Aminooxyacetate and aminoacetonitrile cause increased excretion of glycolate by the cyanobacterium Anabaena cylindrica. Both compounds also reduce NH4-N release induced by methionine sulfoximine in nonnitrogen-fixing cultures. Changes in amino acid pool sizes together with changes in activities of some enzymes related to glycolate metabolism show that glyoxylate to glycine conversion and glycine to serine conversion are inhibited by aminooxyacetate and aminoacetonitrile, respectively. The results also verify that photorespiratory glycolate metabolism via amination of glyoxylate is operative in A. cylindrica.We have recently reported on the stimulation of glycolate excretion by INH2 in cultures of the cyanobacterium Anabaena cylindrica (4). INH inhibits the conversion of glycine to serine in the glycolate pathway of higher plants and eukaryotic algae (27). Evidence has also been presented for the photorespiratory production of ammonia by A. cylindrica (3). As with some green algae (13,23) Medium and Cultivation. Cells were grown in pure culture in either NH4Cl-supplemented (N+) or N-free (N-) BG 11 medium (26) and sparged continuously with sterile air. Cultivation and cell harvesting from N+ and N-media were carried out as detailed earlier (1, 3, 4). The N+-grown cultures were used in experiments on the release of ammonium and amino acid pool sizes, while N-grown cells were used in the remainder of the experiments. Solutions ofall inhibitors were adjusted to pH 7.5 before addition to the medium.Glycolate Excretion. The procedure given earlier (4) was used and excreted glycolate assayed according to Calkins (6). The cultures were grown on air before use and then sparged with 100% 02 from the beginning of the excretion experiments. A photon fluence rate of 200 Mumol. m2 . s' at the surface of the vessels was used during excretion assays, provided by white fluorescent tubes.Release of NH4-N. MSX (0.2 mM)-induced release of NH4-N to the medium was determined as detailed earlier (3), using the phenol hypochlorite method of Chaney and Marbach (7). Cultures were pretreated only with AOA, AAN, and INH as they all, as such, interfere with the colorimetric assay by producing a blue color similar to that of indophenol blue. Cells were centrifuged after exposure to the inhibitors, carefully washed, and resuspended in fresh N-free medium containing 0.2 mM MSX (time zero). The release of NH4-N to the medium was then determined at the time intervals given in Tables III and IV