Abstract
Synechococcus are ubiquitous and cosmopolitan cyanobacteria that play important roles in global productivity and biogeochemical cycles. This study investigated the fine scale microdiversity, seasonal patterns, and spatial distributions of Synechococcus in estuarine waters of Little Sippewissett salt marsh (LSM) on Cape Cod, MA. The proportion of Synechococcus reads was higher in the summer than winter, and higher in coastal waters than within the estuary. Variations in the V4–V6 region of the bacterial 16S rRNA gene revealed 12 unique Synechococcus oligotypes. Two distinct communities emerged in early and late summer, each comprising a different set of statistically co-occurring Synechococcus oligotypes from different clades. The early summer community included clades I and IV, which correlated with lower temperature and higher dissolved oxygen levels. The late summer community included clades CB5, I, IV, and VI, which correlated with higher temperatures and higher salinity levels. Four rare oligotypes occurred in the late summer community, and their relative abundances more strongly correlated with high salinity than did other co-occurring oligotypes. The analysis revealed that multiple, closely related oligotypes comprised certain abundant clades (e.g., clade 1 in the early summer and clade CB5 in the late summer), but the correlations between these oligotypes varied from pair to pair, suggesting they had slightly different niches despite being closely related at the clade level. Lack of tidal water exchange between sampling stations gave rise to a unique oligotype not abundant at other locations in the estuary, suggesting physical isolation plays a role in generating additional microdiversity within the community. Together, these results contribute to our understanding of the environmental and ecological factors that influence patterns of Synechococcus microbial community composition over space and time in salt marsh estuarine waters.
Highlights
We identified 12 Synechococcus oligotypes, which together accounted for an average of 73% of the Synechococcus reads over the entire dataset and comprised 0–100% of the total Synechococcus population depending on sampling location and date
Clear seasonal and spatial patterns were apparent; Synechococcus comprised a greater fraction of the microbial population in the summer than in the winter, and had higher relative abundances in the coastal ocean compared to stations within the estuary (Figure 4)
Two distinct Synechococcus communities emerged that dominated in the early summer (May/June) and late summer (July onward), each composed of different, co-occurring Synechococcus oligotypes (Figure 5)
Summary
The genus Synechococcus is a remarkably diverse group that inhabits aquatic environments ranging from open ocean and coastal waters (Bouman et al, 2006; Zwirglmaier et al, 2007, 2008; Post et al, 2011; Flombaum et al, 2013; Sohm et al, 2015; Hunter-Cevera et al, 2016) to brackish estuaries (Chen et al, 2006; Choi and Noh, 2009), and freshwater lakes (Ivanikova et al, 2008). The diversity of Synechococcus based on 16S rRNA gene phylogeny spans three sub-clusters (5.1, 5.2, and 5.3), and further resolution of the genus is possible using additional genetic markers such as the internally transcribed spacer (ITS) region and the genes cpeA, narB, ntcA, petB, rbcL, and rpoC1 (Scanlan et al, 2009; Ahlgren and Rocap, 2012) These markers partition the genus into at least 20 distinct clades (Scanlan et al, 2009; Ahlgren and Rocap, 2012; Huang et al, 2012; Mazard et al, 2012; Sohm et al, 2015). This may be due to extensive lateral gene transfer or independent parallel evolution of traits that occurred after the marine Synechococcus lineages diverged (Ahlgren and Rocap, 2012; Sohm et al, 2015)
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