Oxygen isotope fractionation (180/160) at the natural abundance level has been measured during photosynthesis of a blue-green and a green alga. When sufficient attention is paid to removal of contaminating air 02 before and during the experiments, then the photosynthetic 02 evolved, as compared to the water 02, had an average difference of -0.36%0 for a blue-green alga and -0.80%0 for a green alga. MATERIALS AND METHODSThe blue-green alga, Agmenellum quadruplicatum, strain PR-6, was grown in modified ASP-2 (9); the green alga, Chlorella pyrenoidosa, high temperature strain TX-71105, was grown in EDTA Knops (6). Axenic growth occurred at 35 C in test tube cultures, with continuous bubbling of 1 % CO2 in air. Illumination consisted of four (two on each side of the bath) F20T12/ CWX fluorescent lamps 11 cm from the growth tubes. Cells for isotopic experiments were purposely harvested at different stages of growth by centrifugation, then injected to give various final cell concentrations (see Table I) into a sealed 4-liter bottle containing 3 liters of fresh medium, magnetically stirred. The bottle was held at 35 C and illuminated on two sides by seven 100-w tungsten lamps, about 22 cm from the center of the bath. Oxygen content of the algal suspension during the degassing phase or during periods of photosynthesis or respiration was monitored with an 02 electrode (Gilson Medical Electronics Clark-Type electrode, OX700, connected to a Keithley 150A microvoltammeter and Brown recorder), see Figure 1. The 02 electrode was routinely calibrated using air-saturated medium (checked by the Winkler method) and an 02-free solution (0.16 M Na2SO3 in 10 mM Na2B407).The removal of atmospheric 02 from an algal suspension was attained by (a) evacuation, using a water aspirator, in con-

Action spectra for photoreactions I and I1 of photosynthesis were obtained for Anacystis nidulans and three of its variants which had altered chlorophyll/phycocyanin ratios. The spectra are properly scaled to each other. They provide information on contributions of phycocyanin and chlorophyll to initial absorption and final distribution of excitation energy to reaction centers I and 11. In normally pigmented cells the light harvesting pigments for photoreaction I include about 40% of the phycocyanin and 84% of the chlorophyll. Both in normal cells and in cells with altered pigmentation excitation energy from phycocyanin is delivered to photoreaction I1 via a small number of chlorophylls. In response to alterations in chlorophyll/phycocyanin ratio Action I spectra showed large variations whereas Action I1 spectra were essentially invariant. The result is taken to mean that alteration in chlorophyll components in Anacystis is attended by a special restriction: there are only small changes in amount of chlorophyll accessible to photoreaction I1 in the face of large changes in amount committed to photoreaction I.

SUMMARY Characteristics of 2 types of pigment mutants of the bluegreen alga, Anacystis nidulans, are described. “Yellow‐green” mutants (YG) which have normal chlorophyll but only half the phycocyanin of the parent are similar to the parent in number of reaction centers/cell, number of chlorophylls/reaction center, maximum turnover rate of the reaction centers, quantum yields at 620 and 686 nm and specific growth rate; they have a reduced action at 620 nm. “Blue” mutants (BL) with somewhat higher phycocyanin but only one‐third the chlorophyll of the parent are dissimilar to the parent. BL's have fewer reaction centers/cell, a smaller number of chlorophylls/reaction center, a higher maximum turnover rate of reaction centers, and a lower specific growth rate. BL's show ca. half the quantum yield of the parent at 620 nm and at 686 nm show a “red rise” rather than a “red drop.” The consequences attending low chlorphyll in the BL's are more drastic than those attending the low phycocyanin of the YG's.

When levulinic acid, a competitive inhibitor of 6-aminolevulinic acid dehydratase, was added to growing cultures of blue-green algae (cyanobacteria), 6-aminolevulinic acid was excreted into the medium and cell growth was inhibited.