Genomic insights into syntrophy: The paradigm for anaerobic metabolic cooperation

Syntrophy is a tightly coupled mutualistic interaction between hydrogen-/formate-producing and hydrogen-/formate-using microorganisms that occurs throughout the microbial world. Syntrophy is essential for global carbon cycling, waste decomposition, and biofuel production. Reverse electron transfer, e.g., the input of energy to drive critical redox reactions, is a defining feature of syntrophy. Genomic analyses indicate multiple systems for reverse electron transfer, including ion-translocating ferredoxin:NAD⁺ oxidoreductase and hydrogenases, two types of electron transfer flavoprotein:quinone oxidoreductases, and other quinone reactive complexes. Confurcating hydrogenases that couple the favorable production of hydrogen from reduced ferredoxin with the unfavorable production of hydrogen from NADH are present in almost all syntrophic metabolizers, implicating their critical role in syntrophy. Transcriptomic analysis shows upregulation of many genes without assigned functions in the syntrophic lifestyle. High-throughput technologies provide insight into the mechanisms used to establish and maintain syntrophic consortia and conserve energy from reactions that operate close to thermodynamic equilibrium.