East Tibetan Lakes Harbour Novel Clusters of Picocyanobacteria as Inferred from the 16S-23S rRNA Internal Transcribed Spacer Sequences

摘要

Planktonic picocyanobacteria abundance and diversity were investigated in nine lakes on the East Tibetan Plateau spanning a salinity gradient of 0.4-22.6 g lp#. The investigation was conducted using epifluorescence microscopy (EFM) and terminal restriction fragment polymorphism analysis of 16S-23S rRNA internal transcribed spacer (ITS) PCR amplicons followed by sequence analyses of large ITS clone libraries of seven selected samples. EFM showed that picocyanobacteria comprised 7-19% of the total prokaryotic cells found in surface water. Most of the clones were classified into six clusters and grouped within the picocyanobacterial clade, which consists exclusively of freshwater Synechococcus. Four new phylogenetic clusters and one new subcluster of Synechococcus spp. were found, none of which are members of any known picocyanobacterial clusters. The new clusters and subcluster were the most abundant picocyanobacteria (about 96% of the sequences) in the samples collected. Sequence analyses indicated that members of the four new Synechococcus groups were only found in freshwater lakes (<1.0 g lp# of total dissolved solid), while members of the new subcluster were found in all the investigated Tibetan lakes, over a large salinity gradient of 0.4-22.6 g l p#. This suggests that there is ecologically significant microdiversity within the observed Synechococcus group as defined by ITS sequences. Collectively our study demonstrated abundant and potentially novel Synechococcus in East Tibetan lakes that are likely the result of evolutionary adaptations to regional conditions.