Ten different strains of marine cyanobacteria were tested for their ability to decolourise and degrade a recalcitrant diazo dye, C.I. Acid Black 1. Of them, Oscillatoria curviceps BDU92191 was able to grow up to a tested concentration of 500 mG L(-1). The organism degraded 84% of the dye at 100 mG L(-1) in 8 days in a medium free of combined nitrogen. The dye degrading ability is attributed to the activities of the enzymes: laccase, polyphenol oxidase and azoreductase. The absence of the doublet amine peak in addition to the overall reduction of absorption in the IR spectra confirmed the mineralisation of the tested azo dye. The nitrogen assimilating enzyme studies along with nitrogenase assay strongly suggested the ability of the non-heterocystous, filamentous marine cyanobacterium, O. curviceps BDU92191 to use C.I. Acid Black 1 as a nitrogen source in an oligotrophic environment. (C) 2011 Published by Elsevier Ltd.