Diversity, biomineralization, and metabolic characteristics of neutrophilic microaerophilic iron-oxidizing bacteria in seafloor hydrothermal environments

Abstract:

Iron is one of the main components of deep-sea hydrothermal activity products and an important driving element for the chemoautotrophic microbial ecosystems at hydrothermal vents. The neutrophilic microaerophilic iron-oxidizing bacteria, represented by Zetaproteobacteria, are the main drivers of biomediated Fe²⁺ oxidation in hydrothermal vents and their surroundings. Iron-oxidizing bacteria acquires the energy essential to maintain their metabolism through Fe²⁺ oxidation, while secrete organic matter to precipitate the oxidized insoluble iron (oxides or hydroxides) outside the cells, forming microstructures with twisted stalks, hollow sheaths, branching hollow tubes or other special morphological features. These microstructures accumulate into iron-rich oxides/hydroxides widely distributed on the seafloor. Increasing studies have demonstrated that cyc2, encoding cytochrome-porin, is the key gene of Fe²⁺ oxidation by Zetaproteobacteria, while c-type cytochromes or other periplasmic cytochromes are the main electron transport carriers in Fe²⁺ oxidation. The metagenome-based studies reveal that Zetaproteobacteria generally possess multiple functional genes and metabolic pathways associated with nitrogen, sulfur, hydrogen, and arsenic cycling, suggesting the potential role of Zetaproteobacteria in the cycling of the above elements. In this paper, we systematically summarized the diversity, physiological characteristics, biomineralization-formed microstructure records, and key genes and electron transport pathways that mediated Fe²⁺ oxidation of neutrophilic microaerophilic iron-oxidizing bacteria in hydrothermal environments. This review facilitates the systematical understanding about the role of these microorganisms in the migration and enrichment of key ore-forming elements, the maintenance of ecological balance, and the mineralization by microorganisms in submarine hydrothermal vents.