Abstract
Surface seawater in the South Pacific Gyre (SPG) is one of the cleanest oceanic environments on earth, and the photosynthetic primary production is extremely low. Despite the ecological significance of the largest aquatic desert on our planet, microbial community composition in the ultra-oligotrophic seawater remain largely unknown. In this study, we collected surface seawater along a southern transect of the SPG during the Integrated Ocean Drilling Program (IODP) Expedition 329. Samples from four distinct sites (Sites U1368, U1369, U1370 and U1371) were examined, representing ∼5400 kilometers of transect line from the gyre heart to the edge area. Real-time PCR analysis showed 16S rRNA gene abundance in the gyre seawater, ranging from 5.96×105 to 2.55×106 copies ml−1 for Bacteria and 1.17×103 to 1.90×104 copies ml−1 for Archaea. The results obtained by statistic analyses of 16S rRNA gene clone libraries revealed the community composition in the southern SPG area: diversity richness estimators in the gyre center (Sites U1368 & U1369) are generally lower than those at sites in the gyre edge (Sites U1370 & U1371) and their community structures are clearly distinguishable. Phylogenetic analysis showed the predominance of Proteobacteria (especially Alphaproteobacteria) and Cyanobacteria in bacterial 16S rRNA gene clone libraries, whereas phylotypes of Betaproteobacteria were only detected in the central gyre. Archaeal 16S rRNA genes in the clone libraries were predominated by the sequences of Marine Group II within the Euryarchaeota, and the Crenarchaeota sequences were rarely detected, which is consistent with the real-time PCR data (only 9.9 to 22.1 copies ml−1). We also performed cultivation of heterotrophic microbes onboard, resulting in 18.9% of phylogenetically distinct bacterial isolates at least at the species level. Our results suggest that the distribution and diversity of microbial communities in the SPG surface seawater are closely related to the ultra-oligotrophic oceanographic features in the Pacific Ocean.
Highlights
The South Pacific Gyre (SPG) is the largest gyre of the world and extends a long way from the Pacific’s swell in the east (20uS) to the oceanic crust of the Cretaceous period in the west (45uS)
Diversity of Cultivated Bacteria The cultivated bacteria were mainly affiliated with the Gammaproteobacteria, accounting for 58.1%, followed by Bacteroidetes (16.2%), Firmicutes (13.5%), Alphaproteobacteria (6.8%) and Actinobacteria (5.4%)
Photobacterium was the dominant group in the members of Gammaproteobacteria (Figure 2 and Figure 3), accounting for 41.9% in Gammaproteobacteria and 24.3% in all the cultivated bacteria
Summary
The South Pacific Gyre (SPG) is the largest gyre of the world and extends a long way from the Pacific’s swell in the east (20uS) to the oceanic crust of the Cretaceous period in the west (45uS). Satellite imaging has shown that the SPG has the lowest surface chlorophyll-a (Chl-a) concentrations of the worlds’ oceans (,0.02 mg l21) [2] and UV absorption measurements have demonstrated that it is the clearest water in the world [3]. Because of its size and low primary productivity indicated by surface Chl-a concentration, this ultraoligotrophic ocean region has been considered the largest oceanic desert on our planet [1]. All these physical and geochemical parameters make the SPG a unique and extreme habitat of life, in contrast to other marine ecosystems. Distribution, phylogenetic diversity and structure of the microbial communities in this ultra-oligotrophic ocean remain largely unkown
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
