PCR analysis of the distribution of unicellular cyanobacterial diazotrophs in the Arabian Sea

An oligonucleotide primer, NITRO821R, targeting the 16S rRNA gene of unicellular cyanobacterial N-2 fixers was developed based on newly derived sequences from Crocosphaera sp. strain WH 8501 and Cyanothece sp. strains WH 8902 and WH 8904 as well as several previously described sequences of Cyanothece sp. and sequences of intracellular cyanobacterial symbionts of the marine diatom Climacodium frauenfeldianum. This oligonucleotide is specific for the targeted organisms, which represent a well-defined phylogenetic lineage, and can detect as few as 50 cells in a standard PCR when it is used as a reverse primer together with the cyanobacterium- and plastid-specific forward primer CYA359F (U. Nubel, F. Garcia-Pichel, and G. Muyzer, Appl. Environ. Microbiol. 63:3327-3332, 1997). Use of this primer pair in the PCR allowed analysis of the distribution of marine unicellular cyanobacterial diazotrophs along a transect following the 67degreesE meridian from Victoria, Seychelles, to Muscat, Oman (0.5degreesS to 26degreesN) in the Arabian Sea. These organisms were found to be preferentially located in warm (>29degreesC) oligotrophic subsurface waters between 0 and 7degreesN, but they were also found at a station north of Oman at 26degreesN, 56degrees35'E, where similar water column conditions prevailed. Slightly cooler oligotrophic waters (<29degreesC) did not contain these organisms or the numbers were considerably reduced, suggesting that temperature is a key factor in dictating the abundance of this unicellular cyanobacterial diazotroph lineage in marine environments.