A study of halotolerance in blue-green algae

General characteristics of blue-green algal halotolerance were studied by growth experiments and selected analyses. Variation in NaCl concentration was used to mimic salinity. Marine isolates were more halotolerant (8 to 10% NaCl) than non-marine isolates (2% NaCl). Although growth experiments showed NaCl saturation at 1 mg/1 for non-marine isolates and 100 mg/1 for marine isolates, the higher NaCl requirements of marine bacteria (18 g/1) or halophilic bacteria (50 to 200 g/1) were not seen in blue-green algae. Intracellular Na+ values were affected by washing; however, bound- K+ values for both marine and fresh-water blue-greens ranged from 1 to 3 μg/mg cells. The reversal of a KCI-inhibition of growth (60 g/1) with small additions of NaCl (1 g/1) indicated a specific Na+ function. This was also implied by the retention after washing of ²²Na+ (0.3 μg/mg cells) by Agmenellum quadruplicatum (PR-6), a marine coccoid blue-green alga. High concentrations of NaCl apparently inhibit growth more by ionic (Na+) stress than by osmotic stress. Changes in light, temperature, or composition of the basal medium failed to alleviate this stress. In contrast to marine bacteria, cells of PR-6 grown in Medium ASP-2 + 90 g NaCl/1 did not undergo lysis when suspended in distilled water. However, viability of cells grown in ASP-2 + 90 g NaCl/1 decreased rapidly compared to cells grown in ASP-2 + 18 g NaCl/1. Cells of PR-6 grown in ASP-2 + 90 g NaCl/1 were larger than normal, formed chains (3 to 16 cells), and appeared bleached. Analyses of such cells revealed an overall decrease in fatty acids, hydrocarbons, and pigment levels. Electron micrographs showed changes in cellular lamellar systems. These data imply that a better understanding of halotolerance may be obtained by further study of NaCl effects on cellular membranes and photosynthetic lamellae. The photosynthetic rate of PR-6 cells was immediately depressed when the cells were transferred from 18 g NaCl/1 to 70 g NaCl/1 medium. When held in the latter for several hours the rate recovered and approached the initial photosynthetic rate maintained before NaCl-shock. This phenomenon was never seen with non-marine isolates. The explanation may lie in the ability of the cell to adjust to sudden Na+ increase via an ion (Na+) pump, for example adenosine triphosphatase (ATPase). Subsequent assays showed more ATPase activity in a marine isolate than in a non-marine isolate. It is suggested that marine blue-green algal isolates are characteristically more halotolerant, perhaps by selection, than fresh-water forms. This difference may be due in part to inherent capacity of the cell to extrude Na+. Alternatively, in fresh-water forms the Na+ sites are more Na+ sensitive than in marine forms