion of nutrients and inorganic carbon. Because of its diffusive interaction with seawater, the lower part of the sea ice matrix is typically characterized by higher nutrient concentrations.