Phylogenetic composition of Prochlorococcus and Synechococcus in cold eddies of the South China Sea

Two mesoscale cyclonic eddies (CE I and CE II) occurred along the Vietnam coast in the South China Sea during summer 2007; CE I was in the decaying stage and CE II was in the intensifying stage. CE II was also influenced by the Mekong River plume. The upwelling effect of the 2 eddies on the phylogenetic diversity of Prochlorococcus and Synechococcus was investigated by clone library construction based on sequences of the internal transcribed spacer (ITS) region, with station SEATS in the oligotrophic basin as a comparison. Phylogenetic analysis revealed the presence of sequences affiliated with 3 Prochlorococcus clades (HL I, HL II & LL IV) and 7 Synechococcus clades (II, III, VII, X, XV, XVI, and sub-cluster 5.2) from the various water bodies, with HL II Prochlorococcus and clade II Synechococcus being the dominant phylotypes. Multivariate analysis demonstrated that spatial distribution of the 2 genera was mostly explained by temperature, but also affected by salinity and chlorophyll a concentration. Upwelling of the nutrient-rich subsurface water at the center of the cold eddy resulted in a significant increase in the abundance of Synechococcus in the young and intensifying CE II, whereas Prochlorococcus dominated in the old and decaying CE I and the surrounding oligotrophic waters. It is intriguing that Synechococcus clades III, XV and XVI, which are known to contain chromatically adapting members, were most numerous in eddy and river plume influenced waters, where light quality changes might be expected, compared to the oligotrophic basin waters. The disparity in cyanobacterial phylogenetic composition between the 2 eddies clearly reflected the differing hydrographic dynamics and developmental stages of these eddy systems.