Distribution and activity of diazotrophs in the Eastern Equatorial Atlantic

The gene abundance and gene expression of six diazotroph populations from the Eastern Equatorial Atlantic in June 2007 were examined using nifH gene quantitative polymerase chain reaction (q PCR) methods. Of all the diazotrophs, Trichodesmium spp. was the most abundant with the highest number of gene copies in the Gulf of Guinea. Trichodesmium also had the highest nitrogenase gene transcript abundance overall with the maximum in samples collected at the equator and in waters influenced by the Congo River plume (> 105 cDNA nifH copies l−1). Both cyanobacterial unicellular groups (A and B) were detected, where group A was the second most abundant in surface samples, in particular at the stations along the equator. Transcript abundance for group A, however, was at the detection limit and suggests that it was not actively fixing N2. Trichodesmium and group B nifH gene abundances co-varied (P < 0.0001). Richelia associated with Hemiaulus hauckii diatoms were detected in 9 of 10 surface samples and the highest abundances (> 104nifH copies l−1) were found north-west of the Congo River plume. In contrast, the Calothrix symbionts (het-3) of Chaetoceros had low abundances at the surface, but were present at 3.7 × 104nifH copies l−1 at 40 m depth in the equatorial upwelling. This is the first report of the Calothrix symbiont in the Atlantic Ocean. This is also the first report of nifH gene copy and transcript abundance in an Equatorial upwelling zone. Although the number of gene copies for Richelia associated with Rhizosolenia were the lowest, the transcript abundance were high (9.4 × 101−1.8 × 104 cDNA nifH copies l−1) and similar to that of Trichodesmium. The distribution of the diazotroph groups, especially the three strains of symbiotic cyanobacteria, was different, and appeared largely controlled by riverine inputs and upwelling.