ANTARCTIC CYANOBACTERIA - LIGHT, NUTRIENTS, AND PHOTOSYNTHESIS IN THEMICROBIAL MAT ENVIRONMENT

The microenvironmental and photosynthetic characteristics of Antarctic microbial mats were measured in a series of ponds near McMurdo Sound. As elsewhere in Antarctica, these cold-water benthic communities were dominated by oscillatoriacean cyanobacteria. Despite large variations in mat thickness, surface morphology, and color, all of the communiti es had a similar pigment organization, with a surface carotenoid-rich layer that overlaid a deep chlorophyll maximum (DCM) enriched in phyco cyanin as well as chlorophyll a. Spectroradiometric analyses showed th at the DCM population inhabited an orange-red shade environment. In se veral of the mats, the deep-living trichomes migrated up to the surfac e of the mat within 2 h in response to a 10-fold decrease in surface i rradiance. The euphotic layer of the mats was supersaturated in oxygen and contained ammonium and dissolved reactive phosphorus concentratio ns in excess of 100 mg N . m-3 or P . m-3. Integral photosynthesis by core samples was saturated at low irradiances and varied two- to three fold throughout the continuous 24-h radiation cycle. Oxygen microelect rode analyses showed that the photosynthetic rates were slow to neglig ible near the surface and maximal in the DCM. These compressed, nutrie nt-rich euphotic zones have some properties analogous to planktonic sy stems, but the integrated photosynthetic responses of the community re flect the strong self-shading within the mat and physiological dominan ce by the motile, DCM populations.