FEMS Microbiology Ecology 45 (1987) 135-142 135 Published by Elsevier FEC 00115 Buoyancy regulation in phosphate-limited cultures of Microcystis aeruginosa Allan Konopka, Jacco C. Kromkamp and Luuc R. Mur Department of Btologtcal Sctences, Purdue University, West Lafayette, IN 47907, U.S.A., and Laboratory for Mtcrobtology, Untverstty of Amsterdam, 1018 WS Amsterdam, The Netherlands Recewed 6 January 1987 Revision received 11 February 1987 Accepted 17 February 1987 Key words: Microcystisaeruginosa; Buoyancy; Gas vesicle; Chemostat; Phosphate limitation 1. SUMMARY Buoyancy regulation was studied in P-limited continuous cultures of Microcyst& aeruginosa grown on light-dark cycles of 8-16 h. Gas-vesicle content did not vary systematically over a range of dilution rates form 0.004 to 0.015 h -1. A reduc- tion in irradiance did not cause a significant change in gas-vesicle content. The proportion of floating cells decreased during the photoperiod and in- creased during the dark period. At three dilution rates, parallel cultures were grown at growth- saturating irradiance and at a lower irradiance. The cultures at low irradiance had a higher pro- portion of floating cells and a smaller decrease in buoyancy during the light period. The buoyancy losses were not due to destruction of gas vesicles but, rather, to the accumulation of heavy sub- stances. However, measured increases in polysac- charide ballast accounted for only 60% of the required ballast. The molecule(s) which comprised the remainder of the ballast are unknown. Upon relief of phosphate limitation, P-limited cultures increased their buoyancy when incubated in the Correspondence to: A. Konopka, Dept. of Biological Sciences, Purdue University, West Lafayette, IN 47907, U.S.A. dark or light. Buoyancy increases in the dark were correlated with a decrease in polysaccharide con- tent, whereas there was an increase in gas vesicle content in the light. 2. INTRODUCTION Many planktonic cyanobacteria contain gas vesicles, which are hollow, proteinaceous struc- tures in the cytoplasm which can provide buoyancy to the cell [1,2]. The regulation of buoyancy can result in stratified layers of organisms in portions of a water column that are not turbulently mixed [3] or in diurnal migration through the water column [4]. The buoyant status of a cell is a consequence of the relative amounts of gas vesicles and molecules denser than water which it contains. Three mecha- nisms to alter buoyancy are known. (1) Pressure- sensitive gas vesicles can be destroyed by high turgor pressures generated in cells [5]. (2) The rate of gas-vesicle formation can be regulated [6]. (3) Accumulation of dense polymers (especially poly- saccharide) can compensate for the buoyancy pro- vided by gas vesicles [7,8]. Light has been the environmental factor which 0168-6496/87/$03.50 © 1987 Federation of European Microbiological Societies Downloaded from https://academic.oup.com/femsec/article-abstract/3/3/135/504951 by guest on 21 May 2020