The microbial adaptations of the cyanobacterial mats of the ponds of the McMurdo Ice Shelf to the environmental extremes experienced at this location, including changes in temperature and salinity, dehydration-rehydration cycles, low nitrogen: phosphorous ratios and high UV radiation was investigated. The fieldwork was mainly experimental with comparisons made between other pond systems from the ... McMurdo Sound area including Mt. Discovery - Minna Bluff and Pyramid Trough (Alph River) areas and from as far south as latitude 80°, the Darwin Glacier region. The Mt Discovery and Darwin Glacier regions underwent a limnological survey where ponds were sampled for conductivity, pH, temperature, size, discharge (L/s), depth, ion content (sodium, potassium, calcium, magnesium, chloride and sulphate), DRP, DOP, NH4, NO3, DON, benthic biomass and species composition. At Bratina Island, 12 ponds were studied at weekly intervals for water quality parameters (relative water level, sediment and water temperature, pH, DIC, conductivity, DRP, DOP, NH4, NO3, DON) and chlorophyll a concentrations. In addition, samples of water and dissolved gases were collected from around and below the benthic mats for dissolved gas composition (CO2, CH4 and N2O) and nutrient composition. The microbial adaptation to extreme Antarctic environments (low temperature/high salinity) was studied from a number of different physiological responses including biochemical adaptations (biochemical mechanisms of membrane lipid composition), the photosynthetic responses and changes in nitrogen fixation rates. A series of four experiments designed to examine the physiological responses of the benthic mats to the prevailing and extreme conditions which occur each year including response to extremes of salinity and temperature, short-term dehydration, ambient UV radiation and nutrient conditions were completed. The effects of salinity on mat metabolism by creating a salinity gradient and measuring nitrogen fixation, photosynthesis and respiration rates. A long term aim of research on these communities is to define the light-capturing and light screening properties of the microbial communities which dominate this and other freshwater ecosystems. To meet this aim, HPLC analysis of microbial communities in the Alph River-Trough Lake region was carried out to evaluate the changes in pigment composition as a function of UVA and UVB manipulations and as a response to desiccation and salinity changes.