Morphological and molecular approaches reveal highly stratified protist communities along Baltic Sea pelagic redox gradients

Abstract

The deep basins of the central Baltic Sea are characterized by anoxic and sulfidic bottom water and steep vertical pelagic redox gradients. The highly active prokaryotic assemblages of this and other redox transition zones have been intensely studied, while knowledge on the protistan communities remains fragmentary. Thus, we conducted a multi-annual microscopybased study, combined in one year with 18S rRNA gene and transcript-based DGGE fingerprints to identify the dominant protist taxa and to assess their vertical distribution. Both approaches, applied in high vertical resolution, demonstrated strong stratifications of the protist community composition along the redox gradient. The suboxic zone was dominated by dinoflagellates and oligotrichous ciliates related to Strombidium, whereas the interface and upper sulfidic zone were dominated by ciliates of the genera Mesodinium and Metacystis. Several flagellate taxa within the jakobids, euglenozoans and choanoflagellates occurred exclusively in sulfidic water. Our morphological approach indicates that the pelagic redoxclines of the central Baltic Sea are inhabited by a stable and characteristic protist community. Incongruously, certain taxa (e.g. Mesodinium and Metacystis sp.) which remained undetected by the molecular fingerprinting technique could be identified and enumerated by microscopic observations, whereas small and virtually amorphous protists (especially flagellates) were detected only by sequencing DGGE bands. Fine-scaled assessment of dominant protists in distinct redox strata is a crucial step in understanding their impact and interactions with the prokaryotic world and the biogeochemical processes they mediate in these zones.

Spatiotemporal variations in microbial diversity across the three domains of life in a tropical thalassohaline lake (Dziani Dzaha, Mayotte Island)

“…Jakobids have been studied primarily because of their evolutionary importance and their uncertain phylogenetic placement among eukaryotes (Archibald, Simpson, & Slamovits, ). This group has been detected in different oxygen‐poor, sulphide‐rich and hypersaline ecosystems (Stock, Jürgens, Bunge, & Stoeck, ; Weber, Anderson, Foissner, Mylnikov, & Jürgens, ), exhibiting H 2 S/HS − concentrations (0 to 100 μM) close to those retrieved in Lake Dziani Dzaha during the unstratified period (67–328 μM H 2 S/HS − above the deep chemocline). When H 2 S/HS − concentrations increased during the stratified period (up to 5,900 μM), jakobids were almost absent from the eukaryotic assemblage, suggesting that high H 2 S/HS − concentrations are toxic to those microorganisms.…”

Strategies of amino acid supply in mesozooplankton during cyanobacteria blooms: a stable nitrogen isotope approach

“…, Weber et al. ). NH 4 + from suboxic waters has an average δ 15 N value of 7.9 ± 1.9‰ according to Frey et al.…”

“…, Weber et al. ). In the southern part of the central Baltic Sea, a heterotrophic diet of ciliates has earlier been suggested for Pseudocalanus based on fatty acid trophic markers (Peters et al.…”

The Genetic Diversity of Mesodinium and Associated Cryptophytes

“…Within the Åland region, up to three size classes (∼20, 40, 60 μm) of M. rubrum -like ciliates are observed, with the smallest being dominant during autumnal red water events (Lindholm, 1978). Deep (>70 m) layers of pigmented Mesodinium have also been found in the Baltic Sea, below the thermocline (Setälä et al, 2005) and near the anoxic boundary of the Gotland Basin (Weber et al, 2014). Our two samples from the Gulf of Finland (GF) revealed a mixed community of M. rubrum subclades A, D, and F. While subclade D ( M. major ) is the largest Mesodinium spp., both subclades A and F may have a range of cell sizes, even within a single strain ( Table 2 ).…”

“…Peaks in cryptophyte abundance have been found to both proceed (Johnson et al, 2013) and co-occur (Kim et al, 2007; Weber et al, 2014) with high levels of M. rubrum -like ciliates in coastal ecosystems. However, most historical accounts of Mesodinium blooms did not note the abundance or composition of co-occurring cryptophyte communities.…”