Carbon dioxide fixation as a central redox cofactor recycling mechanism in bacteria

Abstract

The Calvin-Benson-Bassham cycle (Calvin cycle) catalyzes virtually all primary productivity on Earth and is the major sink for atmospheric CO 2 . A less appreciated function of CO 2 fixation is as an electronaccepting process. It is known that anoxygenic phototrophic bacteria require the Calvin cycle to accept electrons when growing with light as their sole energy source and organic substrates as their sole carbon source. However, it was unclear why and to what extent CO 2 fixation is required when the organic substrates are more oxidized than biomass. To address these questions we measured metabolic fluxes in the photosynthetic bacterium Rhodopseudomonas palustris grown with 13 C-labeled acetate. R. palustris metabolized 22% of acetate provided to CO 2 and then fixed 68% of this CO 2 into cell material using the Calvin cycle. This Calvin cycle flux enabled R. palustris to reoxidize nearly half of the reduced cofactors generated during conversion of acetate to biomass, revealing that CO 2 fixation plays a major role in cofactor recycling. When H 2 production via nitrogenase was used as an alternative cofactor recycling mechanism, a similar amount of CO 2 was released from acetate, but only 12% of it was reassimilated by the Calvin cycle. These results underscore that N 2 fixation and CO 2 fixation have electron-accepting roles separate from their better-known roles in ammonia production and biomass generation. Some nonphotosynthetic heterotrophic bacteria have Calvin cycle genes, and their potential to use CO 2 fixation to recycle reduced cofactors deserves closer scrutiny.Calvin cycle | hydrogen gas | metabolic flux analysis | nitrogenase | RhodopseudomonasA t least 170 y ago, it became widely recognized that CO 2 could serve as the sole carbon source for plants and certain bacteria (autotrophic growth). Using experimental results collected by a number of scientists in the late 1700s and early 1800s, the German chemist von Liebig successfully argued that plants obtain carbon from CO 2 gas rather than from carbon in the soil (the prevailing theory at the time) (1). At this time, photosynthetic bacteria were still considered to be plants and thus were also recognized to fix CO 2 for growth (2).Nearly a century later, it was proposed that CO 2 fixation also plays a role in maintaining redox balance in the group of anoxygenic phototrophs known as purple nonsulfur bacteria (PNSB). PNSB obtain energy from light and carbon from organic substrates to support growth under anaerobic conditions (photoheterotrophic growth). In 1933, Muller reported that CO 2 had to be supplied for PNSB bacteria to grow photoheterotrophically on butyrate, a substrate more reduced than biomass (Table 1) (3). Muller proposed a hypothesis that CO 2 is needed to accept excess electrons and allow butyrate to be oxidized to the redox state of biomass ( Fig. 1) (3). This hypothesis went untested for nearly 50 y. In 1977 Hillmer and Gest demonstrated that Rhodobacter capsulatus would grow on butyrate without CO 2 if growth conditions permitted N 2 ...

New insight into the photoheterotrophic growth of the isocytrate lyase-lacking purple bacterium Rhodospirillum rubrum on acetate

“…A rapid increase in the light intensity would trigger an increase in the photosynthetically produced proton gradient, which would, in turn, increase the rate of NAD + photoreduction to NADH through reverse NADH dehydrogenase activity (Golomysova et al, 2010;Herter et al, 1998;McEwan, 1994). Acetate is a more reduced carbon source than succinate (McKinlay & Harwood, 2010), and the NAD + /NADH ratio could be more difficult to maintain if the intensity of the light received is suddenly increased. In support of this hypothesis, the elemental composition of acetate-grown Rs.…”

“…rubrum S1H Photoheterotrophic growth of purple bacteria has been extensively reported to depend on the Calvin-BensonBassham (CBB) cycle playing the role of an 'electron sink' (Falcone & Tabita, 1991;McKinlay & Harwood, 2010;Wang et al, 2010). In our dataset, we also observed a decreased abundance of RuBisCO when acetate is used as sole carbon source (Rru_A2400; fold change, 0.52; P-value, 0.04; identified peptides, 54) as compared with succinate-containing cultures.…”

Apo-bacteriophytochromes modulate bacterial photosynthesis in response to low light

“…The region amplified from this vector included the ribosomal-binding site to the 3′ end of the coding sequence, and the primers used incorporated an EcoRI restriction site at the 5′ end and an XbaI restriction site at the 3′ end. This fragment was then digested with EcoRI and XbaI and ligated into the plasmid pBBPgdh (47). This construct was mobilized into WT R. palustris or R. palustris ΔHO by conjugation with E. coli S17-1.…”

A Rhodopseudomonas palustris nifA* mutant produces H2 from -containing vegetable wastes

“…palustris strains CGA009 and its derivative NifA* mutant strain CGA676 constructed as described [9]. The strain CGA009 is defective in uptake hydrogenase activity [13].…”

“…The strain CGA009 is defective in uptake hydrogenase activity [13]. CGA676 has constitutive nitrogenase activity allowing it to produce H 2 in the presence of NH þ 4 [9]. Rp.…”