Lars Peter Nielsen

Cable bacteria are filamentous sulphide-oxidising bacteria that transport electrons through conductive periplasmic fibres over centimetre-scale distances in redox-stratified sediments to respire oxygen. Here, we show that the freshwater cable bacterium Electronema aureum GS employs a versatile extracellular electron transfer (EET) system to respire insoluble electron acceptors under anoxic conditions, achieving growth rates comparable to aerobic respiration. Electrochemical and molecular analyses reveal that E. aureum GS performs both direct and riboflavin-mediated EET to electrodes, including at +600 mV vs. Ag/AgCl, an unusually high redox potential typically not accessed by electroactive bacteria. Two distinct cell-surface redox components were identified, which are metal-dependent, pH-sensitive, and heat-labile, consistent with outer-membrane-localised cytochromes. The redox shuttle riboflavin accumulated in bioelectrochemical systems with E. aureum GS, supporting mediated EET. These findings reveal a respiratory flexibility in E. aureum GS and highlight the role of EET in energy conservation in dynamic redox environments.

Cable bacteria are filamentous microbes that couple sulfide oxidation to oxygen reduction over centimeter distances via long-distance electron transport. While their activity creates characteristic biogeochemical gradients that shape sediment ecology, the study of cable bacteria has been constrained by the chemical and physical heterogeneity of the natural sediments they inhabit. To date, laboratory cultivation efforts have relied on these undefined environmental matrices. Here, we established a reproducible enrichment and cultivation platform using an artificial sediment matrix coupled with chemically defined media. This matrix successfully supported the growth of both freshwater and marine cable bacteria and enabled serial propagation over multiple transfers. Microsensor profiling confirmed that the incubations recapitulated hallmark geochemical signatures, including the sulfide, oxygen and pH gradients associated with electrogenic sulfur oxidation. Scanning electron microscopy confirmed the presence of cable bacteria, while 16S rRNA sequencing confirmed enrichment of the cable bacteria together with a stable co-enriched community that included taxa associated with sulfur and iron cycling as well as cellulose decomposition. This defined cultivation system eliminates the variability inherent to natural samples, providing a controlled platform for dissecting the physiology, genetics, and microbial interactions of cable bacteria.

Cable bacteria have previously been shown to conduct electrons over centimeters, unprecedented length scales in the realm of biology, through a network of conducting nanofibers. This is not only of fundamental interest from a biological point of view but also opens long-term perspectives for bioelectronics. Here we investigate the viability of cable bacteria to function as biological electrical wires for the transport of electronic information, through a systematic study of signal speed in direct and alternating signal transmission via current-voltage and impedance measurements. A new equivalent electrical circuit consisting of a network of interconnected parallel resistors and capacitive elements, resembling the fiber network architecture of cable bacteria filaments, is introduced and evaluated. From impedance and frequency response measurements in combination with modelling and simulations, it is found that limited deformation occurs for alternating electrical signals up to at least 5 MHz, in correspondence with the observed signal transmission response time down to 100 ns. These findings highlight that signals can be transported in cable bacteria at unseen speeds for biological materials, far beyond for instance the signal rate in neurons (i.e. frequencies in the order of 100 Hz and rise times in the order of ms). Finally, we created a biohybrid electronic circuit, with which we successfully transmitted for the first time a music signal (“Bad Guy” by Billie Eilish) through a cable bacterium. These results show that cable bacteria have the intrinsic potential to be used as biological electrical wires or interconnects in bioelectronics for high-speed electrical signal transport.

Cable bacteria are filamentous microbes that couple sulfide oxidation to oxygen reduction over centimeter distances via long-distance electron transport. While their activity creates characteristic biogeochemical gradients that shape sediment ecology, the study of cable bacteria has been constrained by the chemical and physical heterogeneity of the natural sediments they inhabit. To date, laboratory cultivation efforts have relied on these undefined environmental matrices. Here, we established a reproducible enrichment and cultivation platform using an artificial sediment matrix coupled with chemically defined media. This matrix successfully supported the growth of both freshwater and marine cable bacteria and enabled serial propagation over multiple transfers. Microsensor profiling confirmed that the incubations recapitulated hallmark geochemical signatures, including the sulfide, oxygen, and pH gradients associated with electrogenic sulfur oxidation. Scanning electron microscopy confirmed the presence of cable bacteria, while 16S rRNA sequencing confirmed enrichment of the cable bacteria together with a stable co-enriched community that included taxa associated with sulfur and iron cycling as well as cellulose decomposition. This defined cultivation system eliminates the variability inherent to natural samples, providing a controlled platform for dissecting the physiology, genetics, and microbial interactions of cable bacteria.

Microbial sulfur cycling is typically divided into an oxidative and a reductive branch, with microbes driving either sulfide oxidation or sulfate reduction distinguished by their genomic setup. Paradoxically, filamentous cable bacteria perform electrogenic sulfide oxidation but contain genes indicative of sulfate reduction, including the reductive type of dissimilatory sulfite reductase (DsrAB), whereas they apparently lack the canonical sulfur transferase DsrEFH essential for sulfur oxidation. AlphaFold2 structure prediction of conserved cable bacteria proteins with unknown functions identified a protein complex resembling canonical DsrEFH (hereafter termed DsrEFH type II). In vitro characterization of heterologously expressed DsrEFH type II confirmed its sulfur transferase function and, together with site-directed mutagenesis, verified that the conserved cysteine, Cys67, is the active sulfur transfer residue. Genes encoding the novel DsrEFH type II were found in 985 prokaryotic genomes. They typically co-occurred with genes for reductive DsrAB in microbes characterized as sulfate reducers or sulfur disproportionators. This study not only fills an important gap in the sulfide oxidation pathway of cable bacteria, but also suggests that a wide range of sulfate reducing bacteria may be more metabolically versatile than currently understood, representing a major shift in the perception of this globally significant physiological group of microorganisms.

