Abstract: Photoautohydrogenotrophic enrichments of wastewater treatment microbiomes were performed to obtain hypothetically high-potential specialist species for biotechnological applications. From these enrichment cultures, ten photoautohydrogenotrophic species were isolated: six Rhodopseudomonas species, three Rubrivivax members and Rhodobacter blasticus. The performance of these isolates was compared to three commonly studied, and originally photoheterotrophically enriched species (Rhodopseudomonas palustris, Rhodobacter capsulatus and Rhodobacter sphaeroides), designated as reference species. Repeated subcultivations were applied to improve the initial poor performance of the isolates (acclimation effect), which resulted in increases in both maximum growth rate and protein productivity. However, the maximum growth rate of the reference species remained 3–7 times higher compared to the isolates (0.42–0.84 d−1 at 28 °C), while protein productivities remained 1.5–1.7 times higher. This indicated that H2-based enrichment did not result in photoautohydrogenotrophic specialists, suggesting that the reference species are more suitable for intensified biomass and protein production. On the other hand, the isolates were able to provide equally high protein quality profiles as the references species, providing full dietary essential amino acid matches for human food. Lastly, the effect of metabolic carbon/electron switching (back and forth between auto- to heterotrophic conditions) initially boosted µmax when returning to photoautohydrogenotrophic conditions. However, the switch negatively impacted lag phase, protein productivities and pigment contents. In the case of protein productivity, the acquired acclimation was partially lost with decreases of up to 44 % and 40 % respectively for isolates and reference species. Finally, the three reference species, and specifically Rh. capsulatus, remained the most suitable candidate(s) for further biotechnological development.

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.

Abstract: Microalgal biomass production is a resource-efficient answer to the exponentially increasing demand for protein, yet variability in biomass quality is largely unexplored. Nutritional value and safety were determined for Chlorella and Spirulina biomass from different producers, production batches and the same production batch. Chlorella presented a similar protein content (47 ± 8%) compared to Spirulina (48 ± 4%). However, protein quality, expressed as essential amino acid index, and digestibility were lower for Chlorella (1.1 ± 0.1 and 51 ± 9%, respectively) compared to Spirulina (1.3 ± 0.1 and 61 ± 4%, respectively). Generally, variability was lower between batches and within a batch. Heavy metals, pesticides, mycotoxins, antibiotics and nitrate did not violate regulatory limits, while polycyclic aromatic hydrocarbon levels exceeded the norm for some samples, indicating the need for continuous monitoring. This first systematic screening of commercial microalgal biomass revealed a high nutritional variability, necessitating further optimization of cultivation and post-processing conditions. Based on price and quality, Spirulina was preferred above Chlorella.