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

Abstract

Chlorella sorokinianaShihira & Krauss, strain 211/8K resuspended in a illuminated mediumshowed continuous growth, and concomitantly NH4 ⁺in the medium depleted at a constant rate. Upon sulfate removal, i) growthdeclined, ceasing within 5 h; ii) NH4 ⁺ depletionbecame almost undetectable over 2 h; and iii) photosynthetic capacity (Pc) wasreduced over 24 h by 80% in the light, but was reduced by only30% in darkness. Over 24 h of S starvation the chlorophyll content ofilluminated cells decreased by 50%, whereas that of darkened cellsdecreased by only 10%. Sulfate-deprivation over 24 h resulted in anoticeable increase in the starch content of the cell and a net increase inthe pools of glutamate, glutamine, serine and asparagine. Cysteine content, incontrast, was decreased. Sulfate addition to cells starved of S for 24 hprovoked a sudden time-dependent increase in Pc and in an immediate renewal ofgrowth in light but not in darkness. Sulphate supply also caused a suddenenhancement of respiratory oxygen consumption and a prompt degradation ofstarch. Starch was degraded at a higher rate in illuminated than in darkenedcells. After the S addition, depletion of NH4 ⁺ inthe medium occurred at slowly increasing rates; amino acids decreasednoticeably over 4 h, where in contrast, cysteine increased noticeably.