The unicellular green halotolerant microalga Dunaliella maritima grown in medium containing 500 mM NaCl and the freshwater microalga Chlorella vulgaris were used as model systems to study adaptation of energy yielding mechanisms of cells in culture to changes in salinity. A microcalorimetric method was used to study the alteration of the heat production rate of microalgae depending on the salt content in the culture medium. It was shown that the heat production of Chlorella at low salt concentration (50 mM) increased and that it decreased at high concentration (500 mM). Chlorella was unable to adapt to concentrations of 1 M NaCl and above - in this sense, this degree of salinity was a critical value for the adaptive processes of this microalga. On the other hand, the halotolerant Dunaliella readily adapts to very high salinity. At 1.5 M salt concentration there was a considerable increase in heat production rate up to 200% compared with the 500 mM control. The light reactions of the microalgae in salinity were measured by a photo-microcalorimetric method. Dunaliella cells absorbed heat to a greater extent than Chlorella vulgaris cells which can be interpreted as more stored light energy in photosynthate. A polarographic method with a Clark-type electrode was employed in the dark to determine the respiration rate from the oxygen uptake rate; and photosynthesis was determined in the light from the oxygen evolution rate. At a salt concentration of 50 mM, there was some increase of the oxygen uptake rate by Chlorella cells and at high concentrations of salt in the medium from 500 mM to 1 M there was a sharp inhibition of this process. Dunaliella cells gave a different response to increasing salt concentrations in terms of the oxygen uptake rate. It considerably increased by 50-60% compared when the salt concentration in the medium was increased to 1.5 M. It is interesting to see that the rates of energy yielding processes in the Dunaliella control cells were significantly higher than those of Chlorella cells, presumably because of the increased rate of pumping out Na+ ions. The data support the hypothesis that an important feature of salt sensitive and salt tolerant microalgae is the increased energetic rate which ensures a quick and effective adaptation of them to stress. (C) 2007 Elsevier B.V. All rights reserved.