(95a) Coculture of Methanotrophs and Microalgae – a Flexible Platform for Biological CH4/CO2 Co-Utilization

Recent findings published in Nature¹ and Nature Geoscience² suggest that the coupling of methane oxidation and oxygenic photosynthesis are prevalent in nature and reduce CH₄ emissions and reuse CO₂. In addition, a 2015 ISME Journal paper reported³ that methane removal in anoxic waters (freshwater lakes depleted of dissolved O₂) is due to true aerobic oxidation of methane, rather than anaerobic oxidation, fueled by in situ O₂ produced from photosynthetic algae. These findings suggest that the coculture of methanotrophs and microalgae represents not only a feasible, but also a highly promising strategy for simultaneous conversion of biogas (both CH₄ and CO₂) into useful products, feed, energy or other products.

Such coupling has been partially validated in laboratory settings. (1) It was reported that coculture of Methylocystis parvus (methanotroph) and Scenedesmus sp. (microalgae) can achieve total microbial conversion of both CH₄ (60%) and CO₂ (40%) in a synthetic biogas without external O₂ supply⁴; (2) coculture of Methylomicrobium alcaliphilum (methanotroph) and Synechococcus PCC 7002 (cyanobacteria) exhibit robust growth on diverse gas mixtures including raw biogas and synthetic natural gas⁵. However, both published research were conducted under continuous illumination, and there has not been any systematic study on the factors that could affect the stability and performance of the cocultures.

In this work, we hypothesize that the coculture of methanotrophs and microalgae offers a flexible platform for simultaneous biological CH₄/CO₂ co-utilization, and cocultures of different methanotroph and microalgae strains can be stabilized if their growth conditions are compatible. To validate our hypothesis, we have established three pairs of methanotroph-microalgae coculture, with one pair preferring high pH and high salt condition, while the other two pairs prefer neutral pH and low salt condition. Multiple transfers of the coculture to new media confirm the stability of the three coculture pairs, all under light-dark cycles (12:12 or 16:8) of the illumination. In addition, one pair of the model coculture was used to gain fundamental understandings on the interactions between the methanotroph and microalgae strains. Specifically, inoculation ratio, gas phase composition and illumination regimes were examined using batch experiments. Individual and total cell concentrations, CH₄ consumption rate, overall CO₂ production/consumption rate, and overall O₂ production/consumption rate, were measured over time to assess the effect of different factors, and evaluate the conversion performance of the coculture under different conditions.

(1) Raghoebarsing, A. A.; Smolders, A. J.; Schmid, M. C.; Rijpstra, W. I. C.; Wolters-Arts, M.; Derksen, J.; Jetten, M. S.; Schouten, S.; Damsté, J. S. S.; Lamers, L. P. Methanotrophic Symbionts Provide Carbon for Photosynthesis in Peat Bogs. Nature 2005, 436, 1153.

(2) Kip, N.; van Winden, J. F.; Pan, Y.; Bodrossy, L.; Reichart, G.-J.; Smolders, A. J.; Jetten, M. S.; Damsté, J. S. S.; den Camp, H. J. O. Global Prevalence of Methane Oxidation by Symbiotic Bacteria in Peat-Moss Ecosystems. Nat. Geosci. 2010, 3, 617.

(3) Milucka, J.; Kirf, M.; Lu, L.; Krupke, A.; Lam, P.; Littmann, S.; Kuypers, M. M.; Schubert, C. J. Methane Oxidation Coupled to Oxygenic Photosynthesis in Anoxic Waters. ISME J. 2015.

(4) Van der Ha, D.; Nachtergaele, L.; Kerckhof, F.-M.; Rameiyanti, D.; Bossier, P.; Verstraete, W.; Boon, N. Conversion of Biogas to Bioproducts by Algae and Methane Oxidizing Bacteria. Environ. Sci. & Technol. 2012, 46, 13425.

(5) Hill, E. A.; Chrisler, W. B.; Beliaev, A. S.; Bernstein, H. C. A Flexible Microbial Co-Culture Platform for Simultaneous Utilization of Methane and Carbon Dioxide from Gas Feedstocks. Bioresour. Technol. 2017.