Salinity responses of the marine microalga Brachiomonassubmarina Bohlin

Abstract

Brachiomonas submarina Bohlin, a green flagellate isolated from a coastal marine environment grows over the salinity range of 0.5 to 400% artificial sea water (ASW: 100% ASW = 3.4% salinity), remains motile at salinities up to 750% ASW, and survives the saturation of sea water. However, the capacity of the alga to tolerate salinity is diminished markedly when the temperature of the growth medium is lowered to 5°C. The alga exhibits greater sensitivity to non-ionic than to ionic osmotic stress. The alga can photoassimilate bicarbonate at salinities as high as 750% ASW when the external salinity is increased gradually. However, photosynthesis is completely abolished when cells are transferred from 5 to 200% ASW. Conversely, cells transferred from 200 to 5% ASW show an increase in the photoassimilation rate. Glycerol is the major osmoregulatory organic solute in this alga. The glycerol content of the cells increases almost linearly with increases in the external salt concentration from 10 to 300% ASW. The cell content of free amino acids, including proline, also increases in B. submarina in response to salinity increases, but this increase is quantitatively much less important than that of glycerol.