Elucidating the key environmental parameters during the production of ectoines from biogas by mixed methanotrophic consortia

Optimization of methane gas-liquid mass transfer during biogas-based ectoine production in bubble column bioreactors

Nowadays, the utilization of biogas for energy generation is hindered by the declining production costs of solar and wind power. A shift towards the valorization of biogas into ectoine, a highly valuable bioproduct priced at 1000 €⸱kg-1, offers a novel approach to fostering a more competitive biogas market while contributing to carbon neutrality. This study evaluated the optimization of CH4 gas-liquid mass transfer in 10 L bubble column bioreactors for CH4 conversion into ectoine and hydroxyectoine using a mixed methanotrophic culture. The influence of the empty bed residence time (EBRTs of 27, 54, and 104 min) at different membrane diffuser pore sizes (0.3 and 0.6 mm) was investigated. Despite achieving CH4 elimination capacities (CH4-ECs) of 10-12 g⸱m-3⸱h-1, an EBRT of 104 min mediated CH4 limitation within the cultivation broth, resulting in a negligible biomass growth. Reducing the EBRT to 54 min entailed CH4-ECs of 21-24 g⸱m-3⸱h-1, concomitant to a significant increase in biomass growth (up to 0.17 g⸱L⸱d-1) and reaching maximum ectoine and hydroxyectoine accumulation of 79 and 13 mg⸱gVSS-1, respectively. Conversely, process operation at an EBRT of 27 min lead to microbial inhibition, resulting in a reduced biomass growth of 0.09 g⸱L⸱d-1 and an ectoine content of 47 mg⸱gVSS-1. While the influence of diffuser pore size was less pronounced compared to EBRT, the optimal process performance was observed with a diffuser pore size of 0.6 mm.

High production of ectoine from methane in genetically engineered Methylomicrobium alcaliphilum 20Z by preventing ectoine degradation

BACKGROUND

Methane is a greenhouse gas with a significant potential to contribute to global warming. The biological conversion of methane to ectoine using methanotrophs represents an environmentally and economically beneficial technology, combining the reduction of methane that would otherwise be combusted and released into the atmosphere with the production of value-added products.

RESULTS

In this study, high ectoine production was achieved using genetically engineered Methylomicrobium alcaliphilum 20Z, a methanotrophic ectoine-producing bacterium, by knocking out doeA, which encodes a putative ectoine hydrolase, resulting in complete inhibition of ectoine degradation. Ectoine was confirmed to be degraded by doeA to N-α-acetyl-L-2,4-diaminobutyrate under nitrogen depletion conditions. Optimal copper and nitrogen concentrations enhanced biomass and ectoine production, respectively. Under optimal fed-batch fermentation conditions, ectoine production proportionate with biomass production was achieved, resulting in 1.0 g/L of ectoine with 16 g/L of biomass. Upon applying a hyperosmotic shock after high-cell-density culture, 1.5 g/L of ectoine was obtained without further cell growth from methane.

CONCLUSIONS

This study suggests the optimization of a method for the high production of ectoine from methane by preventing ectoine degradation. To our knowledge, the final titer of ectoine obtained by M. alcaliphilum 20ZDP3 was the highest in the ectoine production from methane to date. This is the first study to propose ectoine production from methane applying high cell density culture by preventing ectoine degradation.

Recent advances in production and applications of ectoine, a compatible solute of industrial relevance

Extremophilic bacteria growing in saline ecosystems are potential producers of biotechnologically important products including compatible solutes. Ectoine/hydroxyectoine are two such solutes that protect cells and associated macromolecules from osmotic, heat, cold and UV stress without interfering with cellular functions. Since ectoine is a high value product, overviewing strategies for improving yields become relevant. Screening of natural isolates, use of inexpensive substrates and response surface methodology approaches have been used to improve bioprocess parameters. In addition, genome mining exercises can aid in identifying hitherto unreported microorganisms with a potential to produce ectoine that can be exploited in the future. Application wise, ectoine has various biotechnological (protein protectant, membrane modulator, DNA protectant, cryoprotective agent, wastewater treatment) and biomedical (dermatoprotectant and in overcoming respiratory and hypersensitivity diseases) uses. The review summarizes current updates on the potential of microorganisms in the production of this industrially relevant metabolite and its varied applications.

