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Microplastics (MP) constitute a widespread contaminant all over the globe. Rivers and wastewater treatment plants (WWTP) transport annually several million tons of MP into freshwaters, estuaries and oceans, where they provide increasing artificial surfaces for microbial colonization. As knowledge on MP-attached communities is insufficient for brackish ecosystems, we conducted exposure experiments in the coastal Baltic Sea, an in-flowing river and a WWTP within the drainage basin. While reporting on prokaryotic and fungal communities from the same set-up previously, we focus here on the entire eukaryotic communities. Using high-throughput 18S rRNA gene sequencing, we analyzed the eukaryotes colonizing on two types of MP, polyethylene and polystyrene, and compared them to the ones in the surrounding water and on a natural surface (wood). More than 500 different taxa across almost all kingdoms of the eukaryotic tree of life were identified on MP, dominated by Alveolata, Metazoa, and Chloroplastida. The eukaryotic community composition on MP was significantly distinct from wood and the surrounding water, with overall lower diversity and the potentially harmful dinoflagellate Pfiesteria being enriched on MP. Co-occurrence networks, which include prokaryotic and eukaryotic taxa, hint at possibilities for dynamic microbial interactions on MP. This first report on total eukaryotic communities on MP in brackish environments highlights the complexity of MP-associated biofilms, potentially leading to altered microbial activities and hence changes in ecosystem functions. [ABSTRACT FROM AUTHOR]

Throughout the Baltic Sea redoxcline, virus production and the frequency of lytically-infected prokaryotic cells were estimated from parallel incubations of undiluted seawater and seawater that contained prokaryotes with substantially reduced numbers of viruses (virus dilution approach), effectively preventing viral reinfection during the incubation period. Undiluted seawater incubations resulted in much higher estimates of virus production (6–35×104 mL-1 h-1) and the frequency of infected cells (5–84%) than the virus dilution approach (virus production: 1–3×104 mL-1 h-1; frequency of infected cells: 1–11%). Viral production and the frequency of infected cells from both approaches, however, cannot be directly compared, as data obtained from undiluted incubations were biased by viral reinfection and other uncontrollable processes during the incubation period. High in situ viral abundance (1–2×107 mL-1) together with low virus production rates based on the virus dilution approach resulted in some of the longest viral turnover times (24–84 d) ever reported for the epipelagial. Throughout a wide range of environmental conditions, viral turnover time and burst size were negatively correlated. Given that viral decay estimated in ultra-filtered water was below the detection limit and the burst size was low (1–17), we conclude that prokaryotic viruses in the Baltic Sea redoxcline are investing most of their resources into stress defense (strong capsids) rather than proliferation (high burst size). In summary, the Baltic Sea redoxcline constitutes an environment where low virus production is found in combination with low viral decay, resulting in high viral abundance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Machine learning algorithms can be trained on complex data sets to detect, predict, or model specific aspects. Aim of this study was to train an artificial neural network in comparison to a Random Forest model to detect induced changes in microbial communities, in order to support environmental monitoring efforts of contamination events. Models were trained on taxon count tables obtained via next-generation amplicon sequencing of water column samples originating from a lab microcosm incubation experiment conducted over 140 days to determine the effects of glyphosate on succession within brackish-water microbial communities. Glyphosate-treated assemblages were classified correctly; a subsetting approach identified the taxa primarily responsible for this, permitting the reduction of input features. This study demonstrates the potential of artificial neural networks to predict indicator species for glyphosate contamination. The results could empower the development of environmental monitoring strategies with applications limited to neither glyphosate nor amplicon sequence data. • An artificial neural net was able to identify glyphosate-affected microbial community assemblages based on NGS data • Direct comparison of a fully connected artificial neural net vs Random Forest classification • Decision-relevant taxonomic clusters for the ANN can be identified by a stochastic subsetting approach • Just a fraction of present taxonomic clusters is required for classification • Filtering of input data improves ANN classification [ABSTRACT FROM AUTHOR]

