Vertical and horizontal biogeographic patterns and major factors affecting bacterial communities in the open South China Sea.

Microorganisms display diverse biogeographic patterns in the three-dimensional contiguous seawater. The distance-decay relationship, the change in species composition similarity between different communities over a geographic distance, is a commonly observed biogeographic pattern. To study biogeographic patterns and the corresponding driving forces, the bacterial distance-decay patterns along the horizontal and vertical dimensions in the South China Sea (SCS) were investigated through the sequencing of partial 16 S rRNA gene regions. Along the horizontal geographical distances (up to ~1000 km), no significant distance-decay pattern in community compositions was observed in any of the tested seawater layers. However, vertical depths (up to ~4 km) had strong effects on bacterial community variation, which was apparently governed by dispersal barriers due to limited water mass mixing. In addition, community variations in the vertical direction were strongly correlated with the prominent variation of environmental factors. Apparently, the changes in bacterial community compositions along vertical distances were much greater than those along horizontal distances. The results showed that the distance-decay relationship in bacterial communities at the medium spatial scale was associated with vertical depth rather than with horizontal distance, even though the horizontal distance is much larger than the vertical distance in the open SCS.