Ectoines production from biogas in pilot bubble column bioreactors and their subsequent extraction via bio-milking

Despite the potential of biogas from waste/wastewater treatment as a renewable energy source, the presence of pollutants and the rapid decrease in the levelized cost of solar and wind power constrain the use of biogas for energy generation. Biogas conversion into ectoine, one of the most valuable bioproducts (1000 €/kg), constitutes a new strategy to promote a competitive biogas market. The potential for a stand-alone 20 L bubble column bioreactor operating at 6% NaCl and two 10 L interconnected bioreactors (at 0 and 6% NaCl, respectively) for ectoine production from biogas was comparatively assessed. The stand-alone reactor supported the best process performance due to its highest robustness and efficiency for ectoine accumulation (20-52 mgectoine/gVSS) and CH4 degradation (up to 84%). The increase in N availability and internal gas recirculation did not enhance ectoine synthesis. However, a 2-fold increase in the internal gas recirculation resulted in an approximately 1.3-fold increase in CH4 removal efficiency. Finally, the recovery of ectoine through bacterial bio-milking resulted in efficiencies of >70% without any negative impact of methanotrophic cell recycling to the bioreactors on CH4 biodegradation or ectoine synthesis.

Polyhydroxybutyrate Production from Methane and Carbon Dioxide by a Syntrophic Consortium of Methanotrophs with Oxygenic Photogranules without an External Oxygen Supply

Here, a syntrophic process was developed to produce polyhydroxy-β-butyrate (PHB) from a gas stream containing CH₄ and CO₂ without an external oxygen supply using a combination of methanotrophs with the community of oxygenic photogranules (OPGs). The co-culture features of Methylomonas sp. DH-1 and Methylosinus trichosporium OB3b were evaluated under carbon-rich and carbon-lean conditions. The critical role of O₂ in the syntrophy was confirmed through the sequencing of 16S rRNA gene fragments. Based on their carbon consumption rates and the adaptation to a poor environment, M. trichosporium OB3b with OPGs was selected for methane conversion and PHB production. Nitrogen limitation stimulated PHB accumulation in the methanotroph but hindered the growth of the syntrophic consortium. At 2.9 mM of the nitrogen source, 1.13 g/L of biomass and 83.0 mg/L of PHB could be obtained from simulated biogas. These results demonstrate that syntrophy has the potential to convert greenhouse gases into valuable products efficiently.

Methanotrophs as a reservoir for bioactive secondary metabolites: Pitfalls, insights and promises

Methanotrophs are potent natural producers of several bioactive secondary metabolites (SMs) including isoprenoids, polymers, peptides, and vitamins. Cryptic biosynthetic gene clusters identified from these microbes via genome mining hinted at the vast and hidden SM biosynthetic potential of these microbes. Central carbon metabolism in methanotrophs offers rare pathway intermediate pools that could be further diversified using advanced synthetic biology tools to produce valuable SMs; for example, plant polyketides, rare carotenoids, and fatty acid-derived SMs. Recent advances in pathway reconstruction and production of isoprenoids, squalene, ectoine, polyhydroxyalkanoate copolymer, cadaverine, indigo, and shinorine serve as proof-of-concept. This review provides theoretical guidance for developing methanotrophs as microbial chassis for high-value SMs. We summarize the distinct secondary metabolic potentials of type I and type II methanotrophs, with specific attention to products relevant to biomedical applications. This review also includes native and non-native SMs from methanotrophs, their therapeutic potential, strategies to induce silent biosynthetic gene clusters, and challenges.

Production and Recovery of Ectoine: A Review of Current State and Future Prospects

Ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid) is a revolutionizing substance with vast applications in the cosmetic and food industries. Ectoine is often sourced from halobacteria. The increasing market demand for ectoine has urged the development of cost-effective and sustainable large-scale production of ectoine from microbial sources. This review describes the existing and potential microbial sources of ectoine and its derivatives, as well as microbial production and fermentation approaches for ectoine recovery. In addition, conventional methods and emerging technologies for enhanced production and recovery of ectoine from microbial fermentation with a focus on the aqueous biphasic system (ABS) are discussed. The ABS is a practically feasible approach for the integration of fermentation, cell disruption, bioconversion, and clarification of various biomolecules in a single-step operation. Nonetheless, the implementation of the ABS on an industrial-scale basis for the enhanced production and recovery of ectoine is yet to be exploited. Therefore, the feasibility of the ABS to integrate the production and direct recovery of ectoine from microbial sources is also highlighted in this review.

