Simultaneous co-cultivation of the thermoacidophilic methanotroph, Methylacidiphilum sp. RTK17.1, and the microalga, Galdieria sp. RTK37.1, for single cell protein production

Abstract

The verrucomicrobial methanotroph, Methylacidiphilum sp. RTK17.1, and the microalgae, Galdieria sp. RTK37.1 are both thermoacidophilic microorganisms isolated from geothermally heated soils at Rotokawa, Aotearoa-New Zealand. In this work, we used cocultures of Methylacidiphilum sp. RTK17.1 and Galdieria sp. RTK37.1 in batch and continuous systems (45 °C, pH 2.5) to assess their biomass productivity and performance; with the goal of removing methane and carbon dioxide from simulated waste gas streams and assessing the resultant biomass for its potential use as single cell protein. Coculture performance was compared to corresponding axenic cultures and the nutritional suitability of resultant biomass was assessed as a single cell protein feedstock. Stable coculture was achieved in both batch and chemostat systems. In batch experiments, Galdieria sp. RTK37.1 significantly enhanced growth (29 %) and methane oxidation (300 %) rates of Methylacidiphilum sp. RTK17.1, and complete methane removal was achieved without formation of an explosive gas mixture. In steady state chemostat coculture experiments, Galdieria sp. RTK37.1 decreased net volumetric oxygen consumption by 46 %, but its oxygenic activity was unable to supply Methylacidiphilum sp. RTK17.1 with the O₂ required for complete CH₄ removal. Nevertheless, Methylacidiphilum sp. RTK17.1 benefited from the presence of Galdieria sp. RTK37.1 in a low O₂ environment; with O₂ algae-methanotroph cross-feeding playing a fundamental role on their interactions. Methylacidiphilum sp. RTK17.1, Galdieria sp. RTK37.1, and their coculture each displayed similar nutritional profiles, with protein quality comparable to soybean meal and fishmeal feeds used for animals. The biomass needed to meet the daily indispensable amino acid requirements of a 62 kg adult human was 568 g for Methylacidiphilum sp. RTK17.1, 804 g Galdieria sp. RTK37.1, and 754 g for the coculture, with histidine being the limiting amino acid. These thermoacidophilic cocultures, which have not previously been investigated, offer great potential to convert low (or negative) value industrial gas streams into valuable products (e.g. supplementary biofeedstocks).