Microbial Community and Functional Capacity Stratification from Surface to Sediment at the Deepest point of the Baltic Sea. A metagenomic study

Abstract 

From hyposaline surface waters to hypoxic and anoxic deep waters and sediments, the Baltic Sea has distinct and significant environmental features. These conditions result in a particular steep oxygen gradient at Landsort Deep in the Baltic Proper.

We conducted a metagenomic census from the surface to the sediment to investigate if the microbial communities are as stratified as the physical environment.

Functional capacity and taxonomic affiliation show considerable stratification throughout a depth transect, with functional capacity correlating most closely to critical environmental characteristics such as oxygen, salinity, and temperature. We show functional capacity similarities between hypoxic and hadal zone communities, highlighting the Baltic's significant degree of eutrophication. Despite considerable ammonium levels below the chemocline, the reconstructed nitrogen cycle at Landsort deep demonstrates potential for syntropy between archaeal ammonium oxidizers and bacterial denitrification at anoxic depths, but anaerobic ammonium oxidation genes are lacking. In addition, our census demonstrates an enrichment in genetic prerequisites for a copiotrophic lifestyle, as well as resistance mechanisms showing adaptation to widespread eutrophic conditions and the accumulation of environmental contaminants caused by ongoing anthropogenic pressures in the Baltic Sea.

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