Influence of operational conditions on the performance of biogas bioconversion into ectoines in pilot bubble column bioreactors

The application of biogas as a low-priced substrate for the production of ectoines constitutes an opportunity to decrease their production costs and to enhance the viability of anaerobic digestion. The influence of operational conditions on CH₄-biogas biodegradation and on ectoines production yields was assessed in continuous pilot bubble column bioreactors. The rise in biomass concentration from 1 to 3 g L^-1 resulted in a decrease in the specific ectoine content from 42 ± 8 to 30 ± 4 mg_ectoine g_VSS^-1. The concentration of Cu²⁺ and Mg²⁺ did not impact process performance, while the use of ammonium as N source resulted in low CH₄ biodegradation and ectoine yields (13 ± 7 mg_ectoine g_VSS^-1). The increase in CH₄ content from 4.5 to 9 %v·v^-1 enhanced CH₄ removal efficiency. Process operation at NaCl concentrations of 3 %w·w^-1 instead of 6 %w·w^-1 decreased the ectoine yield to 17 mg_ectoine g_VSS^-1. Finally, Methylomicrobiumburyatense was identified as the dominant species.

Biogas bioconversion into poly(3-hydroxybutyrate) by a mixed microbial culture in a novel Taylor flow bioreactor

Biogas-based biopolymer production represents an alternative biogas valorization route with potential to cut down plastic pollution and greenhouse gas emissions. This study investigated for the first time the continuous bioconversion of methane, contained in biogas, into poly(3-hydroxybutyrate) (PHB) by a mixed methanotrophic culture using an innovative high mass-transfer Taylor flow bioreactor. Following a hydrodynamic flow regime mapping, the influence of the gas residence time and the internal gas recirculation on CH₄ abatement was assessed under non nutrient limiting conditions. Under optimal operational conditions (gas residence time of 60 min and internal gas recycling ratio of 17), the bioreactor was able to support a CH₄ removal efficiency of 63.3%, a robust CH₄ elimination capacity (17.2 g-CH₄ m^-3h^-1) and a stable biomass concentration (1.0 g L^-1). The simultaneous CH₄ abatement and PHB synthesis was investigated under 24-h:24-h nitrogen feast/famine continuous operation. The cyclic nitrogen starvation and the Taylor flow imposed in the bioreactor resulted in a relatively constant biomass concentration of 0.6 g L^-1 with PHB contents ranging from 11 to 32% w w^-1 (on a dry weight basis), entailing an average PHB productivity of 5.9 g-PHB m^-3 d^-1 with an associated PHB yield of 19.8 mg-PHB g-CH₄^-1. Finally, the molecular analysis of the microbial population structure indicated that type II methanotrophs outcompeted non-PHB accumulating type I methanotrophs, with a heterotrophic-methanotrophic consortium enriched in Methylocystis, Hyphomicrobium, Rubinisphaeraceae SH PL14 and Pseudonocardia.

Ectoine Production from Biogas in Waste Treatment Facilities: A Techno-Economic and Sensitivity Analysis

The capacity of haloalkaliphilic methanotrophic bacteria to synthesize ectoine from CH₄-biogas represents an opportunity for waste treatment plants to improve their economic revenues and align their processes to the incoming circular economy directives. A techno-economic and sensitivity analysis for the bioconversion of biogas into 10 t ectoine·y^-1 was conducted in two stages: (I) bioconversion of CH₄ into ectoine in a bubble column bioreactor and (II) ectoine purification via ion exchange chromatography. The techno-economic analysis showed high investment (4.2 M€) and operational costs (1.4 M€·y^-1). However, the high margin between the ectoine market value (600-1000 €·kg^-1) and the estimated ectoine production costs (214 €·kg^-1) resulted in a high profitability for the process, with a net present value evaluated at 20 years (NPV₂₀) of 33.6 M€. The cost sensitivity analysis conducted revealed a great influence of equipment and consumable costs on the ectoine production costs. In contrast to alternative biogas valorization into heat and electricity or into low added-value bioproducts, biogas bioconversion into ectoine exhibited high robustness toward changes in energy, water, transportation, and labor costs. The worst- and best-case scenarios evaluated showed ectoine break-even prices ranging from 158 to 275 €·kg^-1, ∼3-6 times lower than the current industrial ectoine market value.