The biology of a new gas-vacuolate cyanobacterium, Dactylococcopsis salina sp. nov., in Solar Lake

Abstract

Several gas-vacuolate microorganisms have been found in Solar Lake, a stratified heliothermal saline pool in Sinai. The most abundant of these organisms is a new cyanobacterium Dactylococcopsis salina . It has been isolated and grown in a defined culture medium, and its biology has been studied in the lake throughout two annual periods of stratification. The Dactylococcopsis dominated the lake plankton between depths of 1 and 4 m during December 1979 to April 1980 but disappeared during summer holomixis. It reappeared in November and again persisted through to May 1981 but during this second period of stratification it was distributed only in a narrow band around a depth of 1 m. By determining the physical and chemical requirements of Dactylococcopsis in culture it has been possible to explain the changes in its abundance and vertical distribution in the lake, as follows. (i) Salinity provides no barrier to its distribution as it is able to grow over a wider range, 5-20% , than that encountered in the lake. (ii) The cyanobacterium is killed by prolonged exposure to temperatures exceeding 46 °C. This temperature was exceeded at depths below 1.5 m as the lake heated up in April 1980 and at depths below 2 m throughout the November 1980 to May 1981 period of stratification Dactylococcopsis was absent from these layers. (iii) While the Dactylococcopsis is able to photosynthesize under anaerobic conditions in the presence of sulphide, its efficiency is less than 4% of the aerobic rate. Its inability to compete effectively with other photosynthetic microorganisms in the presence of sulphide may explain its absence from depths below 4 m during the winter of 1980. (iv) Dactylococcopsis is able to adapt to very high light intensities. The natural population has a photosynthetic I k of 900 µE m -2 s -1 and it survives light intensities of 800 µE m -2 s -1 for prolonged periods. However, the combination of even higher intensities and high oxygen concentrations at the lake surface may contribute to its absence there. (v) The absence of Dactylococcopsis from the top 0.7 m of the lake can be explained by grazing pressure from Artemia salina , the brine shrimp. The shrimp effectively filters the cyanobacterium from the epilimnetic water and is able to assimilate it. Artemia is unable to graze the metalimnetic population during stratification because it cannot withstand the elevated temperatures there. (vi) Dactylococcopsis resides in the stratified metalimnion for many months. Losses by sinking, or floating, are minimized by light intensity-regulated buoyancy control in which gas vacuoles provide an essential part.