COMMUNITY STRUCTURE AND PIGMENT ORGANIZATION OF CYANOBACTERIA-DOMINATED MICROBIAL MATS IN ANTARCTICA

Benthic microbial mat communities were sampled from 20 lakes, ponds an d streams of the McMurdo Sound region, Antarctica. At least five disti nct assemblages could be differentiated by their cyanobacterial specie s composition, pigment content and vertical structure. The most widely occurring freshwater communities were dominated by thin-trichome (0.5 -3 mum) oscillatoriacean species that formed benthic films up to sever al millimetres thick. 'Lift-off mats' produced mucilaginous mats 1-5 c m thick at the surface and edge of certain ponds. Another group of osc illatoriacean communities was characteristic of hypersaline pond envir onments; these communities were dominated by species with thicker tric homes such as Oscillatoria priestleyi. Black mucilaginous layers of No stoc commune were widely distributed in aquatic and semi-aquatic habit ats. Dark brown sheath pigmentation was also characteristic of less co hesive mats and crusts dominated by Pleurocapsa, Gloeocapsa and Caloth rix. High performance liquid chromatography analysis of the lipophilic pigments showed that the upper region of most of the Antarctic mats w as enriched in sheath pigments (scytonemin) and/or certain carotenoids such as myxoxanthophyll and canthaxanthin. Most of the chlorophyll a (Chla), as well as phycocyanin, beta-carotene and echinenone, was loca ted in the lower strata of the mat profiles. In many of these communit ies most of the photosynthetic biomass occurred in a 'deep Chla maximu m' that was well protected from short-wavelength radiation by the surf ace layer of light-screening pigments.