Abstract
Picocyanobacteria of the genus Synechococcus are major contributors to global primary production and nutrient cycles due to their oxygenic photoautotrophy, their abundance, and the extensive distribution made possible by their wide-ranging biochemical capabilities. The recent recovery and isolation of strains from the deep euxinic waters of the Black Sea encouraged us to expand our analysis of their adaptability also beyond the photic zone of aquatic environments. To this end, we quantified the total abundance and distribution of Synechococcus along the whole vertical profile of the Black Sea by flow cytometry, and analyzed the data obtained in light of key environmental factors. Furthermore, we designed phylotype-specific primers using the genomes of two new epipelagic coastal strains – first described here – and of two previously described mesopelagic strains, analyzed their presence/abundance by qPCR, and tested this parameter also in metagenomes from two stations at different depths. Together, whole genome sequencing, metagenomics and qPCR techniques provide us with a higher resolution of Synechococcus dynamics in the Black Sea. Both phylotypes analyzed are abundant and successful in epipelagic coastal waters; but while the newly described epipelagic strains are specifically adapted to this environment, the strains previously isolated in mesopelagic waters are able, in low numbers, to withstand the aphotic and oxygen depleted conditions of deep layers. This heterogeneity allows different Synechococcus phylotypes to occupy different niches and underscores the importance of a more detailed characterization of the abundance, distribution, and dynamics of individual populations of these picocyanobacteria.
Highlights
The Black Sea is a unique marine basin, largely isolated from the global ocean, and characterized by an extensive drainage basin, strong vertical stratification, variable salinity and high concentrations of hydrogen sulfide content below 150–200 m (Konovalov et al, 2005; Stanev et al, 2014)
Expanding on earlier studies of the distribution, abundance, and dynamics of Synechococcus in the Black Sea, which concentrated on the euphotic layer (Uysal, 2000, 2001, 2006; Feyzioglu et al, 2004, 2015), we investigated the correlation of total Synechococcus counts with environmental variables deeper in the water column
We found that total Synechococcus cell numbers, unlike bacteria, were not correlated with Chl-a, which mainly derived from the eukaryotic fraction, as previously reported in other marine systems (Tai and Palenik, 2009)
Summary
The Black Sea is a unique marine basin, largely isolated from the global ocean, and characterized by an extensive drainage basin, strong vertical stratification, variable salinity (from 7.3 in epipelagic to ca. 20–22 PSU in mesopelagic waters) and high concentrations of hydrogen sulfide content below 150–200 m (Konovalov et al, 2005; Stanev et al, 2014). Despite Synechococcus’ remarkable adaptability to different salinities (Fuller et al, 2003; Kim et al, 2018), temperatures, Fe and nutrient concentrations (Fuller et al, 2006; Callieri, 2017; Zwirglmaier et al, 2008), the detection of Synechococcus in an extreme environment like the deep anoxic Black Sea was unexpected – even at relatively low concentrations. This is because, as oxygenic autotrophs, Synechococcus have been usually considered restricted to the photic layer
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