Renewable electricity-driven CO₂-to-protein conversion by hydrogen-oxidizing bacteria: meeting challenges in the recovery and in-situ upcycling of wasted nitrogen

Increasingly more high-quality, protein-rich food is needed by the ever-increasing global population. Traditionally, protein is supplied by agriculture, e.g., plant-based protein from croplands and animal-based protein from pastures and fisheries. However, conventional agriculture requires huge amounts of arable land and water, which puts it in competition with urbanization and industrial manufacturing. Moreover, to ensure high food productivity levels, vast quantities of exogenous matter are introduced into agricultural production, such as chemical fertilizers, pesticides, antibiotics, and hormones. In turn, they can have adverse impacts on both microorganisms, and higher-level life forms found in the ecosystem and thus, impair its overall stability. Moreover, a significant mass of agricultural residue (e.g., straw, stalks, faces, and urine) is generated and has to be disposed of correctly. Furthermore, conventional agriculture is highly dependent on weather and climate. Therefore, novel protein production approaches, which are more sustainable and depend less on climate and weather, are required to meet increasing global demand.

In this context, hydrogen-oxidizing bacteria (HOB)-based power-to-protein conversion offers a promising route to achieving the simultaneous production of alternative microbial protein (MP), mitigation of GHG, and storage of renewable electricity (e.g., solar and wind power). Precisely, the process can assimilate CO₂ using gaseous products (H₂ and O₂) from water electrolysis. To date, several reactors have been developed to produce MP using HOB; however, the nitrogen required for this process is always externally supplied. Besides, nitrogen concentration is usually controlled within a narrow range (0.13 ~ 0.5g N-NH₄⁺/L), due to current limited knowledge on ammonia toxicity.

Hence, in the first part of this Ph.D. project, the ammonia toxicity and nitrogen utilization profiles of a typical HOB strain, namely Cupriavidus necator 335, were examined. The results indicated that an ammonium concentration above 2 g N-NH₄⁺/L inhibits the growth of C. necator 335, while 4 g N-NH₄⁺/L can completely suppress growth. The results indicate that the strain has a higher ammonia tolerance than previously imagined. Furthermore, C. necator 335 can grow on ammonium, nitrate and urea, except for nitrite under autotrophic conditions. In this regard, urea has the highest growth rate, followed by nitrate and ammonium. The results suggest that the strain has the potential to recover and upcycle nitrogen contained in waste streams in various forms and even at high concentrations (up to 2 g N-NH₄⁺/L).

Subsequently, a microbial electrochemical recovery conversion cell (MERC) was designed for simultaneous ammonia recovery and in-situ MP production. The MERC system can be driven by green electricity, and so the effects of several key parameters on system performance were investigated. Applied voltage can affect the growth of C. necator 335 by influencing the production of H₂ and O₂. The MERC system can recover and upcycle ammonium from wastewater across a broad concentration range (0.05 ~ 8 g N-NH₄⁺/L), and when applied to real waste streams, it is highly efficient in terms of MP conversion (74 ~ 142%).

Furthermore, a hybrid biological-inorganic (HBI) system was proposed. In the system, ammonia recovery is self-sustained by wastewater, and no caustic acids/bases are needed for pH control. Simultaneously, in-situ ammonium upcycling for MP production is achieved by C. necator 335 through power-to-protein conversion. A linear accumulation of biomass is achieved in spite of the amounts of supplied CO₂ and operating modes. The HBI system produces MP at the rate of 0.381 ± 0.009 g biomass/L in 72 h (protein content 64.79 ± 4.93%) alongside good wastewater treatment (76.8% ammonium removal and 84.6% COD removal).

Overall, this Ph.D. project investigated the feasibility of using HOB to achieve simultaneous ammonia recovery from wastewater and in-situ power-to-protein conversion in one integrated system. Such system could greatly reduce the environmental footprint during protein production and contribute to GHG mitigation and global climate change alleviation.