(648c) Biofilm-Based Cultivation of Methanotroph-Photoautotroph Coculture – a Highly Effective Biogas Valorization Technology

Each year animal farms in the US generate about 1.1 billion ton wet animal manure.¹ Current manure management practice – open lagoon storage followed by land application as fertilizer, has contributed significantly to GHG emissions and air/water pollutions. Among different waste management approaches, anaerobic digestion (AD) is a commercially proven and arguably the most effective solution for managing animal manure. It breaks down organic matter into biogas (CH₄ and CO₂, and trace of other gases such as H₂S) and nutrient-rich (e.g., N, P, K, etc.) digestate in a controlled and contained manner. Microalgae cultivation has been utilized to convert AD-generated digestate (and CO₂) into biofuels.^2–4 However, biofuels from microalgae cannot compete with inexpensive petroleum fuels and sustainable production levels have not been achieved.^5,6 Algae biofuels also present serious energetic and environmental viability challenges.⁷

In nature, the metabolic coupling of methane oxidation and oxygenic photosynthesis is a major sink of both CH₄ and CO₂ at oxic-anoxic interfaces across various aquatic and terrestrial ecosystems, where the methanotrophic activity is fueled by in situ photosynthetic production of O₂. Recently, we have demonstrated a microalgae-methanotroph coculture offers a highly effective platform for biogas valorization and nutrient recovery from digestate.⁸ When cultured on diluted AD digestate, the coculture showed significantly improved biomass productivity (over 120% improvement) compared to microalgae single culture, plus zero emission without external oxygen supply, and complete recovery of inorganic nitrogen and phosphorus in AD effluent to produce treated clean water.⁸

Despite the huge potential of the methanotroph-photoautotroph for biogas conversion and nutrient recovery, the development of coculture-based biotechnology faces two economic bottlenecks concerning traditional suspended or planktonic cultivation⁵: (1) cost-effective microbial biomass production and (2) efficient biomass recovery. These are the same bottlenecks that hinder the commercialization of microalgae-based technologies. They are caused by the light attenuation in the culture broth and mass transfer resistance of gas substrate into liquid broth, which severely limit the achievable cell density in liquid medium and the scale up of the biotechnology. In addition, the low cell density further results in high energy cost for biomass harvesting which drastically diminishes the biotechnology’s economic feasibility.

Several recent studies have shown that biofilm-based microalgae cultivation increased productivity by 3-7 folds compared to planktonic cultivation.^9–12 However, it has been argued that biomass productivity in biofilm is limited by the light penetration and mass transfer, as biofilms are densely packed microbes and diffusion is the dominant transport mechanism within the biofilm. Therefore, the increased productivity reported for biofilm-based microalgae cultivation could be solely due to vastly increased surface area of biofilm, which overcomes the limitation on light penetration and mass transfer caused by densely packed biofilm.

At the same time, the niche differentiation and biofilm structuring observed in natural biofilm systems suggest that there are other transport mechanisms besides diffusion.^13,14 Also, studies have shown that biofilm communities are typically assembled along physicochemical gradients via optimized metabolic functionality and metabolite exchange.¹⁵ Therefore, we hypothesize that without increased surface area, biofilm-based cultivation, especially for methanotroph-photoautotroph coculture, still offers advantages over suspended cultivation. In this work, we validate our hypothesis by experimentally demonstrate the increased productivity of biofilm-based methanotroph and cyanobacteria single and co-cultures compared to planktonic single and co-cultures of the same strains under the carefully controlled comparable conditions. The biomass productivity and gas substrate consumption rates are systematically and quantitatively compared under various conditions. The results confirm that biofilm-based methanotroph-cyanobacteria co-cultivation can overcome the bottlenecks mentioned above and has the potential to be a highly effective technology for valorizing AD-derived biogas and nutrient recovery.

References:

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