Synechococcus bloom in the Pearl River Estuary and adjacent coastal area-With special focus on flooding during wet seasons.

Based on the field surveys aimed at understanding the variations of Synechococcus (Syn) abundance in the Pearl River Estuary during different seasons. We found that heavy terrestrial precipitation result in significant riverine runoffs and promote Syn growth, extension and blooms during warm and wet seasons. To understand the ecological role of Syn play in this estuary during wet seasons, we combined flow cytometry and high throughput sequencing (HTS) of 16S rDNA to investigate the phytoplankton distribution patterns and the potential shaping mechanisms during a typical wet season. During the cruise, picophytoplankton, especially Syn, and Nano-eukaryotes contributed importantly to the total phytoplankton biomass of the estuary. Syn can be further divided into phycoerythrin (PE)-rich Syn and phycocyanin (PC)-rich Syn, with PC-rich Syn about 1.5 times higher than PE-rich Syn in abundance. Both PE-rich Syn (60.75 × 103 cells ml-1) and PC-rich Syn (604.05 × 103 cells ml-1) reach the highest abundance at the lower part of the estuary. Moreover, PE-rich Syn can be divided into two subgroups which showed different salinity preference, with PE1 distributed in the high salinity area (with salinity >25) while PE2 in the middle salinity area (with salinity 7-20). Our results from the 16S rDNA sequencing also indicated abundant diversity and different niche adaptation of Syn with the operational taxonomic units (OTUs) along the estuary. Besides, analysis also indicated a tight correlation between estuarine Syn and active heterotrophic bacteria, especially groups of Rhodobacteria and Actionobacteria.

Spatial and seasonal distributions of bacterioplankton in the Pearl River Estuary: The combined effects of riverine inputs, temperature, and phytoplankton.

In this study, we used flow cytometry and 16S rRNA gene pyrosequencing to investigate bacterioplankton (heterotrophic bacteria and picocyanobacteria) abundance and community structure in surface waters along the Pearl River Estuary. The results showed significant differences in bacterioplankton dynamics between fresh- and saltwater sites and between wet and dry season. Synechococcus constituted the majority of picocyanobacteria in both seasons. During the wet season, Synechococcus reached extremely high abundance at the mouth of the estuary, and heterotrophic bacteria were highly abundant (>106cellsml-1) throughout the studied region. At the same time, bacterioplankton decreased dramatically during the dry season. Pyrosequencing data indicated that salinity was a key parameter in shaping microbial community structure during both seasons. Phytoplankton was also an important factor; the proportion of Synechococcus and Rhodobacteriales was elevated at the frontal zone with higher chlorophyll a during the wet season, whereas Synechococcus were markedly reduced during the dry season.

Distribution of picoplankton in the northeastern South China Sea with special reference to the effects of the Kuroshio intrusion and the associated mesoscale eddies.

We investigated picoplankton distribution patterns and environmental variables along an east-to-west transect in the northeastern South China Sea (SCS) during late winter 2016, giving us the opportunity to examine the impacts of the Kuroshio intrusion and the associated eddies. The results indicated that the subsurface (50-75m) phytoplankton biomass chlorophyll (Chl a) maximum (SCM) disappeared and was replaced by higher Chl a in the middle part of the transect due to the impacts of the Kuroshio intrusion and mesoscale eddies. Both flow cytometry and pyrosequencing data revealed that picoplankton abundance and community structure were significantly influenced by perturbations in complex physical processes. Picoeukaryotes represented most of the total phytoplankton biomass, and their maximum abundance (>104cellsmL-1) occurred within cyclonic eddy-affected regions (Stations 11 and 12), whereas the abundance of Prochlorococcus was the lowest in these regions. Prochlorococcus showed a higher abundance in the Kuroshio-affected area, while Synechococcus was mostly distributed at the upper well-lit depths, with its maximum abundance observed in surface waters (0-30m) adjacent to the cyclonic eddy center. Heterotrophic bacteria (HBA) displayed high abundance along the transect, consistent with the total phytoplankton biomass. Phylogenetic analysis revealed 26 bacterial phyla, with major components belonging to Proteobacteria, Cyanobacteria, Actinobacteria, and Bacteroidetes, as well as SAR406. Notably, relatively more Rhodobacterales, Flavobacteriales, Alteromonadales, and Vibrionales that were distributed in surface waters of the cyclonic eddy center were specifically associated with the phytoplankton (mainly picoeukaryotes) bloom. Our study highlights the impacts of the Kuroshio intrusion in regulating the microbial ecology of the northeastern SCS and the potential coupling between phytoplankton and bacteria.