The cyanate utilization capacity of marine unicellular Cyanobacteria

Cyanate, a by-product of urea decomposition, is a potential nitrogen (N) source in marine environments, but to date it has received scant attention. Cyanobacteria presumably acquire this compound via a substrate-specific ABC-type transporter (cynABD), and they convert it to ammonium and carbon dioxide by cyanase (cynS) activity. Participation of cyanate utilization genes in N-stress responses in cyanobacteria has been implied previously, but its ecological context has not been studied. We employed polymerase chain reaction protocols for cynA amplification to examine the potential for cyanate recruitment in the Gulf of Aqaba, northern Red Sea. We also monitored growth of cyanobacterial strains with different cynABDS complements on cyanate-containing media. We showed that cyanate is utilized as the sole N source by strains with the full gene complement, and residual growth, fed by natural decay of cyanate to ammonium, was observed in strains lacking any of these genes. Natural abundance of cynA products in the oligotrophic Gulf of Aqaba indicates that cyanate constitutes an essential N source for Prochlorococcus, but not for Synechococcus populations. Noncyanobacterial cynA sequences indicate cyanate utilization by a variety of other phototrophic microorganisms. We hypothesize that in stratified water bodies, cyanate utilization is confined to the N-deplete upper photic zone, where it plays a role in ‘‘regenerated’’ primary production. Cyanate is probably the simplest organic nitrogen (N) compound known for any organism. Cells produce it as a by-product of glutamine metabolism, with carbamoyl phosphate as the intermediate, and of arginine degradation via the urea cycle. Cyanate is potentially toxic to the cell, and neutralization of cyanate occurs via the activity of cyanate lyase (EC 4.2.1.104; synonyms—cyanase, cyanate hydratase). Cyanate lyase is a cytoplasmic holoenzyme composed of 15-kDa monomers and has an approximate 150,000-kDa molecular weight. The enzyme is found in bacteria, fungi, algae, higher plants, and animals. The properties of Escherichia coli cyanate lyase have been reviewed by Anderson and Little (1986). Cyanate lyase efficiently degrades cyanate to ammonium and carbon dioxide. E. coli not only neutralizes cyanate generated intracellularly, it was also reported to take up cyanate from the environment. A cyanate-specific permease was identified, and the knock-out mutants produced were incapable of satisfying their N requirements when cyanate was the sole N source (Sung and Fuchs 1989). Recently, Espie et al. (2007) reported an abundance of cyanate transport genes among bacteria. An ABC-type transporter for cyanate was identified in the freshwater cyanobacteria Synechococcus elongatus strain PCC7942 (Harano et al. 1997) and Synechococcus sp. strain PCC6301. This transporter showed a high degree of similarity to ABC-type transporters for nitrate and bicarbonate uptake in cyanobacteria. The cyanobacterial ability to utilize cyanate has never been evaluated in an ecological context, and nothing is known about cyanate as an N source for cyanobacterial productivity in aquatic systems. Unicellular, non–nitrogen-fixing cyanobacteria of the genera Synechococcus and Prochlorococcus form an abundant fraction of marine phytoplankton, and they contribute up to 65% of primary production in oligotrophic ocean waters (Partensky et al. 1999). They often dominate in N-deplete surface waters, where they engage in regenerated production fueled by urea and ammonium as the most conspicuous N sources. Utilization of these compounds has been characterized for both Synechococcus and Prochlorococcus (Collier et al. 1999; Moore et al. 2002). Ammonium 1 Corresponding author ([email protected]).