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Deep Ocean microbial communities in the South Adriatic Pit

The Adriatic Sea is a semi enclosed basin in the Northeastern Mediterranean with a shelf that extends from its northernmost part to the Split Gargano transect. Its deeper southern part is characterized by the circular, 1243 m deep South Adriatic Pit (SAP). Adriatic ecosystems are influenced by regular water exchange with the Eastern Mediterranean through the strait of Otranto. Levantine Intermediate water and Ionian Surface Water flow into the Adriatic along its eastern border. The volume of this flow is greater in winter but changes from year to year depending on climate oscillations that occur from the Adriatic to the Southeast Mediterranean. In this study we used pyrosequencing of the 16S rRNA gene and DGGE to study the water column above the SAP in comparison to a close station (300 m depth). The total number of bacteria at station P1200 was between 2.6 x 105 at surface and 6.7 x 104 at the bottom with a peak of 3.3 x 105 at 75-100 m. At the shallower P300 station the change in bacterial counts was lower raging from 1.8 x 105 at the surface to 1.1 x 105 at 200 m. Picophytoplankton counts using flow cytometry showed a Prochlorococcus peak at 100m depth in both P1200 and P300. Synechococcus peak was at 10 m in both stations and another peak at 20 m at P300 and 75 m at P1200. DGGE analysis of the P1200 samples showed 4 distinct communities: surface (0 and 10 m), shallow deep (75, 100 and 200 m), intermediate depth (400, 600 and 800 m) and deep (1000 and 1200 m). A similar pattern was obtained by cluster analysis of the pyrosequencing data. Samples form P300 cluster together with those from similar depths from P1200 suggesting that distance from the surface is a significant factor in determining the community within the photic zone (300 m). Taxonomic analysis of the pyrosequnece data showed a depth related distribution of the various phylogentic groups. Interestingly, some of these characteristics are typical to the deep ocean. Flavobacteria were present only in the upper 100 m while Deferribacteres, Acidobacteria and the Sar202 clade (Chloroflexii) were detected in significant numbers only below 200 m. The Alphaproteobacteria class was detected in high abundance throughout the water column ; however, the Rhodobacteraceae and the SAR116 clade were more abundant in the upper 100 m while the Rodospirillaceae were frequent only in deeper waters. Within the Sar11 clade some groups where equally abundant at all depths while other showed clear preference to shallow deep waters. The Sar324 clade, found below 200 m, is the dominant group within the Deltaproteobacteria. Interestingly, the Deltaproteobacteria nitrite oxidizing bacteria family, Nitrospinaceae, are commonly found in the deep waters at P1200 ; however, no bacterial ammonia oxidizer was found in the database. This suggests that ammonia oxidation is probably carried out by Archaea.

bacteria; south adriatic pit; pyrosequencing

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