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anhydrobiosis in antarctic organisms

Blogging on Peer-Reviewed Research I was vaguely contemplating writing about a question in last year's Schol paper, to do with antifreeze proteins in polar fishes, when a journal alert popped up in my in-box. It was for a paper entitled How do terrestrial Antarctic organisms survive in their harsh environment? (Wharton & Marshall, 2009). The uni has an extensive Antarctic research program, & in fact I was lucky enough to go down there myself (as an 'extra' on a botany study) a few years ago, so I was easily distracted. The fish aren't going anywhere, after all :-)

Most people probably think 'penguins!' when asked to name a terrestrial organism that lives on mainland Antarctica. But while penguins breed there, the real stars - the organisms that survive on this harsh, dry, frozen continent all year round - are things like mosses, lichens, springtails & nematodes. While you might not think it, with all that snow & ice around, one of the main difficulties  faced by these organisms (in addition to coping with the cold!) is a shortage of liquid water. There's not much of the stuff - just a bit of melt water in the spring. So how do these organisms survive for the rest of the year?

They enter a state called 'anhydrobiosis' - a dormant condition when their metabolism effectively stops. Reversibly! One of my favourite examples of this is seen in microscopic-but-cute animals called tardigrades, or 'water bears', but species of rotifers, nematodes, & arthropods also possess this adaptation. And nematodes are a significant part of the Antarctic terrestrial ecosystem.

There are at least two physiological adaptations involved in this survival trick. One is the presence of a sugar called trehalose: Wharton & Marshall comment that it replaces water in cells, so that deep-frozen membranes & proteins retain their structure & function. In some animals there's also a suite of proteins, called 'late embryogenesis abundant (LEA) proteins, which may stop other proteins from clumping together as cells & tissues dry out. However, there may be other mechanisms involved - and one way to find out is to use complementary DNA (cDNA) to look for patterns of gene expression related to freezing and thawing. Wharton & Marshall note that this technique, already tried in organisms as different as frogs and springtails, was recently applied to a nematode (Plectus murrayi)  from the Dry Valleys and other sites in the eastern mainland of Antarctica.

The study they report on found that there were a number of gene sequences expressed specifically in desiccated P.murrayiAbout a quarter of these (N=22) were ... associated with metabolism, 15 were involved in environmental information processing (including a homologue of type II antifreeze protein from fish, 23 with genetic information processing, [and] 17 encoded novel proteins...  (Wharton & Marshall, 2009). LEA and a gene involved in trehalose synthesis both showed marked increases in expression in dried-out nematodes - as did a number of genes associated with the production of antioxidants. And so did a gene encoding a type of protein called an aquaporin - a protein that controls water movement across cell membranes.

However, the study described by Wharton & Marshall leaves a number of questions unanswered - not least because the nematodes were treated rather benignly, compared to the conditions they'd experience on the ice: 87% relative humidity (RH) & 23oC, for 2 days. Both RH & temperature are much lower in Antarctica! So we still don't know what goes on in P.murrayi in its natural environment. The authors suggest that it may be more instructive to compare the responses to desiccation between species that are not capable of anhydrobiosis, those that will survive anhydrobiotically if they are dried slowly, and those that will survive immediate exposure to severe desiccation. So there's plenty of work yet for present - & future - Antarctic researchers.

By the way, if you're interested in finding out more about Antarctic organisms, there's some great material here on the International Polar Year website :-)

D.A.Wharton & C.J.Marshall (2009) How do terrestrial Antarctic organisms survive in their harsh environment? Journal of Biology 8: 39 

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