Effects of sulfate-starvation and re-supply on growth, NH4+ uptake and starch metabolism in Chlorella sorokiniana

Chlorella sorokiniana Shihira and Krauss, strain 211/8K resuspended in a illuminated medium showed continuous growth, and concomitantly NH4+ in the medium depleted at a constant rate. Upon sulfate removal, i) growth declined, ceasing within 5 h; ii) NH4+ depletion became almost undetectable over 2 h; and iii) photosynthetic capacity (Pc) was reduced over 24 h by 80% in the light, but was reduced by only 30% in darkness. Over 24 h of S starvation the chlorophyll content of illuminated cells decreased by 50%, whereas that of darkened cells decreased by only 10%. Sulfate-deprivation over 24 h resulted in a noticeable increase in the starch content of the cell and a net increase in the pools of glutamate, glutamine, serine and asparagine. Cysteine content, in contrast, was decreased. Sulfate addition to cells starved of S for 24 h provoked a sudden time-dependent increase in Pc and in an immediate renewal of growth in light but not in darkness. Sulphate supply also caused a sudden enhancement of respiratory oxygen consumption and a prompt degradation of starch. Starch was degraded at a higher rate in illuminated than in darkened cells. After the S addition, depletion of NH4+ in the medium occurred at slowly increasing rates; amino acids decreased noticeably over 4 h, where in contrast, cysteine increased noticeably