Diversity and abundance of “Pelagibacterales” (SAR11) in the Baltic Sea salinity gradient

Abstract

The candidate order "Pelagibacterales" (SAR11) is one of the most abundant bacterial orders in ocean surface waters and, periodically, in freshwater lakes. The presence of several stable phylogenetic lineages comprising "Pelagibacterales" correlates with the physico-chemical parameters in aquatic environments. A previous amplicon sequencing study covering the bacterial community in the salinity gradient of the Baltic Sea suggested that pelagibacteral subclade SAR11-I was replaced by SAR11-IIIa in the mesohaline region of the Baltic Sea. In this current study, we investigated the cellular abundances of "Pelagibacterales" subclades along the Baltic Sea salinity gradient using catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). The results obtained with a newly designed probe, which exclusively detected SAR11-IIIa, were compared to CARD-FISH abundances of the marine SAR11-I/II subclade and the freshwater lineage SAR11-IIIb (LD12). The results showed that SAR11-IIIa was abundant in oligohaline-mesohaline conditions (salinities 2.7-13.3), with maximal abundances at a salinity of 7 (up to 35% of total Bacteria, quantified with a universal bacterial probe EUB). As expected, SAR11-I/II was abundant (27% of EUB) in the marine parts of the Baltic Sea, whereas counts of the freshwater lineage SAR11-IIIb were below the detection limit at all stations. The shift from SAR11-IIIa to SAR11-I/II was confirmed in the vertical salinity gradient in the deeper basins of the Baltic Sea. These findings were consistent with an overlapping but defined distribution of SAR11-I/II and SAR11-IIIa in the salinity gradient of the Baltic Sea and suggested the adaptation of SAR11-IIIa for growth and survival in mesohaline conditions.

Phylogenetic Signals of Salinity and Season in Bacterial Community Composition Across the Salinity Gradient of the Baltic Sea

“…In contrast to S ynechococcus , the SAR11 representatives identified in the marine (OTU-200 and OTU-18) and mesohaline (OTU-14) samples were phylogenetically distinct ( Figure 5 ). In accordance with our previous study in the Baltic Sea, based on fluorescence in situ hybridization (Herlemann et al, 2014), the SAR11-IIIa lineage detected in this study was highly abundant under mesohaline conditions, especially in February. The SAR11-IIIa lineage found in brackish zones was replaced by the marine SAR11-II lineage in marine waters of the Baltic Sea.…”

“…We also identified typical mesohaline bacterial members in the central Baltic Sea, including the verrucomicrobial taxon “ Candidatus Spartobacterium balticum” (Herlemann et al, 2013; Bergen et al, 2014) and the “SAR11-IIIa” clade (Herlemann et al, 2014). Here, we extend these earlier analyses by analyzing a transect dataset sampled in winter and comparing bacterial community composition in winter and summer along the salinity gradient of the Baltic Sea.…”

Genome Reconstruction from Metagenomic Data Sets Reveals Novel Microbes in the Brackish Waters of the Caspian Sea

“…Within the SAR11 clade there are marine and freshwater representatives. The freshwater clade, referred to as LD12, corresponds to subtype IIIb of SAR11 (40), while SAR11 I and SAR11 II are considered to be offshore marine microbes and SAR11 IIIa comprises coastal water (SAR11-HIMB114) and mesohaline water (SAR11-IMCC9063) representatives (60,61). A partial genome was recently reconstructed from a metagenome from brackish Lake Qinghai in China (62).…”

Evolution and adaptation of SAR11 and Cyanobium in a saline Tibetan lake

“…high freshwater inputs with restricted connection to the ocean) in Northern Europe and is characterized by its unique lateral salinity gradient. In the salinity gradient of the distinct marine ecosystem, subclade IIIa was found to be abundant (10%-35% of total bacteria) in the oligosalinemesosaline conditions (3-13 g/l), while subclade I/II and IIIb dominated at seawater (35 g/l) and low salinity level (< 4 g/l) respectively (Herlemann et al, 2014). Accordingly, the selective enrichment of QL1 in the lake community is likely due to, at least in part, the mesosaline condition of Lake Qinghai.…”