1.3 Single-celled consumers (protozoa) in the ice (primary consumers)

Single-celled primary producers in the ice (autotrophic algae and bacteria) form the basis of the food web in the ice: they absorb CO2 and, drawing on the energy of light, use it to produce carbon compounds (photosynthesis). They are referred to as being ‘self-sustaining’, that is, as autotrophic or phototrophic. In contrast, other organisms living in the sea ice take up carbon through their food; they are heterotrophic. These include single-celled organisms, the protists or protozoa, which feed on algae, but also on bacteria and archaea (formerly classified as bacteria). In this regard, the most common group are the ciliates. Flagellates are also common, including heterotrophic dinoflagellates. These also include the so-called kleptoplastic dinoflagellates, which are capable of preserving the chloroplasts from the algae or bacteria they consume, and subsequently using them for photosynthesis. Other flagellates in the sea ice include the choanoflagellates. Other heterotrophic protists include gymnamoebia, foraminifera, acantharians and radiolarians (Caron et al., 2017), which can extend parts of their body to capture prey. Today, the microbial food webs in the sea ice remain poorly understood. However, recent studies indicate that heterotrophic protists employ a wide range of feeding strategies.

Occurrence and distribution

Just like algae, bacteria and archaea, the single-celled consumers (protozoa) also find their way into the sea ice while it’s forming. They are, in a sense, encased in the ice structure. The majority can be found in brine channels, in pressure ridges, in meltwater at the snow-ice interface, on the under-side of the ice, and in meltwater pools on the Arctic sea ice (Caron et. al., 2017). These hetero-trophic protists live under extreme conditions: the salinity varies greatly between the brine chan-nels and the seawater (in the Arctic Ocean the salinity level is circa 32; in the brine channels, it can range from 42 to 93 (Wakatsuchi and Ono, 1983) . The temperature in the ice can vary signifi-cantly, and the light intensity not only varies throughout the annual cycle, but also differs between e.g. the surface and underside of the ice. Although low temperatures generally result in declining feeding behaviour and growth rates, the effect of temperature fluctuations on growth is still only partially understood.

Heterotrophic protists can above all be found in places where there are ample bacteria and algae for them to feed on. Their population reaches maximum growth in summer, which is when the available food is at its highest. Accordingly, the greatest biomass is to be expected when the ice melts in late summer. During this time, the heterotrophic protists’ biomass and waste products enter the seawater as particles, making a substantial contribution to the input of organic material in the water column.For example, in the coastal areas of the Antarctic, single-celled consumers (protozoa) from the sea ice account for nearly 20 percent of total integrated microbial biomass. Up to ten percent of the biomass attached to the bottom of the sea ice (Caron et al., 2017). In Greenland’s pack ice, heterotrophic flagellates alone account for 20 percent of total microbiological biomass on average. The relative distribution of autotrophic and heterotrophic protist species is similar in the Arctic and Antarctic. Their absolute abundance in sea ice (number of cells per given volume of melted sea ice) is often one to two orders of magnitude higher than in seawater (number of cells per given volume of seawater).

The role of single-celled consumers (protozoa)Micro-consumers in sea ice ...

  • consume particulate organic carbon (e.g. EPSs, bacteria or algae) and fulfil an important role in biogeochemical cycles in the polar regions.
  • remineralise the organic compounds they consume, producing inorganic nutrients. These serve as a substrate for bacteria to grow on; in turn, the bacteria drive primary pro-duction.
  • are an important component in food relations (trophic interactions) within the marine organic community.
  • form, together with ice algae, a reserve of organic material, which becomes encased in the ice in winter, offering both a food source for overwintering organisms and a ‘kick-start’ for the ecosystem in spring.

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Caron, D. A., R. J. Gast & M.-E. Garneau (2017): Sea ice as a habitat for micrograzers, In: Sea Ice, D. N. Thomas (ed.), 3rd edition, Wiley-Blackwell, Chichester (UK), Hoboken (NJ), 370-393.
Lizotte, M.P. (2003): The Microbiology of Sea Ice, In: D. N. Thomas & G.S. Dieckmann (eds.) Sea Ice, Oxford: Wiley-Blackwell, 201.
Wakatsuchi M. & N. Ono (1983): Measurements of salinity and volume of brine excluded from growing sea ice, Journal of Geophysical Research Oceans, doi.org/10.1029/JC088iC05p02943