Seasonality of N 2 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 6 0.1 nmol N L21 d21 and 1.9 6 0.2 nmol N L21 d21 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 L21 d21) 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 c proteobacteria had maximum abundances at 60-m depth in the water column in fall 2005 (1.4 3 105 and 2.3 3 102 nifH gene copies L21, respectively). Group A was undetected in all samples, and Trichodesmium and c 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 N2 fixation in the Gulf of Aqaba. There has recently been increased interest in the marine nitrogen cycle, in particular, the diversity, distribution, and activity of nitrogen (N2)–fixing microorganisms. However, N2 fixation rates have not been measured in many areas of the world’s ocean. Studies of the diversity and distribution of marine diazotrophs have largely focused on the openocean subtropical and tropical gyres. Here, we report on N2 fixation rates and the diversity of N2-fixing microorganisms in the Gulf of Aqaba. The Gulf of Aqaba is a semienclosed body of water located in the northeastern extension of the Red Sea. It is 165 km long and approximately 15 km wide and is separated from the northern Red Sea by a shallow sill (240 m) at the Straits of Tiran. Concentrations of inorganic nutrients in the surface waters of the Gulf are oligotrophic (phosphate and nitrate rarely exceed 0.1 and 1 mmol L21, respectively; Fuller et al. 2005), and a shallow but stable thermocline is present for most of the year, except during the winter, when winds drive convective mixing of deep, higher-nutrient water to the surface. Fluvial nutrient inputs are minimal since the gulf is bounded by desert. Chlorophyll a (Chl a) concentrations are lowest during the summer and fall stratification period, and phytoplankton populations during these seasons are composed mostly of Synechococcus and Prochlorococcus, respectively (Lindell and Post 1995). During the winter mixing period, the abundance of picoeukaryotes and larger phytoplankton (i.e., diatoms and the dinoflagellate Ceratium sp.) increase, and Chl a concentrations reach an annual maximum (Genin et al. 1995). In other regions of the world’s oceans where similar oligotrophic conditions persist, diazotrophs can be abundant, contribute significantly to primary production, and provide a source of new nitrogen to the euphotic zone (Capone et al. 1997; Karl et al. 2002). Several groups of diazotrophs have been identified in the Gulf of Aqaba, including the colonial nonheterocystous cyanobacterium Trichodesmium spp. , the unicellular cyanobacterium Synechocystis spp. (Cyanothece spp.), and unicellular and heterocystous cyanobionts of dinoflagellates and diatoms, respectively (Kimor and Golansky 1977; Gordon et al. 1994; Post et al. 2002). Although sporadic occurrences of these diazotrophic groups have been observed by microscopy during summer stratification, and episodic blooms have been reported for Trichodesmium spp. , to our knowledge there has been only one report of N2 fixation rates in the Gulf of Aqaba, which was based on acetylene reduction rate measurements for concentrated Trichodesmium spp. colonies (Post et al. 2002). 1 Corresponding author (rfoster@ucsc.edu).