Industrial agriculture (IA) is detrimental to the environment, contaminates water and fosters climate change. Moreover, IA cannot guarantee food security for the 21st century. Single-Cell-protein (SCP) represents a safe alternative with minimal carbon and water footprints, but does not truly improve sustainability or food security when grown waste products of IA. This concept paper proposes autotrophic SCP bioprocess designs which enable sustainable, fail-safe and efficient “primary production” of edible biomass from CO2 and N2/NH3, while also providing flexible options of biomass upgrading. It can be driven by H2, CO, HCOOH from several sustainable sources. Most promising designs consist of 2-stages. In the 1st stage, homoacetogenic bacteria (HAB) fix CO2 and secrete fixed C as acetate with unrivaled yields. In the 2nd stage, acetate is transformed to edible biomass by selected microbes. Bacteria have versatile features such as O2-tolerant hydrogenases and N2-fixation. Eukaryotic microalgae are approved as food and exhibit oxygenic photosynthesis (O-PS), partly replacing solar-panels, seawater desalination and H2O-electrolyzers. Photoheterotrophic growth on HAB-derived acetate provides the opportunity to benefit from the highly efficient light-reaction, while avoiding the inefficient dark-reaction. Slow gas mass-transfer, poor light distribution and expensive cell harvest are major challenges arising from the cultivation in liquid media. To cope with these, microbes grow as hydrated biofilms, exposed directly to substrate gases and light. Two such bioreactors are presented and adaptations for 2-stage designs are proposed. One bioreactor was designed for algae growth in wastewater and provides a mechanism of dry cell harvesting. By harvesting microalgae from flowing waters, reduced carbon and nutrients can be obtained, while at the same time counteracting eutrophication. Non-food-grade biomass can be fed to anaerobic digesters (AD) with suppressed methanogenesis. Gaseous H2, CO2, NH3, H2S, CO2 and volatile organic acids can be extracted, with the gas phase acting as a “cleaning” barrier. This makes it possible to safely upgrade non-food-grade biomass to food. When food-grade waste is subjected to hydrolysis, diffusible PVA-tubes piercing the AD may allow simultaneous extraction of basic monomers and precious soluble nutrients. As an additional provision of CO2, alkalic pH and hydroxides formed at the cathode during electrolysis may be used to precipitate CO2 from the air as carbonates. 2-stage designs with solar-powered H2 generation from seawater, were estimated to exceed productivity of wheat 20-200 fold. It is speculated that, considering the non-utilization of direct and indirect subsidies, autotrophic SCP is far superior to IA not only in ecological but also in economical aspects.
REVIEWING MAJOR MICROBES AS A SINGLE CELL PROTEIN