Nitrogen (N 2) fixation rates in the brackish Baltic Sea are among the highest per unit of area in the world. However, beyond the filamentous heterocyst-forming cyanobacteria, knowledge about the composition and distribution of N 2 -fixing microbes (diazotrophs) is limited. To address this, we investigated nitrogenase gene (nifH) composition and expression at coastal (<10 km offshore) and offshore (>10 km offshore) stations, at surface (avg. 1.8 m) and at depth (avg. 24 m) and in free-living (0.2–3.0 μm) and particle-associated size fractions (>3 μm). Surprisingly, nifH genes affiliated with Pseudanabaena and non-cyanobacterial diazotrophs (NCDs) dominated the composition whereas filamentous heterocyst-forming cyanobacteria accounted for almost 80% of the nifH transcripts. Salinity had a minor influence on the composition, but Aphanizomenon and Nodularia showed increased relative nifH gene expression at low and higher salinity, respectively. Pseudanabaena only accounted for up to 5% of the nifH transcripts and nifH gene expression by Candidatus Atelocyanobacterium thalassa (sublineage UCYN-A2) was mainly observed in the most saline western part of the Baltic. The only notable expression by NCDs (up to 15% of nifH transcripts at a given station) coincided with an upwelling event at the southern coast and was largely accounted for by a Pseudomonas -like nifH phylotype, recurrently found in the Baltic Sea. NCD relative abundances were dominant in coastal stations, presumably driven by sediment resuspension as evidenced by higher turbidity and DOC levels and the recovery of sediment diazotrophs in the pelagic zone. This study reveals the heterogeneity of the composition and activity of diazotrophs in the Baltic Sea, and underscores the need for future N 2 fixation studies that include coastal and offshore Baltic waters. [Display omitted] • Pseudanabaena and non-cyanobacteria dominate the nitrogenase (nifH) gene pool. • Heterocyst-forming cyanobacteria accounted for 80% of the nifH gene transcripts. • Coastal upwelling affected composition and activity of diazotrophs. • The composition and activity of diazotrophs in the Baltic Sea is heterogeneous. • Future N 2 fixation studies should include coastal and offshore Baltic waters. [ABSTRACT FROM AUTHOR]

Vibrio spp. are ubiquitous in aquatic environments. In temperate regions, including the Baltic Sea, these bacteria, which include facultative pathogenic species, proliferate during warm water periods and may thus pose a risk to human health. However, while present at relatively high abundances in the summer months, Vibrio spp. are rarely detected during cold water periods. Several potential winter reservoirs, such as fish and sediment, have been discussed, but the seeding sources that give rise to Vibrio during the summer months are unknown. In this study, we employed a combination of cultivation-independent genus-specific digital droplet PCR, Illumina 16S rRNA gene sequencing, culture-dependent determinations of colony-forming units (CFU) and genus-specific strain isolation to identify possible winter reservoirs of Vibrio spp. Using these techniques, we demonstrate that in the southern Baltic Sea especially fish can serve as a winter reservoir for distinct Vibrio assemblages, including facultative pathogenic V. vulnificus. Particularly the invasive species round goby (Neogobius melanostomus), with its potential to appear in high individual densities, has the capacity to carry a comparable Vibrio spp. cell number like water and sediment, while only accounting for 1/1000 of the spatial share in the overall coastal environment. Thus, by providing a winter reservoir for distinct Vibrio communities, coastal fish and, especially areas in which round goby densities are high, may be hot spots of pathogenic Vibrio species. [Display omitted] • Fish acts as Vibrio winter reservoir in the Baltic Sea. • Vibrio spp. often highest abundant in connection to invasive species round goby. • Quantitative Vibrio 16S rRNA gene ddPCR without Photobacterium falsification. [ABSTRACT FROM AUTHOR]