Microbial Communities Of The Surface Seawater In The South Pacific Gyre

The South Pacific Gyre (SPG,20°—45°S) is the biggest gyre in the world, theSPG center is far away from the continent, anticyclonic circulation accumulatedquiescent water, oligotrophic conditions, and strong UV radiation etc. make thesurface seawater become the cleanest water all over the word. The concentrations ofchlorophyll-a (Chl-a) and primary productivity were lower than other oceans.Microorganisms play a major role in the biogeochemical cycling. Surprisingly,microbial communities of this ideal “reference clean surface seawater” of the worldremains poorly understood till now. This is the first time to reveal the compositionand diversity of the microbial communities of SPG.Phylogenetic diversity of bacteria and archaea from the center to the edge of theSPG was studied in December2011, following IODP329. Surface samples werecollected in four stations U1368, U1369, U1370and U1371, from the center to theedge of the gyre. Eight16S rRNA gene clone libraries were constructed using theDNA extracted from the filters flitered by water samples. qPCR and traditionalcultivated method were also used. According to the clone libraries results,757bacterial clones and355archaeal clones were picked and sequenced. Phylogeneticanalysis of clones showed that bacterial phylotypes were made up of five phyla, i.e.Proteobacteria (first dominant group), Cyanobacteria (second dominant group),Bacteroidetes, Actinobacteria and Verrucomicrobia. The archaeal phylotypeswere almost only made up of Euryarchaeota. This indicated that the diversity ofSOG was low. The proportions of Proteobacteria decreased progressively(81.1%-54.6%) from the center to the edge. β-Proteobacteria and δ-Proteobacteria only detected in the center of SPG. Bacteroidetes increasedprogressively (0.6%-9.7%) from the center to the edge. All these results indicated the differences between the center and the edge of SPG.According to the results of qPCR, low16S rRNA gene copies densities (rangingfrom (5.96±1.87)×105to (2.55±0.02)×106copies per milliliter for bacteria and(1.17±0.42)×103to (1.90±0.006)×104copies per milliliter for archaea) generated froman absolute quantification analysis indicated the oligotrophic origin. The biggest threegroups were Alphaproteobacteria, Cyanobacteria and Bacteroidetes. Firmicutesaccounted for0.1%-0.5%in the total bacteria and the most common Crenarchaeotawas not be detected, which was further confirmed by qPCR with low abundance(9.9±0.18to22.1±6.2copies per milliliter) of Crenarchaeota.74strains were isolatedfrom the traditional plate cultivation method and their16S rRNA genes weresequenced, which affliating to four dominant groups, i.e. Proteobacteria,Bacteroidetes, Firmicutes and Actinobacteria. What’s worth emphasizing wasthat there was a high percentage (18.9%,14strains) of potential novel bacteriaamong the74strains. These results indicated that the SPG was an extraordinaryarea and biotope system with huge potential. The further survey of SPG will beof great significance to the world.