AN EXTREME ENVIRONMENT THECAMOEBIAN ASSEMBLAGE IN THE ANTARCTIC PERMANENT ICE-COVER LAKE HOARE (TAYLOR VALLEY)

The McMurdo Dry Valleys comprise the largest ice-free area in Antarctica and they are characterised by limited precipitation. They contain many permanent lakes with the presence of permanent ice cover, that limits the amount of radiation reaching the water column and reduces the interaction between atmosphere and lake.. However it provides a shelter from the harsh Antarctic climate, maintaining the lake ecosystem. Two cores were collected in one of these lakes (Lake Hoare, Taylor Valley) and the samples were examined for their thecamoebian-content. Thecamoebians were studied in suspension, and they were generally present in the two cores, even if with low concentration. The tests appeared quite soft and sometimes collapsing. All the specimens were organic tests and no agglutinated species were recovered. Most of the species were referred to the genus Arcella. Moreover, the thecamoebian concentration generally increased in the levels were bacterial mat- fragments were more abundant, while thecamoebians were near absent where the unsorted sand was dominant. This probably reflects the sedimentation pattern occurring in this kind of lakes; that is intervals with mat growth alternated to sediment accumulation on the ice surface and consequent fallout to the ice bottom, resulting in localised mounds on the bottom. This kind of study has been carried out because this environment and its protozoan content may represent the equivalent of past lakes present at lower latitudes during glacial intervals and a component of ancient life possibly present in other planets of the Solar System where the presence of water in the past has been suggested.