Seasonality of N₂ fixation and nifH gene diversity in the Gulf of Aqaba (Red Sea)

Nitrogen (N₂) fixation rates were determined on bulk water collected in the Gulf of Aqaba, Red Sea, during fall (stratified, oligotrophic) and spring (deep mixing, mesotrophic) seasons. N₂ fixation rates were low yet consistently measurable in both seasons, and maximum rates of 1.0 ± 0.1 nmol N L^-1 d^-1 and 1.9 ± 0.2 nmol N L^-1 d^-1 occurred in the fall and spring, respectively. Amendment with inorganic phosphate did not always increase rates. Rates were uninfluenced by dissolved organic phosphorus treatments. The highest rate (2.1 nmol N L^-1 d^-1) was measured in spring, when surface seawater was amended with an aerosol dust filter. The nifH deoxyribonucleic acid (DNA) gene diversity was analyzed in surface and depth profile samples. Most (58) nifH sequences were similar to the previously identified cluster I (containing sequences from proteobacteria and cyanobacteria) sequences. A small subset of sequences (11) was most similar (>96.5) to nifH nucleotide sequences of cyanobacteria, including Trichodesmium spp. and the unicellular cyanobacterial group A. Sequences similar to cluster III (which contains sequences from many anaerobes) lineages were only retrieved from fall libraries. Quantitative polymerase chain reaction (qPCR) assays were used to estimate the nifH abundance for select phylotypes. Trichodesmium and a y proteobacteria had maximum abundances at 60-m depth in the water column in fall 2005 (1.4×10⁵ and 2.3 × 10² nifH gene copies L ^-1, respectively). Group A was undetected in all samples, and Trichodesmium and y proteobacteria were undetected in the spring 2007. The nifH transcription as determined by quantitative reverse-transcription PCR (qRT-PCR) assays was at the detection limit (1-10 transcripts per reaction) for all phylotypes. This study is the first assessment of nifH diversity and rates of ¹⁵N₂ fixation in the Gulf of Aqaba.