Detection of Prokaryotes Involved in Formation of Fast-Growing Ferromanganese Nodules in the Baltic Sea by Culture-Based and Molecular Approaches

Abstract— Ferromanganese nodules (FeMnNs), polymetallic ore deposits widespread in the Global Ocean, are important raw materials for metallurgy. Their largest aggregations are associated with deep-water depressions in the Pacific and Indian oceans, where the rate of nodule growth is several mm per 106 years. Contrastingly, FeMnN formation and growth at the sea shelf occurs three orders of magnitude quicker, and the estimated rate for the fastest-growing Baltic Sea nodules is several mm per 100–200 years; they may therefore be considered a renewable ore resource. The biochemogenic concept of nodule formation developed by the world scientific community implies active involvement of marine microorganisms in FeMnN formation via redox transformations of the iron and manganese minerals, which is especially important in the case of fast-growing shallow-water nodules associated with organic-rich benthic ecosystems. Isolation and characterization of the microorganisms involved in the FeMnN ore genesis is a complex task. We present a description of obtaining psychrophilic primary enrichment cultures from the Baltic FeMnN samples which have been developing for several years with oxidized and reduced manganese minerals as electron acceptors and donors, respectively. The work describes phylogenetic profiles, obtained by 16S rRNA gene amplicon sequencing, for the original FeMnNs, their changes during long-term nodule storage, and the profiles of enrichment cultures, in which we revealed taxonomic groups enriched under the conditions selective for manganese minerals transformation. Bioinformatic screening of genomes of these groups representatives for the ability to carry out extracellular electron transfer revealed the key phylotypes which may determine the processes of FeMnNs formation. Our results provide highlights for the isolation and characterization of pure cultures of the microorganisms affecting FeMnN growth. This, in turn, may open the way to directed, controlled synthesis of these ore formations.