The Arabian Gulf, surrounded by deserts, is an ideal marine basin to study the effects of nutrient input from dust on primary productivity. We studied the microbial community composition of the water column and sediment using 16S r-RNA gene sequencing, and the activity of photosynthetic microorganisms using pigment analysis and microsensors measurements in February and October 2013 and February 2014. We collected samples from three stations (1CD, 2CD & 3CD) along an inshore-offshore transect extending from Doha to the middle of the gulf. Our results showed that although the water column had more pigment diversity than that of the sediment, the latter had higher pigment concentrations. This applies to all the samples collected from different locations during the 3 cruises. In October the pigment content in the water column was ~2 times higher than in February. This indicates higher primary productivity in October than in February, which might have resulted from the nutritive effect of dust after the intensive dust storms during the summer season. The microbial community structure (on class level) of the water column was determined on samples from the 3 sites collected in the two cruises in 2013. The highest contribution of the surface water samples came from members of phyla cyanobacteria and proteobacteria. At deep water, the Proteobacteria dominated over the cyanobacteria. The microbial community structure in the sediment is strongly dominated by members of Proteobacteria (70-78%), while the relative abundance of cyanobacteria dropped to <1%.


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