Some bacteria produce cellular energy using an ancient metabolic strategy that couples the oxidation and the reduction of various substrates (including metals and metalloids) like in an electrical circuit. This process is called anaerobic respiration. It enables bacteria and archaea to thrive in hostile environments poor in organics and characterized by extreme conditions (low and high pH, high salinity, no/low oxygen, etc.). Various biominerals with fundamental and applied significance are produced intra- and extracellularly during this process. The study “Microbial respiration – A biomineral perspective” in FEMS Microbiology Ecology explores the anaerobic respiration process linked to microbial biomineralization.
How bacteria generate energy using metall(oid)s
Microbial respiration is an electrochemical process in which microbes oxidize organic and inorganic molecules – i.e. electron donors, while reducing organic or inorganic molecules – i.e. electron acceptors. Numerous electron donors have been identified such as lactate, acetate, H2, etc.
While oxygen serves as the primary electron acceptor in aerobic respiration, in the absence of oxygen, microbial respiration relies on alternative electron acceptors. These include fumarate, nitrate (NO3⁻), sulfate (SO42-), arsenate (AsO43-), selenate (SeO42-), selenite (SeO32-) and metal ions (e.g. Fe3+), among others. The flow of electrons released by the electron donor is routed to the electron acceptor through a series of protein complexes (i.e. the electron transport chain – ETC) located in the microbial plasma membrane.
This flow generates a proton gradient, which is used by ATP synthase (a molecular machine that acts as an enzyme) to generate adenosine triphosphate (ATP), the main energy currency of cells.
Microbial biominerals
Microbes produce intra- and extracellular biominerals in a genetically controlled (biologically controlled mineralization – BCM) or uncontrolled (biologically induced mineralization – BIM) manner. BCM implies a set of genes involved in the biomineralization, growth, and localization of the biomineral, which is usually intracellular. BIM occurs as a result of the metabolic activity of the microbial cells, which induces local chemical changes (e.g. pH rise), with the minerals precipitating extracellularly. A typical example of microbial BCM is the formation of intracellular crystals of magnetite (Fe3O4) and greigite (Fe3S4) by magnetotactic bacteria. They use these minerals to navigate oxygen gradients in the Earth’s magnetic field. BIM has been described for the production of microbial carbonates or iron oxides.
Arsenic sulfides (AsS) and elemental selenium (Se0) result from extracellular respiratory processes (Fig. 1AB-DEF) in a strain of Shwanella (Shewanella sp. O23S). Thus, the solid biominerals are disposed outside of the bacterial cell, protecting its internal structure. Biogenic sulfur (S0) is produced by the oxidation of H2S, the by-product of the sulfate respiration, being used as an electron donor. Iron minerals have a complex chemistry and are metastable (they transform from one mineral phase to another) (biomienrals produced by the sulfur-disproportionating bacterium Desulfocapsa sulfexigens) (Fig. 1C).
The biological functions of these biominerals are not always known, and more studies are needed to uncover the biomolecular pathways involved in controlling their formation.
Figure 1. Microbial biominerals. A) Extracellular Se0 produced by Shewanella sp. O23S (STEM mode); B) Elemental mapping (S – yellow-green; Se – red); C) Biogenic pyrite (blue) formed in the presence of goethite (orange) during incubation with the sulfur-disproportionating bacterium Desulfocapsa sulfexigens. Tübingen Structural Microscopy Core Facility (Muammar Mansor and Andreas Kappler). Both minerals were pseudocolorized; D) Extracellular AsS produced by Shewanella sp. O23S (STEM mode); E) Elemental mapping (As – blue); F) Elemental mapping (S – yellow). Panels A-B-D-E-F are from Staicu et al. (2025).
Applications of microbial biominerals
Because biominerals form from soluble metals, there is an increasing interest in their potential recovery from metal-rich industrial wastewaters (i.e. biomining). For instance, iron biominerals have been investigated in energy/electron storage (biogeobatteries). On the other hand, S0 biominerals are interesting candidates for use as cathode materials in high-capacity Li-S batteries. In contrast to the natural abundance of iron and sulfur, selenium is a trace element in the Earth’s crust and has numerous applications (e.g. energy production, micronutrient). The recovery of Se0 biominerals from selenium-laden industrial effluents using microbial metabolism is an important goal within the framework of circular economy, and various studies are being focused on this topic.
Read the article “Microbial respiration – A biomineral perspective” by Staicu et al. in FEMS Microbiology Ecology (2025). Authors: Lucian Staicu, Julie Cosmidis, Muammar Mansor, Catarina Paquete, Andreas Kappler.
Geomicrobiology 