This 1967 study examined the photoassimilation of organic compounds by four obligately photoautotrophic blue-green algae (cyanobacteria): Anacystis nidulans, Anabaena variabilis, Nostoc muscorum, and Anabaena flos-aquae. The researchers demonstrated that these organisms could assimilate acetate in a light-dependent process requiring carbon dioxide. Acetate was primarily incorporated into lipid and protein fractions, with radioactivity appearing mainly in four amino acids: glutamate, proline, arginine, and leucine. Analysis of degraded amino acids and enzyme assays on cell extracts from A. nidulans confirmed that established biosynthetic pathways were operating, particularly for glutamate synthesis via the citric acid cycle. Moderate concentrations of fatty acid salts had minimal growth effects, except propionate, which inhibited growth at very low concentrations (10⁻⁴ M), though this inhibition could be reversed by acetate. The findings indicated that despite being obligate photoautotrophs, these cyanobacteria possess metabolic capabilities to utilize organic substrates when light and carbon dioxide are available.

Key findings

• Four obligately photoautotrophic blue-green algae species assimilated acetate through a light-dependent, CO₂-dependent process
• Acetate incorporation occurred primarily in lipid and protein fractions, with radioactivity concentrated in glutamate, proline, arginine, and leucine
• Enzyme assays and amino acid degradations confirmed operation of classical biosynthetic pathways, including citrate synthase and glutamate dehydrogenase
• Propionate was a potent growth inhibitor at 10⁻⁴ M concentrations, but acetate specifically reversed this inhibition in all three tested species
• Other organic compounds tested (butyrate, citrate, glutamate, succinate) were not significantly incorporated by A. nidulans