The Native Bacterioplankton Community in the Central Baltic Sea Is Influenced by Freshwater Bacterial Species

Abstract

The Baltic Sea is one of the largest brackish environments on Earth. Despite extensive knowledge about food web interactions and pelagic ecosystem functioning, information about the bacterial community composition in the Baltic Sea is scarce. We hypothesized that due to the eutrophic low-salinity environment and the long water residence time (>5 years), the bacterioplankton community from the Baltic proper shows a native "brackish" composition influenced by both freshwater and marine phylotypes. The bacterial community composition in surface water (3-m depth) was examined at a single station throughout a full year. Denaturing gradient gel electrophoresis (DGGE) showed that the community composition changed over the year. Further, it indicated that at the four extensive samplings (16S rRNA gene clone libraries and bacterial isolates from lowand high-nutrient agar plates and seawater cultures), different bacterial assemblages associated with different environmental conditions were present. Overall, the sequencing of 26 DGGE bands, 160 clones, 209 plate isolates, and 9 dilution culture isolates showed that the bacterial assemblage in surface waters of the central Baltic Sea was dominated by Bacteroidetes but exhibited a pronounced influence of typical freshwater phylogenetic groups within Actinobacteria, Verrucomicrobia, and Betaproteobacteria and a lack of typical marine taxa. This first comprehensive analysis of bacterial community composition in the central Baltic Sea points to the existence of an autochthonous estuarine community uniquely adapted to the environmental conditions prevailing in this brackish environment.

Pyrosequencing reveals contrasting seasonal dynamics of taxa within Baltic Sea bacterioplankton communities

“…Sampling, DNA extraction and microbial community dynamics, as analyzed by denaturing gradient gel electrophoresis and 16S rRNA gene clone libraries, are detailed in Riemann et al (2008). Table 1.…”

“…For DNA extractions, cells from prefiltered (o3.0 mm) water samples were collected on 0.22 mm Sterivex filters (Millipore, Solna, Sweden) using washed, sterile tubing and utensils. Filters were kept frozen at À20 1C until community DNA was extracted using an enzyme/phenol-chloroform protocol (Riemann et al, 2008). Extracted DNA was stored frozen in Tris-EDTA buffer (10 mM Tris, 1 mM EDTA, pH 8.0) until use.…”

“…In contrast, the central Baltic Sea has a long retention time (45 years (Wulff and Stigebrandt, 1989)) and is conceivably dominated by microbes that have adapted to this specific environment. In the Baltic Sea proper (B6 PSU), Riemann et al (2008) found that surface bacterioplankton communities seemed to be strongly influenced by typical freshwater bacterial groups within Actinobacteria, Verrucomicrobia and Betaproteobacteria, whereas many typical marine taxa appeared to be missing. Hence, although the low salinity of the Baltic Sea surface water seems to be conducive for growth of freshwater bacteria (Kisand et al, 2005;Riemann et al, 2008), typical marine bacterial populations may be constrained by requirements for higher salinity.…”

“…In the Baltic Sea proper (B6 PSU), Riemann et al (2008) found that surface bacterioplankton communities seemed to be strongly influenced by typical freshwater bacterial groups within Actinobacteria, Verrucomicrobia and Betaproteobacteria, whereas many typical marine taxa appeared to be missing. Hence, although the low salinity of the Baltic Sea surface water seems to be conducive for growth of freshwater bacteria (Kisand et al, 2005;Riemann et al, 2008), typical marine bacterial populations may be constrained by requirements for higher salinity. It should be noted that estuarine bacterioplankton communities are exposed to gradients in environmental parameters other than salinity, for example, dissolved organic matter, nutrients and temperature.…”

Towards a better understanding of microbial carbon flux in the sea*

“…SAR11 is known to be the most common prokaryotic group in the surface ocean (Morris et al 2002, Rappé et al 2002, dominating clone libraries (Giovannoni & Rappé 2000) and also as detected by fluorescence in situ hybridization (FISH). However, SAR11 is neither dominant in the Baltic Sea (Riemann et al 2008) nor at some Atlantic coastal sites (e.g. Henriques et al 2004) and, in fact, is a whole clade comprised of many ecotypes (Field et al 1997), probably with different functioning and habitat selectivity.…”