From CO2 to high value-added products: Advances on carbon sequestration by Ralstonia eutropha H16

Due to the large-scale mining and utilization of fossil energy, human not only face the issue of energy shortage but also need to solve the problem of climate change caused by excessive CO2. In order to achieve the goal of green manufacturing and carbon neutrality, it is necessary to develop non-carbon renewable energy sources such as solar, wind, and tidal energy to replace fossil energy, and to capture, utilize, and store the emitted CO2 through artificial technology. Since biological carbon sequestration has the advantages of mild reaction conditions and environmental friendliness, it has become one of the most attractive and promising fields in the future. Among a wide variety of carbon-fixing organisms, Knallgas bacteria are a type of microorganisms that can directly use H-2 as an energy source to fix CO2. H-2 is a kind of clean energy and can be produced by water electrolysis driven by renewable energy. Ralstonia eutropha H16 (also known as Cupriavidus necator H16), as the most in-depth studied Knallgas bacteria, is one of the most effective microorganisms that convert H-2 into biomass. The main attraction of R. eutropha as a platform for microbial carbon sequestration lies in the following three aspects. First of all, in terms of growth conditions, R. eutropha grows faster than photosynthetic microorganisms such as cyanobacteria and will not be affected by factors such as light and weather during large-scale cultivation. Secondly, in terms of genetic manipulation, R. eutropha has genetic maneuverability and availability of genetic tools. Compared with other autotrophic microorganisms such as acetogens and methanogens, its genetic toolbox is more abundant and mature. Finally, in terms of product range, R. eutropha can naturally synthesize high-carbon density bioplastics, namely poly(3-hydroxybutyrate) (PHB), which is biodegradable and has physical properties comparable to petroleum-based thermoplastics. Under gas fermentation conditions, R. eutropha can produce 28 g/L PHB. Moreover, R. eutropha can be engineered to produce biofuels through metabolic pathway design, including alcohols, fatty acids and derivatives, alka (e)nes, and terpenes, as well as other products with economic value, such as acetoin, trehalose, lipochitooligosaccharides (plant growth enhancer), and so on. The output of some products can reach the g/L level under the laboratory-scale fermentation tank, such as isopropanol 3.5 g/L, butanol and 3-methyl-1-butanol similar to 1 g/L, and acetoin 3.9 g/L, which are expected to be industrially produced. It is fully demonstrated that R. eutropha is a promising cell factory that can use renewable energy and CO2 to produce high value-added products. In this review, we first introduce the metabolic characteristics of CO2 fixation and energy utilization of R. eutropha. Under autotrophic conditions, R. eutropha fixes CO2 through the Calvin-Benson-Bassham cycle and utilizes hydrogenase to catalyse H-2 oxidation to provide cells with reducing power and energy. Then, according to the pattern of energy utilization, we summarize the advances and representative works, including R. eutropha directly using H-2 through gas fermentation, indirectly using H-2 and formate through electrosynthesis, and indirectly using H-2 through photosynthesis. In the section of gas fermentation, we summarize the research progress and related strategies of production of natural products and synthetic products by R. eutropha. In the section of electrosynthesis, we introduce the research work of electrosynthesis by R. eutropha according to the electron carrier type (H-2 and formate) and compare the advantages and disadvantages of the two electron carriers. In the section of photosynthesis, we introduce the latest work and analyse the current main problems and solutions. Finally, we compare the pros and cons of the three patterns and prospect the research direction in the future.

Automated summary

The large-scale mining and utilization of fossil energy have led to energy shortages and climate change issues caused by excessive CO2 emissions. To address this, non-carbon renewable energy sources and biological carbon sequestration are being developed for green manufacturing and carbon neutrality. Ralstonia eutropha shows promise as a cell factory that can use renewable energy and CO2 to produce high value-added products.