Fragilariopsis cylindrus live in the ice of Arctic and Antarctic oceans and also sometimes float in open seawater. The species colonises small channels in the ice which are formed when the seawater freezes. Salt concentrations within these channels can be up to 4 times higher than those in normal seawater, and temperatures can reach -30º C. Additionally cells can be stranded in complete darkness for two months. For an alga to survive in this saline soup and withstand an extended period when no light energy can be absorbed, cells must be protected from freezing, tolerate high salt levels and survive without energy production. "During the dark Arctic winter no photosynthesis can take place", explains Kroth.
Ice alga survives by candle light and in permanent frost in a brine soup
Current international research is also based on the genetic code of the species which was recently published. First significant characteristics have already been identified. The bases adenine and thiamine are more common in this species than in two other closely related species. Dr Thomas Mock, who used to lead the international research group at the Joint Genome Center (JGI) in Walnut Creek in California that spent three years sequencing the genetic material, is delighted by these findings. "We are currently investigating the significance of this result for Fragilariopsis”. Mock is particularly fascinated by the fact that the ice alga is still active at -15 ºC and uses minimal amounts of light, i.e. effectively survives ‘by candlelight’ in conditions where no land plants could photosynthesise.
Fragilariopsis modifies its own living space
Mock is part of an international team which currently investigates 15 genes of a novel type of proteins which protect the algae from otherwise lethal freezing. "Some of the anti-freeze proteins can slightly reduce the freezing point of seawater. More importantly, some proteins have the ability to change the shape of ice crystals so that they are no longer harmful to the algae. It appears therefore that Fragilariopsis can modify its own microhabitat and make it more favourable for its own survival" says German algal expert Mock with enthusiasm; Mock teaches and researches at the University of East Anglia in the British city of Norwich.
Fragilariopsis fulfils two important ecological functions in the polar seas. The alga grows underneath the ice and is being nibbled on by krill, small crustaceans and their larvae. Krill, in turn, is a food source for fish and whales. Therefore the ice algae play a key role at the base of the marine food chain. Processes of biomass production, during which oxygen is released, is what biologists call primary production. Together with other diatoms they produce a fifth of all the oxygen we breathe. The contribution of diatoms to the global primary production is therefore as high as that of all rainforests combined. Global warming, which will lead to the melting of polar ice masses, is therefore likely also to have impacts on the extensive populations of diatoms.
Model organism from the polar regions
Fragilariopsis cylindrus is the first higher organism originating from the polar regions for which the genetic code has been sequenced; the results are now being more closely investigated by Mock in an collaborative effort with over 20 colleagues. The researchers have pushed ahead with the sequencing work as the alga is a very suitable model organism. It is one of the most abundant diatoms in both Arctic and Antarctic ecosystems. At the same time it can easily be grown in the laboratory to investigate the genetic control of physiological characteristics. Such research also facilitates experiments simulating elevated carbon dioxide levels and the associated acidification of the oceans.
Like all diatoms, Fragilariopsis reproduces asexually by repeated cell division and is thus as good as ‘immortal’. Because of the reduced metabolic activity due to the low light availability, a cell division cycle takes one to two months. Under more optimal light conditions provided in a laboratory, cell division can occur every two days, Mock says.
Looking forward to new genomic data
To gain a better understanding of the adaptations of Fragilariopsis cylindrus to life in the ice, Mock, together with colleagues from the GenePool Genomecentre in Edinburgh (Scotland), currently also sequence the genome of its relative F. kerguelensis. These results are also eagerly awaited by biotechnologists and physicists who are interested in the structure of the silica shells. Mock and his team are interested in F. kerguelensis because it also lives in polar regions, but only in open and thus slightly warmer waters. By comparing the genome of the two related species, the researchers expect to find new answers regarding the adaptive strategies of Fragilariopsis cylindrus to life in the ice. "To make the invisible visible", is a challenge for Mock. "I’m thrilled by this simple, and yet so beautiful and complex organism".