| As with other extreme environments, the biota of the Antarctic continent is dominated by microorganisms. Benthic mats which form in lakes and ponds are among the most prevalent of these microbial communities, and are dominated by photosynthetic oscillatorian cyanobacteria. Investigation of growth and photosynthetic parameters, versus temperature, of three unusual strains isolated from benthic mats of Antarctica's McMurdo Ice Shelf showed them to be psychrophilic, having temperature optima for growth and photosynthesis below 15°C. In contrast, other benthic oscillatorians described from polar regions are characterized as psychrotolerant, having growth optima above 20°C. That psychrophilic strains do not dominate in polar benthic ecosystems suggests that a selective force other than temperature is exerting the strongest pressure in these communities.; Phylogenetic analysis indicates that psychrophilic oscillatorians share a recent common ancestor. They are not, however, endemic to Antarctica, in that their closest genetic affiliation is with an Arctic strain. The psychrotolerant phenotype, conversely, has arisen multiple times in the cyanobacterial lineage, and psychrotolerant strains are often most closely related to organisms of temperate latitudes. These findings support the hypothesis that Antarctic oscillatorians originated from more temperate species.; Since the development of the Antarctic ozone hole, and the associated increase in transmission of UV radiation, the capacity of Antarctic organisms to respond to changing UV fluxes has become an issue of concern. Field studies examining patterns of migration, photosynthesis, and photoinhibition, of a species of Oscillatoria in response to ambient and modified solar UV, visible irradiance, and temperature were conducted. Solar irradiance >60 W m-2 caused downward migration, whereas high visible irradiance alone, UV-B, and UV-A > 0.26 W m-2 prevented upward migration. Photosynthetic saturation occurred at low visible light levels (∼26 W m-2), and both photo- and UV-inhibition were apparent. Photosynthetic rates increased in the order 2°C < 10°C < 15°C < 20°C. There was no effect of temperature on the magnitude of UV inhibition of photosynthesis. These results are consistent with the hypothesis that UV radiation functions as a primary cue for avoidance of damaging solar radiation in the migratory behavior of motile Antarctic cyanobacteria. This dissertation includes co-authored material. |
