AUTHOR=Xia Xiaomin , Guo Wang , Tan Shangjin , Liu Hongbin 

TITLE=Synechococcus Assemblages across the Salinity Gradient in a Salt Wedge Estuary

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01254

DOI=10.3389/fmicb.2017.01254

ISSN=1664-302X

ABSTRACT=<p><italic>Synechococcus</italic> are the most abundant and widely distributed picocyanobacteria in the ocean. The salt-wedge type of estuary possesses the complete horizontal and vertical gradient of salinity together with other physical and chemical parameters. In order to reveal whether such a complex environmental gradient harbors a high diversity of <italic>Synechococcus</italic>, we investigated the abundance, taxonomic composition and pigment genetic diversity of <italic>Synechococcus</italic> in surface and bottom waters across the salinity gradient in a salt-wedge estuary by flow cytometric analysis and pyrosequencing of the <italic>rpoC1</italic> gene and <italic>cpcBA</italic> operon (encoding phycocyanin). <italic>Synechococcus</italic> were ubiquitously distributed in the studied region, with clear spatial variations both horizontally and vertically. The abundance and diversity of <italic>Synechococcus</italic> were low in the freshwater-dominated low salinity waters. By pyrosequencing of the <italic>rpoC1</italic> gene, we have shown that with the increase of salinity, the dominant <italic>Synechococcus</italic> shifted from the freshwater <italic>Synechococcus</italic> to the combination of phylogenetic subcluster 5.2 and freshwater <italic>Synechococcus</italic>, and then the strictly marine subcluster 5.1 clade III. Besides, the composition of <italic>Synechococcus</italic> assemblage in the deep layer was markedly different from the surface in the stratified waters (dissimilarities: 40.32%-95.97%, SIMPER analysis). High abundance of clade III <italic>Synechococcus</italic> found in the brackish waters may revise our previous understanding that strains of this clade prefers oligotrophic environment. Our data also suggested that both the phylogenetic subcluster 5.3 <italic>Synechococcus</italic>, a lineage that was not well understood, and subcluster 5.1 clade I, a typical cold water lineage, were widely distributed in the bottom layer of the estuary. Clade I detected in the studied region was mainly contributed by subclade IG. Analysis of the <italic>cpcBA</italic> operon sequences revealed niche partitioning between type 1 and type 3 <italic>Synechococcus</italic>, with type 2 distributed broadly across the whole environmental gradients. Our results suggest that the salt wedge estuary provides various niches for different lineages of <italic>Synechococcus</italic>, making it an environment with high <italic>Synechococcus</italic> diversity compared with adjacent freshwater and shelf sea environments.</p>