Effect of salinity on photosynthesis and mannitol synthesis in the green flagellate Platymonas suecica

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Abstract
Platymonas suecica, a brackish-water green flagellate belonging to the Prasinophyceae, grows almost at the same rate over a salinity range of 20–200% normal seawater, and at lower growth rates at salinities as high as 300% or as low as 10% seawater. Both cell size and carbon content increase significantly at salinities above 200% seawater, and there is a linear increase in intracellular mannitol concentration with increasing salinites from 50 to 300% seawater. This relationship between salinity and mannitol content indicates that mannitol is probably involved in osmoregulation in this alga. Photosynthesis is relatively little affected when cells adapted to low salinities are transferred to high salinities, while high-salinity-adapted cells suffer a temporary loss of much of their photosynthetic capacity when transferred to low salinities. It appears that this temporary loss of photosynthetic capacity in response to osmotic shock is due to a non-specific release of soluble cell constituents, including mannitol and amino acids, followed by regain of normal permeability properties and readjustment of cellular composition to the new osmotic environment.