The light intensity reaching the algae will depend on in its position in the ice and the amount of snow cover. In the melt pools at the surface of the ice the light intensity can reach levels where photoinhibition occurs as photosynthetic pigments are adapted to work at low light levels. However in the winter season there can be extended periods of darkness when the algae form spores/cysts or have a resting stage similar to hibernation. Hence an algal cell must have adaptations to protect against damaging effects of high light levels whilst working efficiently at low light levels, and be able to survive long periods of darkness (Thomas 2004).
Low light intensity adaptations
Algae under the ice receives low light intensities and restricted wavelengths, hence many accessory pigments are rapidly synthesised to absorb available light. Algae found in the ice have adapted to low light levels that become worse over the season as more algae grow (Manes & Gradinger 2009). Chlorophyll a is present in all algae, but diatoms in low light levels contain increased concentrations; Meiners et al, (2009) found diatoms at the bottom of the brine channels had the largest concentration of chlorophyll a. Diatoms produce accessory pigments such as chlorophyll c in low light levels to enable maximum utilisation of available wavelengths of light penetrating the ice (Figure 10).
Some photosynthetic organisms have been shown to cope with low light levels by increasing the size of the photosynthetic reaction site and increasing the concentration of chlorophyll a, whilst others maintain the size of the photosynthetic unit and reduce the volume of carotenoids (Wright & van den Enden 2000, Hashihama et al. 2010).
High light intensity adaptations
At the surface of the sea ice there can be high levels of PAR but there are also high levels of UV radiation; because of this the algae in the ice has several mechanisms to avoid photoinhibition of the photosynthetic organs due to high light levels.
Fucoxanthin and β-carotene are xanthophylls from the carotenoid group; they reduce the amount of light reaching the photosynthetic reaction sites, thus helping to prevent photoinhibition. The xanthophyll cycle in diatoms converts diadinoxanthin to diatoxanthin by the removal of the epoxy groups by enzymes (Figure 11). This light energy is dissipated as heat energy within the diatoms (Mangoni et al. 2009, Hashihama et al. 2010). Fucoxanthin traps wavelengths from 450-540nm giving the diatoms their characteristic coffee colour (Thomas 2004).
Another mechanism is chloroplast clumping. Chloroplasts move along cytoplasmic strands to distribute themselves around the cell; however in high light intensities the chloroplasts clump around the nucleus to prevent damage (Thomas 2004).
Algal species can also produce UV absorbing pigments to prevent photosynthetic inhibition by excess UV radiation; this is especially prevalent in Antarctica where there is less protection from the ozone layer (Kirst & Wiencke 1995).
Adaptations to survive in the dark
The algae in Polar Regions have to withstand long periods of darkness. Although algal abundance is decreased by at least one order of magnitude during the polar winter, species diversity does not appear to be affected by this extreme loss of light that would be devastating in most environments (Werner et al. 2007). Survival is achieved by either entering into a resting stage or producing pores/cysts.
Bunt & Lee (1972) found that microalgae in sea ice did not indicate heterotrophy in all algal species, but rather that the respiratory rates slowed and carbon concentrations decreased, aided by a decrease in water temperature, which slowed metabolic rates. Cell division uses carbon and hence models have shown cell division to stop once carbon concentrations reach a critical value; laboratory experiments have shown that cell division only occurrs for a short period after darkness (Jochem 1999, Furusato et al. 2004, Furusato & Asaeda 2009). Reducing metabolic rates conserves energy reserves and sustains the algae through the winter period.
Facultative heterotrophy and absorption of dissolved organic matter are used by some species of diatoms and flagellates to sustain them over the dark period (Palmisano & Sullivan 1985).
Spore and cyst formation, another adaptation to extended periods of darkness, have been seen in species such as diatoms; however the abundance of diatom cysts was only a fraction of the abundance of diatom cells, indicating that alternative survival strategies are more common (Werner et al. 2007).