Beyond magnetosomes: ubiquitous and diverse intracellular inclusions expand the role of magnetotactic bacteria in biogeochemical cycling

Magnetotactic bacteria (MTB) are capable of accumulating and storing intercellular pools containing phosphorus, sulfur, nitrogen and carbon. Yet, the metabolic pathways connected with their intracellular storage of polyphosphate, elemental sulfur, nitrate, and calcium carbonate, and the environmental influence on MTB inclusions and their quantitative contribution to sedimentary chemical budgets remain underexplored. Using a combination of chemical characterization, ultrastructural and compositional analyses, and phylogenetic and genomic insights into microbial assemblages, we investigated the influence of geochemical parameters on the diversity, ecophysiology, and distribution of MTB in freshwater and brackish sediments. Special focus is given to intracellular inclusions and their metabolic pathways to uncover functional traits and ecological roles in elemental cycling. Of the 118 examined MTB cells, polyphosphate granules (75%) and nitrate-containing vacuoles (67%) were the most common inclusions followed by sulfur globules (25%) and calcium carbonate granules (8%). The intracellular phosphorus, nitrogen, and sulfur stored in MTB cells were conservatively estimated to account for 0.60%, 0.91%, and 0.23% of the total sedimentary P, N, and S in the investigated freshwater sediments, respectively. The ubiquitous nature and important ecological role of MTB can be explained by their ability to sequester chemical elements into intracellular inclusions, giving them a metabolic advantage in dynamic chemically stratified environments. The extraordinary phylogenetic diversity of MTB, coupled with their capability to hyperaccumulate and store a wide range of elements and compounds, represents a significant resource for biotechnology innovation.