Abstract
Increasing efforts are directed towards the development of sustainable alternative protein sources among which microbial protein (MP) is one of the most promising. Especially when waste streams are used as substrates, the case for MP could become environmentally favorable. The risks of using organic waste streams for MP production–the presence of pathogens or toxicants–can be mitigated by their anaerobic digestion and subsequent aerobic assimilation of the (filter-sterilized) biogas. Even though methane and hydrogen oxidizing bacteria (MOB and HOB) have been intensively studied for MP production, the potential benefits of their co-cultivation remain elusive. Here, we isolated a diverse group of novel HOB (that were capable of autotrophic metabolism), and co-cultured them with a defined set of MOB, which could be grown on a mixture of biogas and H2/O2. The combination of MOB and HOB, apart from the CH4 and CO2 contained in biogas, can also enable the valorization of the CO2 that results from the oxidation of methane by the MOB. Different MOB and HOB combinations were grown in serum vials to identify the best-performing ones. We observed synergistic effects on growth for several combinations, and in all combinations a co-culture consisting out of both HOB and MOB could be maintained during five days of cultivation. Relative to the axenic growth, five out of the ten co-cultures exhibited 1.1–3.8 times higher protein concentration and two combinations presented 2.4–6.1 times higher essential amino acid content. The MP produced in this study generally contained lower amounts of the essential amino acids histidine, lysine and threonine, compared to tofu and fishmeal. The most promising combination in terms of protein concentration and essential amino acid profile was Methyloparacoccus murrelli LMG 27482 with Cupriavidus necator LMG 1201. Microbial protein from M. murrelli and C. necator requires 27–67% less quantity than chicken, whole egg and tofu, while it only requires 15% more quantity than the amino acid-dense soybean to cover the needs of an average adult. In conclusion, while limitations still exist, the co-cultivation of MOB and HOB creates an alternative route for MP production leveraging safe and sustainably-produced gaseous substrates.
Highlights
The world population is projected to increase to 9.6–12.3 billion people by 2,100 (Gerland et al, 2014)
We investigated the co-cultivation of Methane oxidizing bacteria (MOB) and Hydrogen oxidizing bacteria (HOB) to identify the best combination for the production of microbial protein (MP) (Figure 1)
The general advantages of using HOB for MP production were summarized from Dou et al (2019): 1) compared to other MP types, HOB have higher protein content (i.e., 40-60% for microalgae, 30–45% for fungi and 45–55% for yeasts) (Nasseri et al, 2011); 2) they are metabolically versatile, and can switch from autotrophic to heterotrophic mode; 3) even though they are autotrophic, they do not get limited by light availability; 4) they contain intracellular products with prebiotic functions (i.e., polyhydroxybutyrate (PHB)); and 5) they fix CO2 to protein
Summary
The world population is projected to increase to 9.6–12.3 billion people by 2,100 (Gerland et al, 2014). The parallel rising demand for nutritional food per person (Westhoek et al, 2011), coupled with the Westernization of diets, results in increasing protein consumption (Statovci et al, 2017). Safeguarding its supply can be a difficult task, since the population increase coupled with the global dietary changes (Delgado, 2003) create a yawning gap between food demand and supply (FAO et al, 2019). The global consumption of animal products is ever increasing (WHO, 2012), but their production through current agricultural practices results in detrimental environmental effects. About 92% of global freshwater use is attributed to agricultural practices, while nearly 7.1 Gt CO2equivalents are globally emitted each year due to food production, representing 14.5% of the overall anthropogenic emissions (Gerber et al, 2013). Coupled with the inefficiency of the production of animal products (Apaiah et al, 2006), these facts point out the need for a so-called “protein transition” (Aiking and de Boer, 2018)
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