Abstract
BackgroundThe Black Sea is the largest brackish water body in the world, although it is connected to the Mediterranean Sea and presents an upper water layer similar to some regions of the former, albeit with lower salinity and temperature. Despite its well-known hydrology and physicochemical features, this enormous water mass remains poorly studied at the microbial genomics level.ResultsWe have sampled its different water masses and analyzed the microbiome by shotgun and genome-resolved metagenomics, generating a large number of metagenome-assembled genomes (MAGs) from them. We found various similarities with previously described Black Sea metagenomic datasets, that show remarkable stability in its microbiome. Our datasets are also comparable to other marine anoxic water columns like the Cariaco Basin. The oxic zone resembles to standard marine (e.g. Mediterranean) photic zones, with Cyanobacteria (Synechococcus but a conspicuously absent Prochlorococcus), and photoheterotrophs domination (largely again with marine relatives). The chemocline presents very different characteristics from the oxic surface with many examples of chemolithotrophic metabolism (Thioglobus) and facultatively anaerobic microbes. The euxinic anaerobic zone presents, as expected, features in common with the bottom of meromictic lakes with a massive dominance of sulfate reduction as energy-generating metabolism, a few (but detectable) methanogenesis marker genes, and a large number of “dark matter” streamlined genomes of largely unpredictable ecology.ConclusionsThe Black Sea oxic zone presents many similarities to the global ocean while the redoxcline and euxinic water masses have similarities to other similar aquatic environments of marine (Cariaco Basin or other Black Sea regions) or freshwater (meromictic monimolimnion strata) origin. The MAG collection represents very well the different types of metabolisms expected in this kind of environment. We are adding critical information about this unique and important ecosystem and its microbiome.
Highlights
The Black Sea is the largest brackish water body in the world, it is connected to the Mediterranean Sea and presents an upper water layer similar to some regions of the former, albeit with lower salinity and temperature
Black Sea shotgun metagenome analysis Samples were collected along the Bulgarian coast at two stations (Fig. 1a)
For St. 307, with a maximum depth of 1100 m, samples were from the near-surface at 5 m depth, the deep chlorophyll maximum (DCM) at 30 m, a sample from the redoxcline/pycnocline at 150 m, and a sample at 750 m, corresponding to the euxinic water layer
Summary
The Black Sea is the largest brackish water body in the world, it is connected to the Mediterranean Sea and presents an upper water layer similar to some regions of the former, albeit with lower salinity and temperature. The large watershed and riverine inputs lead to a richer nutrient status (meso-eutrophic) and permanent stratification with a colder, more saline deepwater mass that remains anaerobic and largely euxinic below 150–200 m [1,2,3]. All these properties make the Black Sea a unique brackish-marine environment. Its depth (average 1253 m with a maximum of 2212 m) makes this system much more stable than other brackish inland water bodies like the Baltic Sea in which the anaerobic compartment is only a recent development due to anthropogenic impacts [4]
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