The response of the green halotolerant alga Dunaliella to osmotic stress: effects on pyridine nucleotide contents

Abstract

Dunaliella tertiolecta was subjected to osmotic stress by upward (salt stress) and downward (dilution stress) shifts in NaCl concentration, and the effects on glycerol content, photosynthesis, respiration and pyridine nucleotide contents determined. The increase in glycerol content after salt stress was not dependent on photosynthesis, since it also occurred in the dark and larger stresses were inhibitory to photosynthesis, but was always associated with an increase in the contents of NADPH and NADP +, a decrease in NAD + and an initial increase in the NADH/NAD + quotient. The possible contribution of these changes towards increasing the rate of glycerol synthesis is evaluated. When compared with published contents for dihydroxyacetone phosphate and glycerol phosphate, the results obtained here show that the glycerol phosphate dehydrogenase reaction is far from equilibrium, suggesting that the activity of this enzyme is inhibited in vivo. The decrease in glycerol content after dilution stress was associated with a partial inhibition of photosynthesis, an increase in respiratory oxygen consumption, and an increase in the NADPH/NADP + quotient in the light but not in the dark. It is considered unlikely that changes in NADP contents are directly responsible for the accelerated glycerol dissimilation after dilution stress.