The cyanobacterium Synechococcus 6301 is able to use a limited number of sulfur compounds as the only source of sulfur supply such as sulfate, thiosulfate, thioacetic acid, mercaptoacetic acid, thioacetamide, ʟ-cysteine and glutathione. Compounds containg thioether linkages such as methionin or S-methylcysteine and all compounds investigated so far containing sulfonic acid structures do not support growth. Growth inhibiton was observed by addition of aminomethane- sulfonic acid or cysteamine.When non-growth sustaining sulfur compounds are added as sulfur source, the C-phycocyanin content of the Synechococcus cultures decreased drastically, causing a shift in color from blue- green to yellow-green. An analysis reveals the degradation of C-phycocyanin whereas chlorophyll formation still proceeds to a certain degree in growing sulfur-starved cells. Supplementation of a suitable sulfur source induces a period of intense and preferential C-phycocyanin synthesis prior to resumption of normal growth.

Thiosulfate-reductase activity (TSR) measured as sulfide release from thiosulfate was detected in crude extracts of Chlorella using dithioerythritol (DTE) as electron donor. Purification of this activity by ammonium-sulfate precipitation between 35% and 80% followed by Sephadex G-50 gel filtration, diethylaminoethyl-cellulose chromatography, and gel filtration on Biogel A 1.5 M led to four distinct proteins having molecular weights of: TSR I, 28000; TSR II, 26500; TSR IIIa, 55000; TSR IIIb, 24000 daltons. These thiosulfate reductases were most active with DTE; the monothiols glutathione, L-cysteine, and β-mercaptoethanol had little activity towards this system. The following pH optima were obtained: for TSR I and TSR II, 9.0; for TSR IIIa, 8.5; and for TSR IIIb, 9.5. The apparent-Km data for DTE and thiosulfate were determined to: [Formula: see text] TSR I, 0.164 mmol·l(-1) and TSR II, 0.156 mmol·l(-1); KmDTE TSR I, 1.54 mmol·l(-1) and TSR II 1.54 mmol·l(-1). The thiosulfate reductases IIIa and IIIb were further stimulated by addition of thioredoxin. All TSR fractions catalyzed SCN formation from thiosulfate and cyanate and thus had rhodanese activity; this activity, however, could only be detected in the presence of thiols.

A cysteine desulfhydrase was purified 110-fold from the green alga Chlorella using conven­tional techniques. The isolated cysteine desulfhydrase was specific for ᴅ-cysteine having no activity towards ʟ-cysteine. ᴅ- and ʟ-cysteine desulfhydrase activities can be separated using DEAE-cellulose chromatography techniques. The isoelectric point of this enzyme was deter­mined to be around a pH of 4.5 using a chromatofocussing column. The pH-optimum for the ᴅ-cysteine desulfhydrase was found to be in the range of 8.5 to 9 and the apparent Kᴍ for ᴅ-cysteine was determined to 0.16 mᴍ. The enzyme was active without addition of metal ions and EDTA or citric acid did not inhibit this activity.