The life cycle of cable bacteria

Cable bacteria are multicellular filamentous microorganisms that perform electrogenic sulphur oxidation, coupling the oxidation of sulphide in deeper sediments to oxygen reduction at the sediment-water interface via long-distance electron transport. Despite reshaping our understanding of sedimentary microbial processes, key aspects of cable bacteria biology remain unresolved, particularly regarding their life cycle, dispersal mechanisms, and ecological interactions. This thesis aims to address these gaps by investigating the growth, dispersal, and decline of cable bacteria populations in natural sediment environments. We demonstrate that cable bacteria can disperse via small filament fragments transported through the oxygenated water column, despite oxygen previously being shown to inhibit their electron transport capabilities. This dispersal is facilitated by sediment particles that offer partial protection, enabling colonisation of new sediment patches. Following colonisation, cable bacteria populations exhibit characteristic boom-and-bust dynamics, with a rapid expansion of filaments and high rates of electron transport, followed by stagnation and gradual decline. Interestingly, multiple cable bacteria strains can coexist through desynchronised growth cycles, although these dynamics exert limited influence on the broader microbial community, except in the oxic zone where cable bacteria suppress single-cell sulphur oxidisers. Manipulative experiments revealed that severing filaments disrupts connectivity to oxygen, leading to increased activity and possible upward migration of disconnected cable bacteria. This highlights the importance of uninterrupted electron pathways for maintaining population integrity. Additionally, the study explored the occurrence of “ghost cells” and found that their presence does not impair filament motility or electron transport. Microscopy further revealed signs of predatory interactions, including viral attachments, bacterial invasions, and ciliates feeding on cable bacteria, suggesting previously underappreciated ecological pressures. Finally, we isolated and characterised a novel cable bacterium strain, YB6, expanding the known diversity of the genus Ca. Electrothrix and underscoring the ecological and evolutionary complexity of this group. Overall, this research provides critical insights into the life cycle of cable bacteria, from dispersal and colonisation to population collapse. It reveals how these organisms, despite their ecological dominance, remain subject to environmental constraints and microbial interactions, particularly in the oxic zone where cells serve as sacrificial electron sinks. These findings contribute to a deeper understanding of the ecological strategies and resilience of cable bacteria in sedimentary environments.