Effects of solar UV and visible irradiance on photosynthesis and vertical migration of Oscillatoria sp. (Cyanobacteria) in an Antarctic microbial mat

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Abstract
The migratory patterns of an Oscillatoria sp, in a hypersaline microbial mat on Antarctica's McMurdo Ice Shelf, 78 degrees S, 166 degrees E, were examined under the natural solar irradiance of austral summer. Upward and downward migration was monitored in response to different intensities of full solar irradiance and of selected wavelengths achieved using a series of filter and screening treatments. [C-14] photoincorporation rates, using freshly collected cell material, were also measured under different intensities and spectral regions of solar irradiance, as well as at several temperatures of incubation. Our objectives were two-fold: (1) to determine whether this cyanobacterium displays a pattern of migration, photosynthesis, and photoinhibition in response to solar UV and visible irradiance similar to that displayed by motile cyanobacteria in hypersaline ponds of middle latitudes (similar to 28 degrees N), and (2) to examine the impact of temperature on these activities. Oscillatoria sp. migrated completely to the surface under low visible irradiance (60 W m(-2) and blue light, which included 0.94 W m(-2) W-A, caused complete downward migration. Photosynthetic saturation occurred at low visible light levels (similar to 26 W m(-2)), and both photo- and UV-inhibition was apparent. Photosynthetic rates increased in the order 2, <10, <15 and <20 degrees C, and there was no apparent effect of temperature on the magnitude of UV inhibition on photosynthesis. Overall trends in photosynthesis and migration patterns of Oscillatoria sp. parallel those described for benthic cyanobacteria of middle latitudes, though the Antarctic species appears to have a lower response threshold to visible light and W. These results are consistent with the hypothesis that UV radiation is functioning as a primary cue for avoidance of damaging solar radiation in the Oscillatoria sp. population, and suggests that W is involved in the migratory behavior of motile cyanobacteria in microbial mats worldwide.