Genetic Evidence for Two Carbon Fixation Pathways (the Calvin-Benson-Bassham Cycle and the Reverse Tricarboxylic Acid Cycle) in Symbiotic and Free-Living Bacteria

Abstract

Primary production on Earth is dependent on autotrophic carbon fixation, which leads to the incorporation of carbon dioxide into biomass. Multiple metabolic pathways have been described for autotrophic carbon fixation, but most autotrophic organisms were assumed to have the genes for only one of these pathways. Our finding of a cultivable bacterium with two carbon fixation pathways in its genome, the rTCA and the CBB cycle, opens the possibility to study the potential benefits of having these two pathways and the interplay between them. Additionally, this will allow the investigation of the unusual and potentially very efficient mechanism of electron flow that could drive the rTCA cycle in these autotrophs. Such studies will deepen our understanding of carbon fixation pathways and could provide new avenues for optimizing carbon fixation in biotechnological applications.

Identification of a Novel Fumarate Reductase Potentially Involved in Electron Bifurcation

“…Furthermore, the known NADH-dependent FRDs are not involved in central energy metabolism, while P. thermosuccinogenes, a strictly anaerobic fermentative organism and likely to be constrained by its ability to generate metabolic energy, produces succinic acid as one of its main products of fermentation. It seems therefore credible that it can couple succinic acid production to additional energy conservation, in the form of reduced Something very similar was proposed recently for the reverse TCA-cycle of sulfur-oxidizing bacteria found in symbiosis with tubeworms (24). In those bacteria, the HdrABC-FlxABCD complex colocalizes on the genome with a FRD (homologous to the methanogenic thiol-driven FRD, TfrAB) as well as a 2oxoglutarate:ferredoxin oxidoreductase (KorABCD).…”

Photo-driven CO2 Reduction to Hydrocarbons: Carbon Flux and Energy Transforming by Marine Microbial Community

“…Particularly, gamaproteobacteria with obviously higher percentage (39.88% in S1, 85.91% in S2, 93.56% in S3, and 96.35% in S4) was increased along with culturing, which have been extensively reported that most of this subdivision are capable of photoautotrophic pathway or chemolithoheterotrophic pathway for CO 2 xation [21][22][23][24]. Cyanobacteria, Ignavibacteriae, Chlorobi, and Bacteroidetes were only detected in sample S1, which have been well-elaborated the capacity of CO 2 xation, nitrogen xation, and hydrogen production via photoautotrophy or chemolithoautotrophy, yet the phylum (like Ignavibacteriae, Chlorobi, etc) were sensitive to oxygen [22][23][24][25][26]. Oxygen, possibly generated from the oxygenic photoautotroph like Cyanobacteria, was detected (data shown in Fig.…”

“…S6). Key genes of these completed pathways, namely RubisCO for CBB cycle [23,[27][28][29], ATP citrate lyase for rTCA cycle [22,23,25] and CO dehydrogenase/acetyl-CoA synthase for WL pathway [23,30] were detected and showed the signi cant abundance in all samples ( Fig. 4a), demonstrating that the microbial communities embodied the ability of autotrophic carbon xation.…”

“…Chlorobium sp. (bin 8), as anaerobic phototrophs xing CO 2 via a rTCA cycle [25], was detected with increasing abundance in the enrichment starting from 17 th day, however showed the negligible percentage at the beginning of culturing. The archaeon Halobacterium (bin 13) was detected which can capture light energy to assimilate CO 2 [37].…”

Horizontal acquisition of a patchwork Calvin cycle by symbiotic and free-living Campylobacterota (formerly Epsilonproteobacteria)

“…These bacteria could be using both cycles depending on the environmental setting, as suggested for the sulfide-oxidizing gammaproteobacterial symbionts of vestimentiferan tubeworms 46–48 found at hydrothermal vents, the large sulfur bacteria Beggiatoa and Thiomargarita spp. 49–51 , and recently the cultivable sulfur oxidizer Thioflavicoccus mobilis 52 . The tubeworm symbiont Ca.…”