Introduction
Sea ice covers 13% of the Earth`s surface with the largest extent in the Southern Ocean at 20 million km2 (Thomas and Dieckmann, 2002), therefore, it is a major driver of marine ecosystems (Norkko et al., 2007). Brine channel networks within the ice create complex three-dimensional structures, providing a habitat for a number of microorganisms (Ewert and Deming, 2013). Sea ice has the ability to significantly change the surrounding environment resulting in extremes in temperature and salinity (Arrigo and Thomas, 2004). Therefore, adaptions to membranes, transport systems, and proteins are vital for survival (Cavicchioli et al., 2002).
This paper aims to explore the community composition of microorganisms within sea ice, identify their biochemical adaptations and highlight their ecological significance.
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Community Composition
As sea ice forms microalgae, bacteria, protists, and metazoa become trapped in a brine channel matrix, and a sympagic community similar to that of the source water is created (Arrigo and Thomas, 2004).
The bacteria are integrated into the sea ice attached to algae (Deming and Collins, 2017) and typically inhabit the upper layers of ice (figure 1; Ewert and Deming, 2013). As the sea ice ages the diversity of bacteria decreases and psychrophilic bacteria populations become prominent (Arrigo and Thomas, 2004). This is bacteria with an optimal growth temperature of 15º c or less (Cavicchioli et al., 2002). Bacteria groups often found in polar ice are Gammaproteobacteria, Alphaproteobacteria, and Bacteroidetes (Collins et al., 2010).
Sea ice algae flourish in the skeletal layer at the bottom of the ice (Arrigo and Thomas, 2004). Here, conditions are stable, and nutrients are available (Michel et al., 2019). The ice is beneficial to the microalgae, suspending it in the upper ocean where light is not limited to growth (Arrigo and Thomas, 2004). As sea ice ages larger centric diatoms are replaced by smaller pennate diatoms e.g. Fragilariopsis culta (Arrigo and Thomas, 2004).
Protists and metazoans graze on the assemblages of bacteria and microalgae found within the brine channels (Thomas and Dieckman, 2002). Here only a small amount of energy expenditure is needed to exploit a rich food source (Arrigo and Thomas, 2004). However, grazers must be small as the size of the channels limits access to food (Arrigo and Thomas, 2004).
Archaea are single-celled organisms that makeup about 7% of the microbial community in sea ice (Deming and Collins, 2017). Little is known about archaea in sea ice; however, they have the potential to play an important role in ecological functions (Deming and Collins, 2017).
Biochemical Adaptations
The survival of microorganisms in sea ice relies on a number of adaptations that help deal with low temperatures, high salinities, and varied irradiances.
As temperature drops, cell membrane fluidity decreases limiting its structural integrity and ability to control what enters and exits the cell. Bacteria and algae respond by increasing the presence of unsaturated fatty acids within the membrane (Barria et al., 2013) as the composition of fatty acids within phospholipids control membrane fluidity (Thomas and Dieckmann, 2002). Decreases in chain length and increases in polyunsaturated fatty acids (PUFA) also occur (Arrigo and Thomas, 2004). Diatoms also benefit from an increased proportion of PUFAs in thylakoid membranes (Thomas and Dieckmann, 2002). This results in an increased velocity of electrons flowing into photosystem II (Thomas and Dieckmann, 2002). The biosynthesis of PUFAs is facilitated by the production of polyketide synthases a cold-active enzyme (Thomas and Dieckmann, 2002).
Cold-active enzymes favor flexible amino acids with structural adaptations, therefore increasing access to the catalytic site (Ewert and Deming, 2013). This means that at low temperatures cold-active enzymes have a high catalytic efficiency to compensate for the reacting molecules' low kinetic energy (Cavicchioli et al., 2002). Other cold-active proteins help with translation, transcription, and ribosome function (Horn et al., 2007), staying active within the cold brine channels.
Extracellular polymeric substances (EPS) coat bacteria and are released by ice algae (Collins et al., 2004). EPS has the ability to bind to ice crystals or inhibit recrystallization (Deming and Collins, 2017). Which has been seen to increase the habitable area within brine channels by 15% (Ewert and Deming, 2013). EPS can also protect bacteria against ice crystal damage (Collins et al., 2004) and have an affinity for ice which prevents expulsion from brine channels (Deming and Collins, 2017).
The Antarctic ice community protects itself from high UV radiation by producing protective pigments such as Stoneman and by performing rapid DNA repair (Vincent et al., 2004; Ewert and Deming, 2013). To deal with changing salinity ice microorganisms accumulate osmolytes such as organic solutes and inorganic ions, releasing them when salinity is low and breaking them down when salinity is high to create an osmotic balance (Thomas and Dieckman, 2002).
Ecological Significance
Organic matter is released from brine channels into surrounding waters where larger grazers such as amphipods, euphausiids, and fish feed at the sea-ice boundary (Arrigo and Thomas, 2004). This is a rich source of PUFA which is an important component of the diet (Thomas and Dieckmann, 2002). Without sea-ice algae, a number of organisms including krill will struggle to meet energy demands (Michel et al, 2019). Krill are a primary source of food for higher trophic levels, including penguins and seals (Panwar et al., 2020). Atkinson (et al., 2004) have demonstrated a relationship between sea ice extent and declining krill stocks, therefore impacting the Antarctic food web (Loeb et al., 1997).
Biological matter released from sea ice is incorporated into the sequestration and recycling of carbon and nitrogen (Huston et al., 2000; Ewert and Deming, 2013).
Conclusion
Sea ice has a rich microscopic community consisting of bacteria, microalgae, protists, and metazoans. Polyunsaturated fatty acids, cold-induced proteins, and extracellular polymeric substances are some of the adaptations that allow these microorganisms to thrive in extreme abiotic conditions. The survival of these communities ensures food availability to upper trophic levels, therefore, supporting the whole food web.
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