Living on thinning ice: survival mechanisms for extremophilic algae

Extremophiles are organisms that live in physically or geochemically extreme environments. Typically, one might think that only the simplest of life forms such as bacteria may survive such conditions but examples are found in all three domains. Psychrophiles live in low temperatures, known to be inhospitable to most other organisms.

In 2010, on my first expedition to the Greenland Ice Sheet (GrIS), I observed pigmented areas present on the ice surface giving it, in places a purplish-brown hue. Microscopic examination of melted ice scraped from these areas revealed the presence of photosynthetic organisms, some of which were heavily pigmented.

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Purple-brown areas interspersed with bare ice on the GrIS surface.

The organisms found on the ice sheet have also been recorded in snow fields and glacial surfaces in many other locations, world-wide. Some species belong to the phylum Chlorophyta, the green algae. Common species include cysts of Chlamydomonas-like organisms, coloured red by the presence of a carotenoid pigment, also a common feature of ‘water-melon’ coloured snowfields. Other species, may contain purple-brown coloured secondary pigments in vacuoles around the green chloroplasts.

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Chlamydomonas cf. nivalis cysts. The red pigment ‘astaxanthin’ shields the cells from excessive ultra-violet radiation and photosynthetically active radiation, (PAR)

Mesotaenium cf berggrenii. The purple pigments may shield chloroplasts from excess PAR.

It was evident that darker areas of ice would have a much lower surface reflectivity (albedo) compared to bare ice patches and therefore could play an important role in ice sheet melt. On researching the literature, it became apparent that I was not the first to have made this observation. About 140 years ago, Nordenskiold, together with botanist Berggren, first recorded the presence of heavily pigmented algae and cyanobacteria on glacial ice, during expeditions to the GrIS in the 1870’s. Collectively, these organisms were considered, at that time, to be a ‘dangerous enemy’ to the ice as the dark masses could absorb a much greater amount of heat than white ice, thereby promoting melting. This positive feedback is exactly what the algae need as liquid water provides the trigger for growth of these organisms. In particular, 2010 was a year of unprecedented high temperatures leading to a much longer period when surface water was present. Was this just an anomalous year in terms of algal growth or are blooms a common occurrence in the ablation zone of the Greenland ice sheet? Expeditions to the GrIS have been relatively infrequent. Survey work subsequent to 2010 indicates limited visible surface coverage of photosynthetic organisms in colder summers.

These extremophilic algae must be able to cope with a plethora of stresses including constantly low temperatures, periodic freeze-thaw cycles and desiccation, high irradiance (PAR and UVR), low nutrients and potentially might be consumed by grazers on ice sheets and glaciers. Decades of research on bloom formation in waterbodies has identified many factors that lead to the formation of blooms of photosynthetic algae and bacteria. As these factors may show inter-annual variation, the prediction of bloom occurrence is far from straightforward.

We have now identified a number of extremophilic algae capable of growing on ice sheets and glacial surfaces. However, our understanding of the factors that enable their survival and promote bloom formation, is limited. During my Leverhulme Fellowship I plan to fill in some gaps in our knowledge of these extremophiles. Focussing on species from the phylum Chlorophyta, I will investigate thermos-acclimatory responses, examine how their pigment composition changes in response to selected light-temperature scenarios and will begin to examine genomic traits that potentially play a role in cold adaptation.

Dr Marian Yallop
University of Bristol