On the evolution and physiology of cable bacteria

Kasper U Kjeldsen, Lars Schreiber, Casper A Thorup, Thomas Boesen, Jesper T Bjerg, Tingting Yang, Morten S Dueholm, Steffen Larsen, Nils Risgaard-Petersen, Marta Nierychlo, Markus Schmid, Andreas Bøggild, Jack van de Vossenberg, Jeanine S Geelhoed, Filip J R Meysman, Michael Wagner, Per H Nielsen, Lars Peter Nielsen, Andreas Schramm

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaperJournal articleResearchpeer-review

Abstract

Cable bacteria of the family Desulfobulbaceae form centimeter-long filaments comprising thousands of cells. They occur worldwide in the surface of aquatic sediments, where they connect sulfide oxidation with oxygen or nitrate reduction via long-distance electron transport. In the absence of pure cultures, we used single-filament genomics and metagenomics to retrieve draft genomes of 3 marine Candidatus Electrothrix and 1 freshwater Ca. Electronema species. These genomes contain >50% unknown genes but still share their core genomic makeup with sulfate-reducing and sulfur-disproportionating Desulfobulbaceae, with few core genes lost and 212 unique genes (from 197 gene families) conserved among cable bacteria. Last common ancestor analysis indicates gene divergence and lateral gene transfer as equally important origins of these unique genes. With support from metaproteomics of a Ca. Electronema enrichment, the genomes suggest that cable bacteria oxidize sulfide by reversing the canonical sulfate reduction pathway and fix CO2 using the Wood-Ljungdahl pathway. Cable bacteria show limited organotrophic potential, may assimilate smaller organic acids and alcohols, fix N2, and synthesize polyphosphates and polyglucose as storage compounds; several of these traits were confirmed by cell-level experimental analyses. We propose a model for electron flow from sulfide to oxygen that involves periplasmic cytochromes, yet-unidentified conductive periplasmic fibers, and periplasmic oxygen reduction. This model proposes that an active cable bacterium gains energy in the anodic, sulfide-oxidizing cells, whereas cells in the oxic zone flare off electrons through intense cathodic oxygen respiration without energy conservation; this peculiar form of multicellularity seems unparalleled in the microbial world.

Original languageEnglish
JournalProceedings of the National Academy of Sciences
Volume116
Issue38
Pages (from-to)19116-19125
Number of pages10
ISSN0027-8424
DOIs
Publication statusPublished - 17 Sept 2019

Keywords

  • Cable bacteria
  • Electromicrobiology
  • Microbial evolution
  • Microbial genome
  • Microbial physiology

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