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It is not surprising to encounter heterotrophic bacteria that move towards organic nutrients or phototrophic bacteria that move towards light, but what about bacteria that seek a chemical even more essential to life – water? In very dry environments, an organism that is able to follow a receding pocket of water would possess an enormous advantage. Does this ability exist?

Ferran Garcia-Pichel (Arizona State University) and Olivier Pringault (Universite Bourdeaux) recently reported the behavior of cyanobacteria that appear to have just this ability [Nature (2001) 413:380-381]. Monitoring the vertical movement of an Oscillatoria population within soil samples from an arid region of Spain, they found that filaments accumulated at the surface over the course of minutes when dry soil was wetted and, conversely, filaments disappeared when the wet soil was allowed to dry.

Movement was not towards light, as it occurred upon wetting equally in the dark as in the light, nor was it the result of a purely physical force (e.g. surface tension), as it was blocked by inhibitors of metabolism. Rather, the movement appears to be towards either wetness itself or towards some environmental condition associated with it, e.g. salinity or the resumed metabolic activity of other microorganisms in the soil.

As with any tactic response, there is the intriguing question as to how a microorganism can sense what is the proper direction in which to head. Chemical or activity gradients over the length of a single cell must be infinitesimal – can a filamentous cyanobacterium compare environmental conditions present at one end of its length with that present at the other? Alternatively, does the filament compare conditions as they change over time, continuing its path if conditions improve but changing directions if they don't?