Methanotroph-microalgae cocultures have recently emerged as promising candidates for integrated biogas valorization and nutrient recovery. Methanotrophs are microorganisms that utilize methane (under atmosphere pressure and temperature) as their sole carbon and energy source to grow. Aerobic methanotrophs require oxygen to oxidize methane and exhibit higher growth rates than anaerobic methanotrophs. Although biogas is rich in methane, it does not contain the oxygen required by the aerobic methanotroph to grow. It has been reported that the coupling of methane oxidation (by methanotrophs) and oxygenic photosynthesis (by photoautotrophs or plants) is prevalent in nature to reduce CH4 emissions and reuse CO2 (Kip et al., 2010; Milucka et al., 2015; Raghoebarsing et al., 2005). The methanotroph-microalgae coculture converts the carbon contained in both CH4 and CO2 into biomass, which can serve as feedstock for animal feed or bioplastic production (Wang and He, 2023). Exploring the synergistic relationship between methanotrophs and microalgae, several groups have demonstrated that methanotroph-microalgae cocultures can significantly increase biomass production and nutrient recovery for wastewater treatment (Badr et al., 2022; Rasouli et al., 2018; Roberts et al., 2020; Wang et al., 2022; Zhang et al., 2023).
The choice of the biocatalyst, i.e., the methanotroph-microalgae coculture, has arguably the biggest impact on the performance of the coculture waste-to-value (W2V) platform. However, existing research on methanotroph-microalgae cocultures all focused on the performance of selected coculture pairs in biogas conversion or biomass production. There has not been any report on a comprehensive comparison or screening of different species. In addition, there is a lack of quantitative evaluation on the effect of interspecies interactions within a given coculture.
In this study, seven methanotrophs and five microalgae previously indicated as promising biocatalysts for wastewater treatment were screened in a parallel bioreactor system developed in-house. Based on the individual strains growth on wastewater, six methanotroph-microalgae coculture pairs were further evaluated for integrated biogas valorization and nutrient recovery. To systematically assess the growth performance of different monocultures and cocultures, mathematical models (modified Gompertz models) that describe the microbial growth under batch cultivation were developed to determine the maximum growth rate, delay time, and carrying capacity from growth data, allowing for consistent and systematic assessment of different species, as well as the identification of the coculture pairs with synergistic and inhibitory interactions. Using biological relevant parameters estimated from the modified Gompertz model, different types of inter-species interaction within different coculture pairs were identified.
Reference
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