**2. Natural gas fermentation to value-added products**

Fermentation processes are ubiquitous in nature. They can be aerobic (see e.g. [19, 20] for methane) or anaerobic (see e.g. [21–24] for methane) and they have been used by mankind for thousands of years [25–27]. Methane is produced by anerobic fermentation, as it is well known from and deployed in biogas plants [28] or happening in landfills [29]. Also, methane is released from waste-water treatment plants, manure storage pits, ruminants (e.g., cattle) or natural soil such as wetlands. It is in these habitats where also methane-consuming microorganisms, so-called methanotrophic bacteria [30–33] can be found. They can be isolated [34] and be deployed for various target products. The methanome contains methanogenic and methanotrophic bacteria. Basically, methanotrophs are found in all habitats where methane is available [30, 35]. However, methane-oxidizing bacteria only constitute a fraction of the microbial community in most soils, in general <4% of all bacteria [36]. Anaerobic methane fermentation constitutes an important process in the deep subsurface, e.g., in marine sediments [37], as a biogeochemical process that limits the release of methane into the atmosphere. For industrial use, aerobic methanotrophs are more relevant. There are also other bacteria which feed on (higher) hydrocarbons, e.g., as they are found in contaminated land [38].

Methanotrophic bacteria (methanotrophs, methane-oxidizing bacteria (MOB)) [39] belong to the wider "methylotrophic" group that subsumes microorganisms which can use C1 substrates like methane or methanol [32, 40, 41]. Methane is found in nature; In the atmosphere, its concentration is slightly below 2 ppm. Biogenic methane is emitted from rice fields, various wetlands, termites and herbivores. Also, methanogenic archaea liberate methane from organic matter decomposition [42]. Aerobic methanotrophs are estimated to consume 10–90% of the methane that comes out of the deep anoxic layers of wetlands before it reaches the atmosphere [43], hence they have a strong impact on the climate, as methane is a very potent greenhouse gas with a greenhouse warming potential (GWP) of 24 relative to CO2 [44].

Said methanotrophs are gram-negative bacteria that use methane as their only source of carbon and energy [31], making them unique and interesting for biotechnological applications. Apart from being hosts for biotechnological production with native and engineered microorganisms in industry, they could be used for geoengineering purposes to fight global warming by methane, e.g., from thawing permafrost soil, fugitive anthropogenic emissions or direct air capturing of methane. Methanotrophs have also been described to co-metabolize toxic compounds [31]. **Table 2** lists several methane-utilizing bacteria.

*Value-Added Products from Natural Gas Using Fermentation Processes: Fermentation of Natural… DOI: http://dx.doi.org/10.5772/intechopen.103813*


#### **Table 2.**

*Selection of methane-oxidizing bacteria. Source: [45].*

Methanotrophs are an important biological sink for methane, and thereby are very relevant for the global carbon cycle [46]. The very first methanotrophic bacterium, *bacillus methanicum*, was discovered in the year 1906. To date, more than 100 different methanotrophs are known. Aerobic methanotrophs comprise >20 [47] recognized genera that belong to three major phylogenetic groups (type I, type II and type X). That classification is done on the microorganisms' characteristics such as morphology or carbon assimilation pathways [31]. Apart from the described bacteria, also some yeasts were found to feed on methane [48–50].

### **2.1 Motivation for microbial methane fermentation**

Biocatalytic conversion of methane [51] is a promising route for both commodities and for specialty materials. There is consensus on the need for alternative proteins other than meat. Projections say that alternatives for meat can account for up to 60% of the market in 2040 [52]. Also, there is significant consumer interest in such alternatives [53].

Agriculturally-produced alt protein, e.g., from peas or soy, is one option, noncrop-based proteins are another one. However, there are significant cost hurdles when non-agricultural protein is to be made. For insect protein [54], the price (2021 level) is between 4250 and 6066 USD/ton [55]. Single cell protein (SCP) from side streams of lignocellulose was estimated at requiring a minimum selling price of 5160–9007 €/ton to be economically viable [56]. A ubiquitous raw material is methane, but its use is not straightforward either; one of the most invidious molecules for the organic chemist is CO2. Almost next in line comes CH4. One requires 438.8 kJ/mol [30] to activate the C▬H bond. As a result, thermochemical processes with methane are energy-intensive, requiring high temperatures and pressures, and/or costly catalysts, see **Table 3**.

By contrast, biocatalysts operate at mild conditions. Also, their carbon conversion efficiency tends to be higher. Therefore, biotechnological methane oxidation using methanotrophic bacteria offers an interesting route.

#### **2.2 Methanotroph product range and cultivation**

When we look at the products accessible through methanotrophic bacteria, wild types can be used to obtain compounds from the natural microorganisms' metabolic pathways. Generic engineering allows to expand that range, see **Figure 12**.


#### **Table 3.**

*Comparison of process conditions and carbon conversion efficiencies of chemical and biochemical catalysts. Source: [30].*

#### **Figure 12.**

*Potential fuels and chemicals accessible through methane fermentation. Methanotrophic bacteria can generate all relevant 1-, 2- and 3-carbon intermediate compounds. "Virtually all biosynthetic modules for the production of advanced fuels or chemicals, developed for glucose-based fermentation in E. coli, could potentially be implemented in methane-utilizing strains". PHB = polyhydroxybutyrate; PHV = polyhydroxyvalerate; FAEEs = fatty acid ethyl esters; FAMEs = fatty acid methyl esters; source: [57].*

Genetic engineering is useful when the target products are chemicals. Obtaining approval for feed and food made with GMO (genetically modified organisms) will be significantly harder and is therefore discouraged. To date, most metabolic engineering was achieved with type I methanotrophs [58]. Synthetic methanotrophs [59, 60] offer
