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REVIEW: OCEAN SCIENCE
The pack ice of Earth's polar oceans appears to be frozen white desert, devoid of life. However, beneath the snow lies a unique habitat for a group of bacteria and microscopic plants and animals that are encased in an ice matrix at low temperatures and light levels, with the only liquid being pockets of concentrated brines. Survival in these conditions requires a complex suite of physiological and metabolic adaptations, but sea-ice organisms thrive in the ice, and their prolific growth ensures they play a fundamental role in polar ecosystems. Apart from their ecological importance, the bacterial and algae species found in sea ice have become the focus for novel biotechnology, as well as being considered proxies for possible life forms on icecovered extraterrestrial bodies.
Sea ice is an ephemeral feature of polar regions, but also of the Baltic, Caspian, and Okhotsk Seas. At its maximum, it covers 13% of the Earth's surface area (1), making it one of the major biomes on the planet, similar, in terms of area, to that of deserts and tundra (2). The largest expanse of sea ice occurs in the Southern Ocean where, during winter, 20 million square kilometers are blanketed by an ice cover approximately 1 m thick. Unlike freshwater ice, frozen seawater forms a semisolid matrix, permeated by a network of channels and pores. These vary in size from a few micrometers to millimeters, and are filled with brine formed from expelled salts as the ice crystals freeze together (3). It is within this labyrinth that the sea-ice organisms live (Fig. 1).
Sea ice is dominated by strong gradients in temperature, salinity, space, and light (Fig. 2). These properties and the morphology of the brine channel system are highly variable, being ultimately governed by air temperature and snow cover. Large seasonal and even diurnal differences in ice properties occur (3). Small-scale variations in ice morphology are compounded by rafting of ice floes and deformation, which impart a tremendous spatial heterogeneity to any sea-ice zone, even within a single floe.
Change in salinity is the dominant factor in external chemistry to influence the biological assemblages within the sea ice. Owing to the closed or semiclosed pore system within the ice, diffusion rates of dissolved...