DIEL MIGRATIONS OF MICROORGANISMS WITHIN A BENTHIC, HYPERSALINE MAT COMMUNITY

We studied the diel migrations of several species of microorganisms in a hypersaline, layered microbial mat, The migrations were quantified by repeated coring of the mat with glass capillary tubes. The resultin g minicores were microscopically analyzed by using bright-field and ep ifluorescence (visible and infrared) microscopy to determine depths of coherent layers and were later dissected to determine direct microsco pic counts of microorganisms. Microelectrode measurements of oxygen co ncentration, fiber optic microprobe measurements of light penetration within the mat, and incident irradiance measurements accompanied the m inicore sampling. In addition, pigment content, photosynthesis and irr adiance responses, the capacity for anoxygenic photosynthesis, and gli ding speeds were determined for the migrating cyanobacteria. Heavily p igmented Oscillatoria sp. and Spirulina cf. subsalsa migrated downward into the mat during the early morning and remained deep until dusk, w hen upward migration occurred. The mean depth of the migration (not mo re than 0.4 to 0.5 mm) was directly correlated with the incident irrad iance over the mat surface. We estimated that light intensity at the u pper boundary of the migrating cyanobacteria was attenuated to such an extent that photoinhibition was effectively avoided but that intensit ies which saturated photosynthesis were maintained through most of the daylight hours. Light was a cue of paramount importance in triggering and modulating the migration of the cyanobacteria, even though the mi grating phenomenon could not be explained solely in terms of a light r esponse. We failed to detect diel migration patterns for other cyanoba cterial species and filamentous anoxyphotobacteria. The sulfide-oxidiz ing bacterium Beggiatoa sp. migrated as a band that followed low oxyge n concentrations within the mat during daylight hours. During the nigh ttime, part of this population migrated toward the mat surface, but a significant proportion remained deep.