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

The migratory patterns of an Oscillatoria sp, in a hypersaline microbial ma t on Antarctica's McMurdo Ice Shelf, 78 degrees S, 166 degrees E, were exam ined under the natural solar irradiance of austral summer. Upward and downw ard migration was monitored in response to different intensities of full so lar irradiance and of selected wavelengths achieved using a series of filte r 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 o f incubation. Our objectives were two-fold: (1) to determine whether this c yanobacterium displays a pattern of migration, photosynthesis, and photoinh ibition in response to solar UV and visible irradiance similar to that disp layed by motile cyanobacteria in hypersaline ponds of middle latitudes (sim ilar to 28 degrees N), and (2) to examine the impact of temperature on thes e activities. Oscillatoria sp. migrated completely to the surface under low visible irradiance (<8 W m(-2)), green light, and complete darkness; none of these treatments included UV-B or more than 0.26 W m(-2) UV-A. Red light , which included similar to 0.5 W m(-2) UV-A, promoted a partial upward asc ent. W-A (6.9 W m(-2) UV-A, similar to 0.1 Wm(-2) UV-B), visible irradiance >60 W m(-2) and blue light, which included 0.94 W m(-2) W-A, caused comple te downward migration. Photosynthetic saturation occurred at low visible li ght levels (similar to 26 W m(-2)), and both photo- and UV-inhibition was a pparent. Photosynthetic rates increased in the order 2, <10, <15 and <20 de grees C, and there was no apparent effect of temperature on the magnitude o f UV inhibition on photosynthesis. Overall trends in photosynthesis and mig ration patterns of Oscillatoria sp. parallel those described for benthic cy anobacteria of middle latitudes, though the Antarctic species appears to ha ve a lower response threshold to visible light and W. These results are con sistent with the hypothesis that UV radiation is functioning as a primary c ue for avoidance of damaging solar radiation in the Oscillatoria sp. popula tion, and suggests that W is involved in the migratory behavior of motile c yanobacteria in microbial mats worldwide.