Diagram illustrating the sea ice showing the pores and channels with the distribution of microorganisms in them
Figure 1 Diagram illustrating the sea ice showing the pores and channels with the distribution of microorganisms in them (Taken from Experience Science)

Sea ice is an important ephemeral feature of the polar regions, covering 13% of the Earth’s surface area and creating habitats for marine mammals and sea birds. Sea ice in the Antarctic can reach up to 20 million km2 during the winter and decrease to 4 million km2 during the summer. During the sea ice formation, when seawater freezes it creates a porous semisolid matrix which traps numerous microorganisms who were suspended in the water column (shown in Figure 1). This matrix is made of interconnected channels and pores filled with a hypersaline fluid containing solutes, organic and inorganic particles which are expelled during the ice formation. Organisms who become trapped in the brine fluid have to be highly adapted to living in the extreme cold temperatures (typically ranging from -2°C to -30°C) and extreme hypersaline conditions (often reaching 3 times the salinity of seawater) for several months in order to survive. Such organisms include bacteria, viruses, algae, protists, flatworms and small crustaceans who have been caught in or stuck to the ice crystals as they rise to the surface during the sea ice formation. Concentrations of some organisms can be extremely high such as pennate diatoms. These unicellular photosynthetic microalgae have been found in concentrations of up to 1000 mg of chlorophyll per litre in the ice, compared to the 0-5 mg of chlorophyll per litre found in water causing a brown discolouration of the Antarctic sea ice. The primary production of pennate diatoms in the sea ice contributes to 5% of total annual primary production of Antarctic sea ice.

                                                                             Living at sub-zero temperatures

A microphoto of a diatom (D) surrounded by an extracellular (EPS) matrix in a pore in the sea ice. The diatom has been stained using Alcian blue to identify the EPSs
Figure 2 A microphoto of a diatom (D) surrounded by extracellular polymeric substances (EPS) in a pore (P) in the sea ice. The diatom has been stained using Alcian blue to identify the EPSs. (Picture taken from Krebs et al., 2002)

The microorganisms found in the sea ice are either psychrotolerant, meaning they can grow in low temperature conditions but their optimal growth temperature is > 20°C, or psychrophilic, meaning they are ‘cold-loving’ and grow best at temperatures of < 15°C. In all organisms, physiological processes are hindered at the low temperatures, so in order to survive organisms have to be highly adapted. The most vital metabolic requirement is to maintain the function of lipid membranes for transport of nutrients and waste products. The fatty acid structure of membrane phospholipids regulates fluidity, and in the cold temperatures cytoplasmic membranes and enzymes tend to rigidify. In order to keep their fluidity, increasing the proportion of unsaturated fatty acids decreases the average chain length which in turn increases the polyunsaturated fatty acids (PUFAs). PUFAs are also an essential part of the diet for grazing fish because of the inability to produce PUFAs themselves. Though the research in this area is lacking. The ability to synthesise antifreeze proteins (AFPs) has also been found in bacteria. The AFPs can bind to the ice crystals and create thermal hysteresis, lowering the temperature at which the organism can grow.

Other adaptations to the cold include increasing extracellular polymeric substances (EPS) such as trehalose and exopolysaccharides which are cryoprotectants and are thought to aid in avoiding protein denaturation and have a colligative effect. High concentrations of EPSs (Figure 2) found in sea ice bacteria help the cells retain water, nutrients and protect extracellular enzymes from the cold. An increase in concentration has also been observed in microalgae who increase the key enzyme used in photosynthesis (ribulose-1,5-biosphosphate carboxylase/oxygenase; RUBISCO) which has a poor catalytic efficiency but at higher concentrations seems to accommodate for this.

Living in the hypersaline channels and pores

Living in the brine solution of the sea ice pores and channels produces a risk of dehydration due to the osmotic up-shift. The intensity of which is made worse with decreasing temperatures. Organisms also have to adapt to the ice melting, which would mean they have to encounter water with a decreased salinity making it almost freshwater. For this reason, bacteria regulate the proportions of fatty acids and have salt tolerant enzymes, which are able to work over a wide range of salinities.

For bacteria, a common response to salinity stress is accumulation of potassium and glutamate, partnered with the release of putrescine, which balances the intracellular charges and thus reduces the osmotic effect. In other microorganisms, to avoid growth limitations from the permanent intracellular accumulation of salts, compatible solutes are synthesised in the cell instead of potassium. These compatible solutes are small and water soluble organic molecules that increase osmolality of cytoplasm but without the disrupting salt ion effects.

Photosynthetic rates are also reduced with increasing salinity.  Dimethylsulfonioproprionate (DMSP) is an osmolyte and a cryoprotectant found at relatively high concentrations in sea ice algae. It decomposes into vaporous dimethyl sulfide (DMS), which is oxidised to methane sulphonate (SO2) thus causes cloud condensation nuclei. When salinity increases this occurs at higher rates and can impact the Earth’s climate, due to the sulphuric acid acting as an aerosol. The decomposition of DMSP also produces acrylic acid, which at high levels, disrupts growth of protozoan and metazoan herbivores. Bacteria can tolerate DMSP, DMS and acrylic acid but at higher levels affects their growth too.

Jupitar's moon, Europa, is covered in ice kilometers thick. Yet, it has a very similar colour to the discolouration of sea ice in the polar regions caused by pennate diatoms. (Photo taken from NASA
Figure 3 Jupiter’s moon, Europa, is covered in ice kilometres thick. Yet, it has a very similar colour to the discolouration of sea ice in the polar regions caused by pennate diatoms. (Photo taken from NASA)

Why do we care? 

The highly specific adaptations that these sea ice organisms have to go through, such as the physiological and biochemical acclimations, have the potential to improve biotechnological studies. Furthermore, PUFA production is used to improve food for aquaculture and livestock and thus improve food items for humans. However, by far the most enthusiastic research is from astro-biologists. Due to the findings of life in the sea ice, astro-biologists are currently researching whether there could also be life on other planets such as Mars and Jupiter’s moons Europa and Ganymede. Europa’s sea ice seems to have the same colour (Figure 3 shows the colouration of Europa) as the pennate diatom-coloured ice in the polar regions which largely sparks the interest in this area. So far it seems very unlikely that there is life other planets.

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