Microbial mat in the 'liftoff' region of Lake Hoare in the McMurdo Dry Valleys of Antarctca. This image shows the mat just below the ice at a depth of about 5m (ice-cover was about 4 m thick). Note the gas bubbles forming on and withing the mat. The gas is composed primarily of air (N2, O2, and Ar). About 10% of the O2 is produced by photosynthesis, the remainder is present as a result of physical processes. Microbial mat can also be observed floating towards and just beneath the ice cover. Once the mat accretes into the ice it will eventually move towards the surface as more ice is added to the bottom and ice is ablated at the surface.