Buoyancy regulation of Microcystis aeruginosa

Abstract

Cyanobacteria often dominate the algal community of eutrophic waterbodies during summer months. In part, their success can be attributed to the presence of rigid, gas-filled, proteinaceous cellular inlusions, gas vesicles (WALSBY 1994). Gas vesicles allow Cyanobacteria to avoid losses by sedimentation during periods of low mixing and also allow them to regulate their buoyancy (OLIVER 1995), migrate vertically in the water column and thereby overcome the vertical separation between light and nutrients (GANF & OLIVER 1982). There are three mechanisms evident in gas-vacuolated Cyanobacteria which allow them to regulate their buoyancy: (i) dilution of gas vesicles by growth; (ii) collapse of gas vesicles by increased turgor pressure, (iii) change in the concentration of cellular constituents which act as ballast. Gas - vacuolated Cyanobacteria with gas vesicles too strong to be collapsed by turgor pressure regulate their buoyancy by the accumulation and loss of carbohydrate by photosynthesis and respiration. KROMKAMP & WALSBY (1990) developed a model for Oscillatoria agardhii incorporating density change, Stoke's law of sinking velocity, diel irradiance and light attentuation. This paper investigates the regulation of ballast as a mechanism for diel buoyancy changes in Microcystis aeruginosa KUTZ emend. ELENKIN. The model of KROMKAMP & WALSBY (1990) was used to determine the influence of colony size and water clarity on the amplitude and frequency of diel migrations. These simulations were used to verify whether or not colony size influenced the ability of M. aeruginosa to overcome the vertical separation of light and nutrients.