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 (and together) forms a distinct band within soft microbial mats. Upward and downward migration in excised mats was measured in response to different intensities of the full solar spectrum and of selected wavelengths, using various filters and screens. 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 14C incorporation under different intensities and spectral regions of solar irradiance. Upward migration to or near the surface by either Spirulina or Oscillatoria occurred under low visible light (20–90 W m−2), green light (∼250 W m−2), red light (∼470 W m−2) and in complete darkness. It was prevented by intensities of UV-A above ∼1.5 W m−2 and by broad visible light above ∼100 W m−2. Incident 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 response to visible light over ∼400 W m−2 and to relatively high levels of UV-A alone (>∼10 W m−2). If forced experimentally to remain on the surface during periods of high solar irradiance, both cyanobacterial species suffered photo and UV inhibition of photosynthesis, with UV-B and UV-A being particularly effective. Attenuation measurements within the mat showed that UV-A and UV-B wavelengths can penetrate deeply enough into mats to potentially act as positioning cues for these motile cyanobacteria, and as a consequence allow them to take refuge from higher intensities of damaging radiation.