Synechococcus are the most abundant and widely distributed picocyanobacteria in marine environments, playing important roles in the planktonic food web. Several studies have demonstrated the global distribution pattern of Synechococcus lineages. However, few systematic studies on the diversity, abundance, evolution, and ecological importance of Synechococcus were carried out in the western Pacific Ocean.

In this thesis, we studied the phylogenetic and pigment diversity, abundance, and genome of Synechococcus in the western Pacifica Ocean by 454 pyrosequencing, clone library construction, flow cytometer and genomic sequencing methods. We also studied seasonal variations of Synechococcus abundance and assemblage composition in Hong Kong water. Our studies revealed that different...[ Read more ]

Synechococcus are the most abundant and widely distributed picocyanobacteria in marine environments, playing important roles in the planktonic food web. Several studies have demonstrated the global distribution pattern of Synechococcus lineages. However, few systematic studies on the diversity, abundance, evolution, and ecological importance of Synechococcus were carried out in the western Pacific Ocean.

In this thesis, we studied the phylogenetic and pigment diversity, abundance, and genome of Synechococcus in the western Pacifica Ocean by 454 pyrosequencing, clone library construction, flow cytometer and genomic sequencing methods. We also studied seasonal variations of Synechococcus abundance and assemblage composition in Hong Kong water. Our studies revealed that different Synechococcus taxonomic lineages and pigment genetic types had different niche preferences in the western Pacific Ocean. A clear latitudinal separation of major lineages was observed. The East China Sea (ECS) had a higher Synechococcus abundance than most other studied regions in the western Pacific Ocean. By applying a new designed primer set, we discovered a novel Synechococcus pigment genetic type KORDI that dominates in oceanic waters. Our results also revealed that Synechococcus community composed significantly different pigment genetic compositions in ECS and SCS (South China Sea).

A monthly study at two contrast sites revealed that Synechococcus are important primary producers in Hong Kong waters with strong seasonal variations in abundance and community composition. Using pyrosequencing of rpoC1 gene, we discovered that the PC Synechococcus, which contain only phycocyanin, found in Hong Kong estuarine waters were mainly Cyanobium, S5.2 and freshwater Synechococcus, while PE Synechococcus that dominated in coastal waters were featured with clear temporal variations of dominant lineages.

We isolated more than twenty Synechococcus strains from Hong Kong waters. Most of them are classified as clade VIII, IX and S5.2 that are not abundant in marine environments. Different from typical strictly marine Synechococcus, some phycoerythrin-containing (PE-containing) strains isolated from Hong Kong waters showed high ability to deal with salinity variations. We sequenced the genome sequence of a novel euryhaline PE Synechococcus strain, which revealed the strategies developed by the strain to deal with salinity variations and to adapt to coastal environments.

Vertical profiles of Synechococcus community composition over a salinity gradient in a salt wedge estuary was also studied by sequencing cpcBA gene and rpoC1 gene. Surface water and bottom layer of the Pearl River estuary harbored distinct Synechococcus communities. We also revealed that three major PC Synechococcus lineages, S5.2, freshwater Synechococcus and Cyanobium, occupied different niches. PC Synechococcus dominated in the surface water, while PE Synechococcus were the major Synechococcus in the deep water.

Over all, these studies are the first systematic study of the ecophysiology and phylogenetic diversity of Synechococcus in the western Pacific Ocean, as well as the subtropical estuarine environment, by using a suite of modern biological research techniques. The findings of this study provide a better understanding of the effects of abiotic and biotic factors on Synechococcus community diversity and abundance, and extend our knowledge on the strategies developed by different Synechococcus lineages to adapt to specific niches.