Bioenergetic and metabolic processes for the survival of sulfur-deprived Dunaliella salina (Chlorophyta)

The work addressed bioenergetic, metabolic and physiological responses of the green alga Dunaliella salina to sulfur (S)-deprivation. Photo-autotrophically grown cells were suspended in a medium in which the sulfates were replaced by chloride salts. Growth characteristics, pigment content, rates of photosynthesis and respiration, as well as endogenous substrate (starch and protein) accumulation were monitored as a function of time under S-deprivation. Lack of S from the growth medium had a differential effect on photosynthesis and respiration. The rate of light-saturated photosynthesis declined semi-exponentially with time, whereas the activity of respiration remained fairly constant over a period of up to 100 h in S-deprived medium. Cell division and 'packed cell volume increase' declined in tandem with the decline in the rate of photosynthesis. There was gradual loss of chlorophyll from the cells and a concomitant loss of photochemically competent system-II reaction centers, whereas the concentration of system-I remained largely unaffected under S-deprivation. Cells altered the partition of photosynthate between starch and protein so that control steady-state starch/protein ratios in the light (0.1: 1, w: w) gradually increased up to about 1: 1 as a function of S-deprivation. Sealed Dunaliella salina cultures, in which the capacity of photosynthesis declined to levels lower than that of respiration, consumed dissolved oxygen and became anaerobic in the light. These cultures, however, did not activate the reversible 'hydrogenase pathway' and did not produce H-2 gas. Instead, under extended S-deprivation, cells maintained a low-level cycling of O-2 and CO2 between photosynthesis and respiration that resulted in no net exchange of gases. Such low-level cycling of photosynthesis and respiration was sufficient to ensure the generation of ATP needed for survival of the organism under protracted S-deprivation conditions.