Carbon roadmap from syngas to polyhydroxyalkanoates in Rhodospirillum rubrum

Abstract

The gasification of organic waste materials to synthesis gas (syngas), followed by microbial fermentation, provides a significant resource for generating bioproducts such as polyhydroxyalkanoates (PHA). The anaerobic photosynthetic bacterium, Rhodospirillum rubrum, is an organism particularly attractive for the bioconversion of syngas into PHAs. In this study, a quantitative physiological analysis of R. rubrum was carried out by implementing GC-MS and HPLC techniques to unravel the metabolic pathway operating during syngas fermentation that leads to PHA production. Further, detailed investigations of the central carbon metabolites using (13) C-labelled substrate showed significant CO2 assimilation (of 40%) into cell material and PHA from syngas carbon fraction. By a combination of quantitative gene expression and enzyme activity analyses, the main role of carboxylases from the central carbon metabolism in CO2 assimilation was shown, where the Calvin-Benson-Bassham cycle (CBB) played a minor role. This knowledge sheds light about the biochemical pathways that contribute to synthesis of PHA during syngas fermentation being valuable information to further optimize the fermentation process.

Synthesis Gas (Syngas)-Derived Medium-Chain-Length Polyhydroxyalkanoate Synthesis in Engineered Rhodospirillum rubrum

“…This indicated that CO 2 present in syngas can be more readily converted into acetyl-CoA, which is then subjected to fatty acid synthesis, than can CO, which has to be oxidized by the CO dehydrogenase in R. rubrum to CO 2 at first (42). Recently, Revelles et al (16) showed an upregulation of genes coding for additional carboxylases, other than RubisCO, which play a major role in the fixation of CO 2 in R. rubrum when acetate is present in the cultivation medium but not with malate. Therefore, the use of another citric acid cycle intermediate, such as succinate, as an auxiliary carbon source should likewise not lead to an upregulation of the acetate assimilation route ethylmalonyl-CoA pathway or acetylCoA carboxylation via PFOR (Fig.…”

“…The purple nonsulfur alphaproteobacterium Rhodospirillum rubrum is able to utilize CO and CO 2 and has been a host and model organism for several studies focusing on the synthesis of PHA SCL from various feedstocks (14)(15)(16). Applying the water-gas shift reaction, R. rubrum oxidizes CO with H 2 O to CO 2 and H 2 , employing a carbon monoxide dehydrogenase (CODH), thereby gaining energy and carbon that is channeled into the Calvin cycle via ribulose 1,5 bisphosphate carboxylase (17).…”

“…Applying the water-gas shift reaction, R. rubrum oxidizes CO with H 2 O to CO 2 and H 2 , employing a carbon monoxide dehydrogenase (CODH), thereby gaining energy and carbon that is channeled into the Calvin cycle via ribulose 1,5 bisphosphate carboxylase (17). Moreover, recent investigations have shown that other carboxylases, which are specifically active when acetate assimilation is required, play a major role in CO 2 fixation (16). These include carboxylases of the ethylmalonyl-CoA pathway and a ferredoxin-dependent pyruvate synthase (PFOR).…”

“…Synthesis of poly(3-hydroxybutyrate) [poly(3HB)] is deactivated by the deletion of phaC1 R. rubrum and phaC2 R. rubrum , which code for PHA SCL synthases (6). Additional carboxylases, which might be also involved in assimilating additional CO 2 (16), include a pyruvate synthase (Rru_A2398 [7]), a crotonyl-CoA reductase (Rru_A3063 [8]), a propionylCoA carboxylase (Rru_A1943 [9]), and a 2-oxoglutarate synthase (Rru_A2721 [10]). HS-CoA, coenzyme A thiol; HS-ACP, acyl carrier protein thiol.…”

About how to capture and exploit the CO2 surplus that nature, per se, is not capable of fixing

“…By selecting the appropriate microorganism, a wide range of valuable products can be synthesized, such as PHAs, isoprene, lactic acid and methane (www.celbicon.org). For example, Rhodospirillum rubrum, a purple non-sulfur bacterium, can produce PHAs from CO as carbon and energy source (Revelles et al, 2016). This feature and its metabolical versatility make this species interesting as biological tool for CO 2 fixation.…”