Iron–Manganese Concretions Sustaining Microbial Life in the Baltic Sea: The Structure of the Bacterial Community and Enrichments in Metal-Oxidizing Conditions

Abstract

The abundant deposits of spherical iron-manganese concretions in the Gulf of Finland are colonized by bacteria in vast numbers. Communities on the surface and in the porous interior have formed two separate clusters, in accordance with their genetic differences. The overall bacterial community in the concretions was highly diverse, representing 12 phyla. Half of the bacteria were affiliated with the most common classes of Proteobacteria, while a third of the bacteria were unclassified. Cloned 16S rRNA-gene sequences of the concretion bacteria showed high scores for similarity to the sequences obtained from sea sediments, metal-rich environments, and ocean crust. The clone library of native concretions was not dominated by known Fe- and Mn-oxidizing species. Known Mn-oxidizing bacteria Sphingomonas, Pseudomonas, and Bacillus were enriched in experiments with Mn2+-containing liquid media, whereas Prosthecobacter (Verrucomicrobia) and Rheinheimera were enriched in semisolid media possibly better simulating the natural conditions in the concretions. In a corresponding experiment, the Fe2+-oxygen gradient favored the enrichment of Shewanella baltica and Thalassolituus oleivorans, which are known to reduce Fe and to degrade petroleum hydrocarbons, respectively. An individual spherical concretion forms a microcosm for a diverse microbial community having potential to oxidize Fe and Mn as shown in cultivation experiments. Therefore, bacteria may significantly affect the formation of the concretions in the Gulf of Finland.