Cupriavidus necator grows on CO₂ as carbon source and it simultaneously needs hydrogen as energy source as well as oxygen as electron acceptor in the gas mixture, with strain H16 being the most studied one. Carbon dioxide is assimilated through the Calvin-Benson-Bassham (CBB) pathway (Figure 1). Different optimal H₂/O₂/CO₂ ratios have been reported, but the best values are often claimed to be around 7/2/1 (Amer and Kim, 2023). Anaerobic respiration with reduction of nitrate and full denitrification down to molecular nitrogen (N₂) is also possible. Additionally, this bacterium can also metabolise other substrates besides CO₂ and heterotrophic growth has been observed with different carbon sources such as fructose (Figure 1). Glucose is not metabolised by native strains of that species though. This organism has been renamed after being originally known as Alcaligenes eutrophus, Hydrogenomonas eutropha and Ralstonia eutropha. Because of the potentially explosive aerobic gas mixture, it is also known as knallgas bacterium. Cupriavidus necator has mainly been studied for its ability to accumulate Poly-Hydroxy-Alkanoates (PHA), i.e., biopolymers. In terms of biopolymer content, percentages as high as around 80% have been reached in that species in media with nutrient (e.g., nitrogen) limitation (Lambaue et al., 2023). Besides numerous studies with native strains, the organism has more recently been genetically modified and has also been used in microbial electrosynthesis (MES) systems in order to try to broaden its product spectrum. In that sense, a wide range of products have now recently been reported to be obtainable from C. necator cultivation, which include acetoin, 2,3-butanediol, isopropanol, n-butanol, iso-butanol, 3-methyl-1-butanol, fatty acids and derivatives, isoprene and terpenes, lycopene, methyl-ketones, alkanes, alkenes, α-humulene, 2-hydroxy isobutyric acid, trehalose, sucrose, lipochitooligosaccharides, among others (Pan et al., 2021). The potential application of such bacteria in full-scale processes is, however, still considered to have several limitations such as the reported low yields, high cost, or significant explosion issues, among others.

Microalgae are able to assimilate carbon dioxide through photosynthesis and use it as carbon source. Under phototrophic conditions, they would only need CO₂ and light as carbon and energy sources, respectively. Interestingly, photosynthetic effciency is often higher than what is commonly observed in plants. Microalgae biomass contains about 50% carbon and some studies claim that around 1.83 kg CO₂ can be fixed for every kilogram of biomass produced (Chisti, 2007). They allow to valorise carbon dioxide through its conversion into high value products. Some of first reports focussed mainly on their potential to accumulate lipids, which is then suitable to produce biofuels such as biodiesel and for which one of the most studied organisms belongs to Chlorella spp. (e.g., Chlorella vulgaris). In terms of biofuels, some microalgae are also rich in carbohydrates, though their levels are high in only some specific species, and it then represents an interesting feedstock for third generation bioethanol production after biomass harvesting, pretreatment/hydrolysis and subsequent sugar fermentation. Biogas is another interesting biofuel. In that sense, macroalgae (seaweed) are suitable for biogas production through anaerobic digestion of macroalgae feedstock. The same is possible with microalgae characterised by a high biomass productivity together with low ash content.

Additionally, the product spectrum of microalgae has been broadened over the years and they are now studied for their accumulation of compounds such as proteins, carotenoids, extracelular polymers and PHA. Chlorella species have been reported to be able to accumulate about 50%–60% proteins in the best cases, though some other species such Spirulina may even reach higher values of around 70%, containing all the essential amino acids. Microalgae have proportionally higher protein contents than most plants or crops.

