Nitrate is essential for lindane dechlorination by the cyanobacteria Anabaena sp. strain PCC7120 and Nostoc ellipsosporum, as it is for dechlorination of other organic compounds by heterotrophic microorganisms. Based on analyses of mutants and effects of environmental factors, we conclude that lindane dechlorination by Anabaena sp. requires a functional nir operon that encodes the enzymes for nitrate utilization.Cyanobacteria are photoautotrophic microorganisms common to a variety of environments including polluted ones. Earlier, we reported that two filamentous nitrogen-fixing cyanobacteria, Anabaena sp. strain PCC7120 and Nostoc ellipsosporum transformed lindane (14) first to ␥-pentachlorocyclohexene and then to a mixture of chlorobenzenes (Fig. 1). This process was cometabolic and depended on the presence of nitrate (14).Nitrate-dependent dehalogenation of organic compounds by different heterotrophic bacteria has been described in the past (1, 9, 21), but no mechanism for this process or link to genetic systems has been proposed. For some microorganisms dehalogenation was coupled with denitrification (9). In both cyanobacteria and anaerobic denitrifying microorganisms, nitrate uptake and reduction are initial processes of nitrate utilization, and the genes for these processes are organized in similar operons (4,17,19,24,26). At the level of nitrite reduction, metabolic pathways diverge and lead to the assimilatory chain for cyanobacteria (8), algae (3), fungi (12), and plants (27) and the dissimilatory chain for heterotrophic anaerobic microorganisms (28). In Anabaena sp. strain PCC7120, genes organized in an operon as 5Ј-nirA-nrtABCD-narB-3Ј encode nitrite reductase, nitrate transport proteins, and nitrate reductase (2, 7), similar to other cyanobacteria (17, 23).We report that Anabaena sp. nirA, nrtC, nrtD, or narB mutants cannot dechlorinate lindane in the presence of nitrate. Dechlorination is also inhibited in the dark and in the presence of ammonium, both of which are environmental inhibitors of the function(s) encoded by the nir operon. Fifteen strains of wild-type cyanobacteria screened by us degraded lindane.Effect of mutations in the nir operon on lindane dechlorination by an Anabaena sp. We analyzed lindane degradation by Anabaena sp. transpositional mutants TLN10 (insertion in the nirA gene), TLN12 (insertion in the nrtC gene), TLN21 (insertion at the 3Ј end of the nrtD gene), and DR796 (site-directed interposition in the narB gene), which have been described in detail by Cai and Wolk (2). Growth of the cultures at 28°C was monitored by measuring chlorophyll content. Experimental procedures were as described by Kuritz and Wolk (14). Addition of lindane to the cultures to a final concentration of 0.5 mg/liter allowed us to monitor the kinetics of the disappearance of lindane associated with the cells. To measure concentrations of cell-associated lindane, 2 ml of each culture was sampled, washed twice with 2 ml of sterile water, resuspended in 1 ml of sterile water, subjected to sonication in an ice...

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