THE EFFECT OF SOLAR UV AND VISIBLE IRRADIANCE ON THE VERTICAL MOVEMENTS OF CYANOBACTERIA IN MICROBIAL MATS OF HYPERSALINE WATERS

The vertical gliding motility patterns of Oscillatoria cf. laetevirens and Spirulina cf. subsalsa from hypersaline ponds near Guerrero Negro , Mexico, were monitored under natural solar irradiance. Each of these two filamentous cyanobacteria land together) forms a distinct band wi thin soft microbial mats. Upward and downward migration in excised mat s was measured in response to different intensities of the full solar spectrum and of selected wavelengths, using various filters and screen s. Positions of the cyanobacteria were quantified at the beginning and end of the treatments, using microscopic examination of minicores. In addition, photosynthetic rates of these cyanobacteria, using freshly collected cell material, were measured by C-14 incorporation under dif ferent intensities and spectral regions of solar irradiance. Upward mi gration to or near the surface by either Spirulina or Oscillator ia oc curred under low visible light (20-90 W m(-2)), green light (similar t o 250 W m(-2)), red light (similar to 470 W m(-2)) and in complete dar kness. It was prevented by intensities of UV-A above similar to 1.5 W m(-2) and by broad visible light above similar to 100 W m(-2). Inciden t UV-B intensities as low as 0.1 W m(-2) may also have slowed movement upward. Downward migration of Spirulina cf. subsalsa was promoted by high UV or visible radiation. With a larger data set for Oscillatoria cf. laetivirens, it was apparent that downward migration occurred in r esponse to visible light over similar to 400 CV m(-2) and to relativel y high levels of W-A alone (> similar to 10 W m(-2)). If forced experi mentally to remain on the surface during periods of high solar irradia nce, both cyanobacterial species suffered photo and UV inhibition of p hotosynthesis, with UV-B and UV-A being particularly effective. Attenu ation measurements within the mat showed that UV-A and W-B wavelengths can penetrate deeply enough into mats to potentially act as positioni ng cues for these motile cyanobacteria, and as a consequence allow the m to take refuge from higher intensities of damaging radiation. (C) 19 98 Published by Elsevier Science B.V. All rights reserved.