AME 20:231-243 (1999)  -  doi:10.3354/ame020231

ABSTRACT: The migratory patterns of an Oscillatoria sp. in a hypersaline microbial mat on Antarctica's McMurdo Ice Shelf, 78°S, 166°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. [¹⁴C] 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 (~28°N), and (2) to examine the impact of temperature on these 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 ~0.5 W m^-2 UV-A, promoted a partial upward ascent. UV-A (6.9 W m^-2 UV-A, ~0.1 W m^-2 UV-B), visible irradiance >60 W m^-2, and blue light, which included 0.94 W m^-2 UV-A, caused complete downward migration. Photosynthetic saturation occurred at low visible light levels (~26 W m^-2), and both photo- and UV-inhibition was apparent. Photosynthetic rates increased in the order 2, <\10, <\15 and <\20°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 UV. 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 UV is involved in the migratory behavior of motile cyanobacteria in microbial mats worldwide.

KEY WORDS: Cyanobacterial mats · Vertical migration · Photosynthesis · Oscillatoria sp. · UV · Solar irradiance · Antarctica