Cable bacteria are filamentous sulphide-oxidisers performing long-distance electron transfer in redox-stratified sediments by transporting electrons over centimetre-scale distances to reduce oxygen. Here, we show that the freshwater cable bacterium Electronema aureum GS can respire insoluble electron acceptors under anoxic conditions via a versatile extracellular electron transfer (EET) system, supporting growth rates comparable to those under aerobic conditions. Using electrochemical and molecular biology analyses, we demonstrate that E. aureum GS engages in both direct and mediated electron transfer to electrodes, including at +600 mV vs. Ag/AgCl—an unusually high redox potential typically not accessed by electroactive bacteria. Two distinct cell-surface redox components were identified, which are metal-dependent, pH-sensitive, and heat-labile, consistent with outer-membrane-localised cytochromes. Moreover, the redox shuttle riboflavin accumulated extracellularly and enhanced current production in bioelectrochemical systems, indicating a role for soluble mediators in cable bacteria. Together, these findings reveal a previously unrecognised respiratory flexibility in E. aureum GS and highlight EET as a key alternative strategy for energy conservation in fluctuating redox environments.

Various morphotypes of large filamentous bacteria were isolated through micromanipulation from sulphidic sediment mats in the Bay of Concepción, central Chile. This study employed DNA amplification, whole‐genome sequencing and bioinformatics analyses to unveil the taxonomic and genomic features of previously unidentified bacteria. The results revealed several novel genera, families and species, including three specimens belonging to Beggiatoales (Beggiatoaceae family), five to Desulfobacterales (Desulfobacteraceae family), two to the Chloroflexi phylum and one to the phylum Firmicutes. Metabolically, Beggiatoaceae bacteria exhibit a flexible and versatile genomic repertoire, enabling them to adapt to variable conditions at the sediment–water interface. All the bacteria demonstrated a mixotrophic mode, gaining energy from both inorganic and organic carbon sources. Except for the Firmicutes bacterium, all others displayed the ability to grow chemolithoautotrophically using H ₂ and CO ₂ . Remarkably, the reverse tricarboxylic acid (rTCA) and Calvin–Benson–Bassham (CBB) pathways coexisted in one Beggiatoaceae bacterium. Additionally, various defence systems, such as CRISPR‐Cas, along with evidence of viral interactions, have been identified. These defence mechanisms suggest that large filamentous bacteria inhabiting sulphidic sediments frequently encounter bacteriophages. Thus, robust defence mechanisms coupled with multicellularity may determine the survival or death of these large bacteria.

Microbial perception of spatial electromagnetic fields is essential for navigation and communication on Earth's surface system, but current understanding of this phenomenon is limited. At present, cable bacteria of the Desulfobulbaceae family have the longest known range of electron transport. In fact, the flow of electrons along these long filamentous bacteria generates an external electrostatic field, suggesting a potential for electromagnetic induction mirroring that of metallic wires. In this study, we measured the responses of cable bacteria to externally applied electric waves. We noted the formation and disappearance of square waves caused by a pair of spatially variable electric fields, generating negative and positive mirror-symmetric inductions (±1.20 mV in marine sediment) along the horizontally filamentous bacterial layer. Both seawater Candidatus Electrothrix and freshwater Ca. Electronema exhibited this electric induction. The distinct spatial boundary of bacterial induction was strictly confined within 12.5 mm below the surface of the seawater sediment. The results of this study open further avenues of research into understanding how bacteria sense and respond to spatial electromagnetic information.

Bacterial cells can vary greatly in size, from a few hundred nanometers to hundreds of micrometers in diameter. Filamentous cable bacteria also display substantial size differences, with filament diameters ranging from 0.4 to 8 µm. We analyzed the genomes of cable bacterium filaments from 11 coastal environments of which the resulting 23 new genomes represent 10 novel species-level clades of Candidatus Electrothrix and two clades that putatively represent novel genus-level diversity. Fluorescence in situ hybridization with a species-level probe showed that large-sized cable bacteria belong to a novel species with the proposed name Ca . Electrothrix gigas. Comparative genome analysis suggests genes that play a role in the construction or functioning of large cable bacteria cells: the genomes of Ca . Electrothrix gigas encode a novel actin-like protein as well as a species-specific gene cluster encoding four putative pilin proteins and a putative type II secretion platform protein, which are not present in other cable bacteria. The novel actin-like protein was also found in a number of other giant bacteria, suggesting there could be a genetic basis for large cell size. This actin-like protein (denoted big bacteria protein, Bbp) may have a function analogous to other actin proteins in cell structure or intracellular transport. We contend that Bbp may help overcome the challenges of diffusion limitation and/or morphological complexity presented by the large cells of Ca . Electrothrix gigas and other giant bacteria. IMPORTANCE: In this study, we substantially expand the known diversity of marine cable bacteria and describe cable bacteria with a large diameter as a novel species with the proposed name Candidatus Electrothrix gigas. In the genomes of this species, we identified a gene that encodes a novel actin-like protein [denoted big bacteria protein (Bbp)]. The bbp gene was also found in a number of other giant bacteria, predominantly affiliated to Desulfobacterota and Gammaproteobacteria, indicating that there may be a genetic basis for large cell size. Thus far, mostly structural adaptations of giant bacteria, vacuoles, and other inclusions or organelles have been observed, which are employed to overcome nutrient diffusion limitation in their environment. In analogy to other actin proteins, Bbp could fulfill a structural role in the cell or potentially facilitate intracellular transport.