Growth responses of blue-green algae to sodium chloride concentration

Summary

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 (8–10% NaCl) than non-marine isolates (2% NaCl). The Na⁺ requirement for growth was saturated at 1 mg NaCl/l for non-marine isolates and 100mg NaCl/l for marine isolates. Intracellular Na⁺ values were affected by washing; however, bound-K⁺ values for both marine and fresh-water blue-green algae were fairly constant, 1–3 μg/mg cells. A specific Na⁺ function was implied by the retention after washing of ²²Na⁺ (0.1 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, pH, 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/l did not undergo lysis when suspended in distilled water. However, viability of cells grown in Medium ASP-2+90 g NaCl/l decreased rapidly compared to cells grown in Medium ASP-2+18 g NaCl/l.

Cells of PR-6 grown in ASP-2+90 g NaCl/l were larger than normal, formed chains (3–16 cells), and appeared bleached. Analyses of such cells revealed an overall decrease in fatty acids, hydrocarbons, and pigment levels. Electron micrographs showed that NaCl stressed cells were little altered in morphology.

The photosynthesis of PR-6 cells was immediately depressed when the cells were transferred from 18 g NaCl/l to 70 g NaCl/l 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 suggested more ATPase activity in a marine isolated than in a nonmarine isolate. The ATPase was not, however, ouabain sensitive.

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 freshwater forms rhe Na⁺ functional sites may be more Na⁺ sensitive than in marine forms.