Acetogens are mainly comprising Clostridium species, although they also include some other ones such as Acetobacterium woodii, among others. Native organisms metabolise C₁-gases such as CO₂ and/or CO to produce acetic acid as their main end metabolite, where CO₂ is used as carbon source and hydrogen is needed as an energy source. However, some other products can also be found in some non-engineered strains, though not all of them, such as longer chain fatty acids, e.g., butyrate and hexanoate, ethanol, butanol, hexanol, 2,3-butanediol (Fernández-Naveira et al., 2017). Those metabolites are produced through the so-called Wood-Ljungdahl pathway (WLP) (Figure 2). In the Eastern branch of the WLP, CO₂ is first reduced to formate to subsequently lead, stepwise, to acetyl-CoA. On the other side, in the Western branch, CO₂ can be converted into CO or, otherwise, carbon monoxide can be used directly as single substrate for the production of acetyl-CoA. Both in the Eastern as well as the Western branch, acetyl-CoA will then yield fatty acids (i.e., acetate, butyrate, or hexanoate) or alcohols (e.g., ethanol, butanol, hexanol). Although CO can be used as sole carbon and energy source, the presence of hydrogen is recommended, as CO is partly converted to CO₂ and thus subsequent removal of produced CO₂ would benefit from the availability of hydrogen as electron donor. These aspects are shown hereafter in some examples of reactions of bioconversion of CO and CO₂ into acetic acid (Fernández-Naveira et al., 2017). 4   C O + 2   H 2 O   → C H 3 C O O H + 2   C O 2 2   C O 2 + 4   H 2   → C H 3 C O O H + 2   H 2 O 2   C O + 2   H 2   → C H 3 C O O H

Fatty acids are the main end metabolites and are generally produced first, while alcohols are mostly obtained from the bioconversion of those fatty acids. Though the production of carboxylic acids is a commun feature of acetogens, only very few of them can produce alcohols and they include only a scarce number of identified species, i.e., mainly Clostridium aceticum, Clostridium autoethanogenum, Clostridium carboxidivorans, Clostridium ljungdahlii, Clostridium ragsdalei (Kennes-Veiga et al., 2024). Mixotrophic as well as heterotrophic growth is also possible and, similarly as with other bacteria (e.g., C. necator), growth under autotrophic conditions is generally more limited or lower than with organic carbon sources. The low solubility and slow mass transfer of C₁ gases and hydrogen in liquid (aqueous) phase contribute to such limitation in all autotrophic bacteria.

Besides the natural production of fatty acids and/or alcohols, more recent studies have been performed aiming at broadening the range of products that can be obtained from C₁ gases with acetogenic bacteria. Basically two approaches can be considered, consisting either in engineering bacteria in order to obtain other new products or, otherwise, use produced fatty acids (e.g., acetate) and alcohols (e.g., ethanol) to ferment and convert them into other, higher value, compounds.

Acetone and iso-propanol are the most typical and probably most studied examples of metabolites produced by genetically modified acetogenic organisms (GMO). In recent research, production of acetone was studied in engineered acetogen, i.e., A. woodii, in which the acetone biosynthesis pathway was constructed by combining genes from Clostridium acetobutylicum and C. aceticum (Arslan et al., 2022). Acetate, as by-product, was however, still detected. In another study, efficient combined production of acetone and isopropanol was also assessed at pilot-scale based on the engineered autotrophic acetogen C. autoethanogenum (Liew et al., 2022).

The other alternative consists in using acetate and/or ethanol obtained from gas fermentation for their further bioconversion into other valuable products. Acetate and ethanol produced by acetogenic bacteria from C₁ gases act as electron donor and electron acceptor, respectively, in a chain elongating process, yielding C₆ and, eventually, C₈ medium chain fatty acids. Chain elongation has been studied both with mixed bacterial cultures as well as with pure cultures, mainly focusing on the species Clostridium kluyveri. It was shown that co-cultures of an acetogenic strain, fermenting C₁ gases to produce acetate + ethanol, e.g., C. aceticum, together with a chain elongator, e.g., C. kluyveri, will yield medium chain carboxylic acids (caproate mainly) (Fernández-Blanco et al., 2022).

Acetate, obtained from gas fermentation, is also a suitable, cost-effective alternative to other more conventionl carbon sources such as glucose, for the production of a wide range of biofuels and bioproducts. Some examples include lipids (microbial oils) accumulated by oleaginous yeasts during fermentation of acetate obtained from gas fermentation, or the production of nutraceuticals such as ß-carotene produced under similar conditions with non-conventional yeasts grown on C₁ gas derived acetate (Naveira-Pazos et al., 2023).