A Metagenomics Transect into the Deepest Point of the Baltic Sea Reveals Clear Stratification of Microbial Functional Capacities

Abstract

The Baltic Sea is characterized by hyposaline surface waters, hypoxic and anoxic deep waters and sediments. These conditions, which in turn lead to a steep oxygen gradient, are particularly evident at Landsort Deep in the Baltic Proper. Given these substantial differences in environmental parameters at Landsort Deep, we performed a metagenomic census spanning surface to sediment to establish whether the microbial communities at this site are as stratified as the physical environment. We report strong stratification across a depth transect for both functional capacity and taxonomic affiliation, with functional capacity corresponding most closely to key environmental parameters of oxygen, salinity and temperature. We report similarities in functional capacity between the hypoxic community and hadal zone communities, underscoring the substantial degree of eutrophication in the Baltic Proper. Reconstruction of the nitrogen cycle at Landsort deep shows potential for syntrophy between archaeal ammonium oxidizers and bacterial denitrification at anoxic depths, while anaerobic ammonium oxidation genes are absent, despite substantial ammonium levels below the chemocline. Our census also reveals enrichment in genetic prerequisites for a copiotrophic lifestyle and resistance mechanisms reflecting adaptation to prevalent eutrophic conditions and the accumulation of environmental pollutants resulting from ongoing anthropogenic pressures in the Baltic Sea.

Biomarkers in the stratified water column of the Landsort Deep (Baltic Sea)

“…3). The same trend has been described for PMI in the Black Sea (Wakeham et al, 2007), and the presence of Euryarchaeota in Landsort Deep anoxic waters has recently been proven by Thureborn et al (2013).…”

“…Unidentified ciliates also occurred in the anoxic waters of the Landsort Deep (Anderson et al, 2012). Members of the genus Rhodopseudomonas, a possible alternative source of tetrahymanol, have so far not been identified in the suboxic zone (Labrenz et al, 2007;Thureborn et al, 2013). We therefore regard bacterivorous ciliates living under suboxic to anoxic conditions as the most likely source of tetrahymanol in the suboxic zone and below.…”

“…However, 10-me16 : 0 PLFA may also be contributed by sulfate-reducing bacteria (see Sect. 4.6), and no evidence for anammox has been observed in the water column of the Landsort Deep from molecular biological studies so far (Hietanen et al, 2012;Thureborn et al, 2013). Regardless of the biological source, BHT II was described in stratified water columns of the Arabian Sea, Peru Margin and Cariaco Basin (Sáenz et al, 2011), and the Gotland Deep and has therefore been proposed as a proxy for stratified water columns.…”

“…Furthermore, published genetic studies reporting on prokaryotes and the related metabolisms in the water column of the Landsort Deep (Labrenz et al, 2007;Thureborn et al, 2013) provide a background to which the organic geochemical results can be advantageously related. The depth profiles of biomarkers from this setting not only reveal how actual biogeochemical processes are reflected by lipid abundances, distributions, and stable carbon isotope signatures, they also provide reference data for the reconstruction of past water columns using biomarkers from the sedimentary record.…”

DNA Sequencing as a Tool to Monitor Marine Ecological Status

“…Increased exposure to anthropogenic chemicals promotes production and dissemination of these genes. Increases in the abundance of resistance genes and of integrons, genetic elements involved in horizontal gene transfer, are documented in polluted sediments (Thureborn et al, 2013). Resistance to multiple antibiotics is more frequent at polluted sites, and resistance to antibiotics and heavy metals can co-occur (Henriques et al, 2016).…”