Spatio-Temporal Variation of Synechococcus Assemblages at DNA and cDNA Levels in the Tropical Estuarine and Coastal Waters.

Ting Wang,Hongmei Jing,Jiawei Chen,Hongbin Liu,Xiaomin Xia

doi:10.3389/fmicb.2022.837037

Abstract

Synechococcus is a major contributor to global marine primary production. Here, its spatio-temporal variations in abundance and phylogenetic structure were studied at three stations of the South China Sea at both DNA and cDNA levels. Synechococcus cell abundance was lowest in March, but highest in October at two coastal stations. Its abundance was higher at the estuarine station, which reached a peak value of 1.36 × 105 cells/ml in April, owing to the nitrogen nutrients discharged from the Sanya River. Gene and gene transcript abundances of four Synechococcus lineages, clades II, III, VIII, and S5.3, were studied by quantitative PCR, which showed that clade II was the most abundant lineage at both DNA and cDNA levels. High-throughput sequencing revealed that, at the DNA level, Synechococcus assemblage was dominated by clade SY4 (a novel clade defined in this study), S5.2, and clade II in the coastal waters and was dominated by freshwater/S5.2 Synechococcus, reaching a value up to 88.61% in June, in estuarine waters. Changes in salinity and nutrient concentration caused by seasonal monsoonal forcing and river discharge were the key determinants of the spatio-temporal variation in Synechococcus assemblages at the DNA level. In comparison, high dissimilation among samples at the same stations and in the same seasons leads to the imperceptible spatio-temporal variation pattern of Synechococcus assemblages at the cDNA level. Furthermore, co-occurrence networks disclosed that Synechococcus community had closer and more complex internal interactions at the cDNA level. These discrepancies highlighted the necessity to study Synechococcus assemblages at both DNA and cDNA levels.

Highlights

Marine Synechococcus represents one of the most abundant phytoplankton in the global ocean (Flombaum et al, 2013)
We found highly abundant Synechococcus at the estuarine station, especially more than 105 cells/ml in April and July, which is probably due to the eutrophic environment in estuarine waters caused by the discharge of freshwater and sanitary sewage
Many previous studies have shown that Synechococcus cell abundance is the highest in summer and the lowest in winter because the temperature changes the relationship between Synechococcus growth and its grazing mortality (Paerl et al, 2012; Chen et al, 2020; Wang et al, 2021b)

Summary

Introduction

Marine Synechococcus represents one of the most abundant phytoplankton in the global ocean (Flombaum et al, 2013). Marine Synechococcus exhibits high genetic diversity (Farrant et al, 2016; Xia et al, 2019) Gene markers, such as the 16S rRNA gene (Mackey et al, 2017), the 16S–23S internally transcribed spacer (ITS) (Li et al, 2021; Nagarkar et al, 2021), the RNA polymerase gene (rpoC1) (Kent et al, 2019; Wang et al, 2021a), the nitrate reductase gene (narB) (Robidart et al, 2012), the cytochrome b6 gene (petB) (Mazard et al, 2012), and the ribulose–1,5–bisphosphate carboxylase oxygenase gene (rbcL) (Paerl et al, 2012), have been commonly applied to study the genetic diversity of Synechococcus. Clade I is more abundant prior to spring, whereas clades II and III only appear in late summer and winter in the California Current (Tai and Palenik, 2009); clades II and VI are the major lineages when the summer monsoon prevailed, but clades II, IX, and miyav are the dominant clades during winter in subtropical coastal waters of Hong Kong (Xia et al, 2015)

Results

Discussion

Conclusion