EFFECTS OF MACRONUTRIENTS UPON BUOYANCY REGULATION BY METALIMNETIC OSCILLATORIA-AGARDHII IN DEMING LAKE, MINNESOTA

Gas-vacuolate filaments of Oscillatoria agardhii form a metalimnetic l ayer in Deming Lake, Minnesota. The environmental factors which affect buoyancy and the physiological processes which mediate changes in buo yancy were determined. Buoyant filaments lost their buoyancy in a few hours when incubated at light intensities above those found in situ (a pproximately 15 mumol photons m-2 s-1, or 1% of the surface value). Th e rate of buoyancy loss was accelerated by the addition of 10 muM phos phate at irradiances >200 mumol photons m-2 s-1. The effect of nutrien t additions on buoyancy was also investigated over a longer time perio d by incubating metalimnetic samples in situ. The samples were deploye d for 6 days at a depth where the irradiance was 8% of the surface val ue. As found in short-term experiments, the addition of phosphate resu lted in the largest decrease in buoyancy. However, the addition of amm onia in addition to phosphate attenuated the buoyancy loss on day 2, a nd on day 6 the filaments in these treatments were almost completely b uoyant. The physiological status of the filaments in these treatments was assayed by analysis of elemental ratios of C, N and P, and by meas urement of cellular chlorophyll, polysaccharide and protein. In additi on, the cellular content of gas vesicles was determined. The construct ion of ballast balance sheets from these data indicated that changes i n buoyancy were primarily due to differences in the amount of polysacc haride ballast in the cells. However, in another set of in situ experi ments, the increase in measured ballast molecules did not explain the observed loss of buoyancy. We hypothesized that another, undetected ba llast-providing molecule had accumulated in the cells.