Tuesday, March 08, 2005

Life on the edge

This month's Trends in Microbiology has a review by David Thomas about the recent discovery of polar microbial communities which are capable of photosynthesis under extremely cold and dry conditions, and moreover with extremely low light levels. Work reported last September documented a "green band" of microbes colonizing the undersides of rocks near the edges of glaciers, where they are protected from UV irradiation, wind scouring, and drying out. There is only a narrow band along the bottom of the rock where they can live. Too far out, and they're exposed; too far under, and there's not enough light.

Many of the species of bacteria found here have also been seen in other very cold niches. For example, similar cyanobacteria also grow inside sandstone rocks in the Ross Desert in Antarctica. The microbes living in such extreme niches must develop extensive specializations such as antifreeze strategies. It would be interesting to know how these photosynthetic bacteria manage to survive during the long, dark winter. In Antarctic lakes, the protozoa just switch to eating bacteria or each other when light is short, allowing them to grow year-round.

Learning more about these cold-tolerant communities could lead to
new enzymes for molecular biology research, because their enzymes move are active at low temperatures (relative to humans' preference!) and can be easily inactivated at "room temperature." For example, my personal bugaboo, ligation, requires two DNA molecules to dock together long enough for an enzyme to join them permanently. The DNA moves less at low temperatures, but the current tradeoff is that the enzymes move less too. A ligase which prefers cold temperature might reduce my migrane levels.


A second interesting feature of these communities is their razor-thin niche. Any disturbance could result in the whole community getting dried out. However, this also means that they generate instant fossils , similar to the mummies discovered in present-day Peru, which are also preserved by the extreme dry and cold. Fossil bacterial mats of the sort described here would be sitting out in the open, unchanged, until perhaps the rock got jiggled back to an advantageous configuration. It might be that searching for life on Mars would not require drilling or astronauts, but rather just turning over the right kind of rock.

more KT science

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