Studies on the Ecology, Growth and Physiology of Toxic Microcystis Aeruginosa in South Africa

Abstract

Toxic blooms of Microcystis aeruginosa are a regular feature of numerous impoundments throughout southern Africa. In the more eutrophic impoundments, bloom formation can reach serious proportions, often resulting in cattle kills. The extent of the problem in South Africa is taken into perspective. As a result of the variability in appearance of the natural populations it is not possible to identify the toxic strains from microscopical identification alone, but samples collected from various impoundments over a period of four years indicated that toxicity, when present, was always associated with a form described by Kom£rek (1958) as Microcystis aeruginosa forma aeruginosa and never with the form described as M. aeruginosa forma flos-aquae. Laboratory experiments supplemented with field measurements indicated that M. aeruginosa can grow well over a range of light intensities, giving it a competitive advantage over other algae. The alga adapts physiologically to different light intensities by changing its gas vacuole and pigment composition. At high light intensities there is an increase in gas vacuole content, while the reverse is true at low light intensities. The pigment and gas vacuole changes enable M. aeruginosa to grow rapidly under a variety of conditions and partly explains the success of this alga in many turbid South African impoundments.

Part of the work contained in this report was presented by the first author under the title of “Observations on the ecology, growth and physiology of Microcystis aeruginosa in the laboratory and in the field” at the Symposium on “Health Aspects of Water Supplies” Pretoria, November 15, 1979.