It does not always take two to tango: “Syntrophy” via hydrogen cycling in one bacterial cell

Abstract

Interspecies hydrogen transfer in anoxic ecosystems is essential for the complete microbial breakdown of organic matter to methane. Acetogenic bacteria are key players in anaerobic food webs and have been considered as prime candidates for hydrogen cycling. We have tested this hypothesis by mutational analysis of the hydrogenase in the model acetogen Acetobacterium woodii. Hydrogenase-deletion mutants no longer grew on H 2 + CO 2 or organic substrates such as fructose, lactate, or ethanol. Heterotrophic growth could be restored by addition of molecular hydrogen to the culture, indicating that hydrogen is an intermediate in heterotrophic growth. Indeed, hydrogen production from fructose was detected in a stirredtank reactor. The mutant grew well on organic substrates plus caffeate, an alternative electron acceptor that does not require molecular hydrogen but NADH as reductant. These data are consistent with the notion that molecular hydrogen is produced from organic substrates and then used as reductant for CO 2 reduction. Surprisingly, hydrogen cycling in A. woodii is different from the known modes of interspecies or intraspecies hydrogen cycling. Our data are consistent with a novel type of hydrogen cycling that connects an oxidative and reductive metabolic module in one bacterial cell, "intracellular syntrophy."

Homoacetogenic Conversion of Mannitol by the Thermophilic Acetogenic Bacterium Thermoanaerobacter kivui Requires External CO2

“…The essential principle here is the involvement an electron-bifurcating hydrogenase operating reverse (confurcating) direction, and concomitantly oxidizing NADH and Fd red , as originally described in the thermophilic fermentative bacterium T. maritima ( Schut and Adams, 2009 ). In acetogens, the intermediate accumulation of only small concentrations of H 2 during sugar metabolism has been demonstrated, for the thermophile M. thermoacetica ( Kellum and Drake, 1984 ), and more recently in the mesophile A. woodii ( Wiechmann et al, 2020 ). In the latter, the electron-bifurcating hydrogenase has been shown to be involved in a variation of H 2 cycling (see below), which had also been proposed for M. thermoacetica ( Wang et al, 2013 ).…”

Towards continuous industrial bioprocessing with solventogenic and acetogenic clostridia: challenges, progress and perspectives

“…The generated chemiosmotic gradient can be used for energy generation by a membrane-bound ATPase. Electron bifurcating hydrogenases like HydABCD are essential for the supply of reduced ferredoxin during growth on mixtures of H 2 and CO 2 and may also serve for redox balancing during heterotrophic growth [ 258 , 316 ]. During acidogenic growth, solventogenic clostridia like C. acetobutylicum balance surplus NADH by forming H 2 [ 218 ].…”

Synthetic Biology on Acetogenic Bacteria for Highly Efficient Conversion of C1 Gases to Biochemicals

“…Nevertheless, plasmid-based gene expression methods developed for Clostridium species have been shown to be functional in other acetogens. In addition, many metabolic engineering efforts have recently been made using homologous recombination (HR) [ 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ], ClosTron [ 28 , 36 , 37 , 38 , 39 ] and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) along with its CRISPR-associated (Cas) protein (CRISPR-Cas) system [ 32 , 40 , 41 , 42 , 43 , 44 , 45 ] to improve the production of value-added biochemicals from C1 gases.…”

“…Consequently, double-crossover mutants can be readily isolated in uracil-free medium as they restore uracil protrophy [ 74 ]. This approach has been employed in A. woodii [ 25 , 26 , 27 ], C. autoethanogenum [ 28 ], Moorella thermoacetica [ 33 ], and Thermoanaerobacter kivui [ 34 , 35 ]. For example, a uracil auxotrophic mutant, ∆ pyrE , was first generated in A. woodii to successfully delete rnf genes [ 25 ].…”