It is well documented that microbes can survive and even thrive in extreme heat. On the other end of this spectrum are organisms that live in extremely cold environments. During winter months in the Polar Regions large areas of ocean become covered by sea ice as sea water freezes. The temperature in this ice ranges between -1.8oC at the sea-ice interface and >-20oC at the air-ice interface. As the ice forms, salt separates from the water and small brine channels are created that act as drainage through the ice. Despite the extreme cold and high salinity, the brine channels are home to many micro-organisms. An important adaptation these organisms possess is the ability to produce a form of antifreeze. Different names for these molecules exist depending on the authors, but these include antifreeze proteins, ice binding proteins or ice structuring proteins.
One of the most dominant microbes in the Antarctic sea ice belongs to the diatom genus Fragilariopsis . In a study by Krell et al (2008) a species belonging to this genus called F. cylindrus was one of the first discoveries of a diatom to produce antifreeze proteins (AFP). Its discovery was actually an accident as they were originally examining this species ability to with stand salt stress. An increase in salt concentration caused the up regulation of four genes that encode for antifreeze proteins. Three of these genes were almost identical and the proteins they produce are believed to secrete from the cell and prevent the formation of ice crystals around them.
Bayer-Giraldi et al (2010) continued research on this topic to see whether more AFPs could be identified in F. cylindrus and another species, F. curta. Using PCR amplifications with different primers they were able to isolate the genes identified by Krell et al (2008) plus some additions. In total there were 10 in F. cylindrus and 11 in F. curta making up a multigene family in each species. Majority of the genes had very little variation between them and sometimes only resulted in a single amino acid replacement but as the protein structures have not been studied in detail, it is unknown if this has any relevance. The authors suggested that having multiple genes that are essentially the same would compensate for reduced kinetic activity within cells and their components. In other words, these cells can continue to produce AFPs in relatively high amounts even when enzyme activity has slowed right down as a result of low temperature.
A second objective of this study was to compare these genes with other groups of organisms. Fragilariopsis AFPs appear to be significantly different to other diatom AFPs suggesting that they were acquired through horizontal gene transfer from different microorganisms. Similarities with other microbes indicate Fragilariopsis acquired AFPs from bacteria or fungi but archaea were possibly the source for other diatoms. There were also remarkable similarities between some of the microbe AFPs and AFPs found in a species of copepod called Tephos longipes which also lives in Antarctica. This indicates that AFP genes are highly mobile and seem to play a very important role in polar adaptations.
A Review of:
Bayer-Giraldi M, Uhlig C, John U, Mock T and Valentin K (2010) Antifreeze proteins in polar sea ice diatoms: diversity and gene expression in the genus Fragilariopsis. Environmental Microbiology. 12: 1041-1052.
Additional Reference:
Krell A, Beszteri B, Dieckmann G, Glockner G, Valentin K and Mock T (2008) A new class of ice-binding proteins discovered in a salt-stress-induced cDNA library of the psychrophilic diatom Fragilariopsis cylindrus (Bacillariophyceae). European Journal of Phycology. 43: 423-433.
7 comments:
Over the Christmas break I saw a repeat of Prof Brian Cox's Wonders of the solar system. One of the episodes talks about the possibility of alien life and he mentions very briefly the microbes that live in sea ice and argues that such organisms might exist on the ice moon Europa that orbits Jupiter. Gravitational forces from Jupiter causes the moon to distort and give off heat from it core which maintains an ocean hundreds of miles deep beneith the ice. Wouldnt it be a great day for science if some form of microbial life was found in the ice or ocean of Europa. Unlikley I guess but you never know. It would be a fantastic PhD.
I reviewed a paper back in December about a related topic - mixotrophic microbial activity in the polar regions. It seems that there is alot to learn about the marine environments in these regions both from molecular and ecological aspects alike. This could be of particular significance because of the high profile these regions have in the debate surrounding anthropogenic climate change.
would it be possible to actually put some life on Europa and see what happens? If we learn how these environments work then we might get life to survive there! might be ethical issues though.
Hey, I was just wondering whether it was mentioned in the paper about fish that live in the same regions which also have this antifreeze protein. Do fish produce this protein themselves? or do they have symbiotic relationships with bacteria which do produce the antifreeze proteins? or are they two separate adaptations?
Hi David, I read your review on mixotrophic microbes. It seems to make good sense to me as every 6 months or so their environment switches from complete darkness to 6months of continuous sunshine then back to 6 months of darkness. It must be a real advantage to be able to maybe switch from phototrophic to heterotrophic or vice versa. I must admit that until recently I had no idea that anything could be active in ice. Even though they dont freeze solid they live in such a cold environment it seems so unlickly a place to find anything living.
Hi Alice,
I think a major barrier for doing that is the ice itself. On average the surface is about 100 Kelvin far colder than anywhere on earth and the ice is rock hard as a result. Maybe one day we will find out more about the ocean beneith but not for a long time I guess. I dont even know if any craft are heading there at the moment.
Hi Arainna
The paper does mention species of fish that produce antifreeze proteins but I think they produce it themselves. It would be cool if there was some degree of homology between theirs and the antifreeze proteins from the sea ice microbes. sadly this paper did noy cover that. They did however find alot of similarity with a copepod found in the same environment.
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