The anaerobic photodissimilation of acetate by Chlamydomonas reinhardii F-60 adapted to a hydrogen metabolism was studied utilizing manometric and isotopic techniques. The rate of photoanaerobic (N(2)) acetate uptake was approximately 20 mumoles per milligram chlorophyll per hour or one-half that of the photoaerobic (air) rate. Under N(2), cells produced 1.7 moles H(2) and 0.8 mole CO(2) per mole of acetate consumed. Gas production and acetate uptake were inhibited by monofluoroacetic acid (MFA), 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) and by H(2). Acetate uptake was inhibited about 50% by 5% H(2) (95% N(2)). H(2) in the presence of MFA or DCMU stimulated acetate uptake and the result was interpreted to indicate a transition from oxidative to reductive metabolism. Carbon-14 from both [1-(14)C]- and [2-(14)C]acetate was incorporated under N(2) or H(2) into CO(2), lipids, and carbohydrates. The methyl carbon of acetate accumulated principally (75-80%) in the lipid and carbohydrate fractions, whereas the carboxyl carbon contributed isotope primarily to CO(2) (56%) in N(2). The presence of H(2) caused a decrease in carbon lost from the cell as CO(2) and a greater proportion of the acetate was incorporated into lipid. The results support the occurrence of anaerobic and light-dependent citric acid and glyoxylate cycles which affect the conversion of acetate to CO(2) and H(2) prior to its conversion to cellular material.