Bacterial and archaeal community composition in the deep chlorophyll maximum of the South Pacific gyre.

The South Pacific gyre is the largest oceanic province on Earth. It covers 15% of the total ocean area and 33% of the total oligotrophic ocean areas. It has been described as Earth''s largest oceanic desert (1). Its center is farther from continents and productive oceanic regions than any other site on Earth. It is rarely visited by scientific expeditions and its microbial community composition is largely unknown. To assess the microbial community composition and diversity of a continuous and biologically significant horizon, we sampled the deep chlorophyll maximum along a 7500-km ship track through the western and central portion of the gyre. All samples were taken in southern-hemisphere summer (December 2006-January 2007). We analyzed bacterial and archaeal V6-tag populations from the deep chlorophyll maximum of eight sites. Over this sequence of sites, the depth of the deep chlorophyll maximum varied from 110 to 200 meters below the sea surface. The temperature of the sampled horizon varied from 11 to 20C, its salinity ranged from 34.3 to 35.6 psu, and its clorophyll-a concentration ranged from 0.22 to 0.40 mg/m3. The deepest sample was taken in the heart of the gyre, where the water is clearer and the deep chlorophyll maximum deeper than anywhere else in the world (2). The shallowest samples were taken at the western and southern edges of the gyre. We analyzed 9900 to 32600 bacterial V6-tag sequences and 9200 to 35500 archaeal V6-tag sequences per site (totaling ca. 178,000 bacterial V6 sequences and ca. 168,000 archaeal V6-tag sequences for the eight sites combined). Across all sites, the most heavily represented bacterial groups are unassigned Alphaproteobacteria, Cyanobacteria, unassigned Gammaproteobacteria, and SAR11 and its relatives. 95% to 99% of the Archaea at each site are unassigned Euryarchaeota. Unassigned Gammaproteobacteria predominate at the sites where the deep chlorophyll maximum is shallowest (100-110 mbsl) and unassigned Alphaproteobacteria predominate at the sites where the horizon is deeper (120-200 mbsl). Cyanobacteria are relatively most abundant at four of the five sites where the deep chlorophyll maximum is shallowest (100-120 mbsl). Diversity of bacterial OTUs (<=97% similarity) exceeds diversity of archaeal OTUs at all eight sites. Over the range of sequences sampled at each site (9200 to 35500), bacterial diversity increases with increasing number of sequences ca. 2 to 5 times as fast as archaeal diversity