Seawater circulates through new oceanic crust formed at mid-ocean ridges,
and the subsequent reactions can add or remove elements from the oceans.
Bacteria may also mitigate crustal alteration at low temperatures
(<120 ºC)-- microscopic observations have revealed small, micrometer-sized
channels or tubes extending into rocks at the alteration front even deep in
the crust, as well as the nearby presence of organic matter and DNA.

A detailed examination of an alteration front by Alt et al. with
transmission electron microscopy now reveals that many of the small tubes
are mostly filled with clay minerals produced by the divitrification of the
volcanic basaltic glass (an ion exchange process in which potassium is
leached from seawater). Thus, the available room in many tubes would seem
to present a tight squeeze for most bacteria. In addition, the amount of
potassium consumed would have required the circulation of large volumes of
seawater. -- BH

Details in: Earth Planet Sci. Lett. 181, 301 (2000).