Optimizing growth and biological methanation by Methanococcus maripaludis in continuous stirred-tank reactors

Abstract

In recent years, the demand for renewable energy sources has increased due to the growing human population and the impact of climate change. Biological methanation (BM) is a promising green method to replace fossil fuels, by employing microorganisms such as methanogenic archaea to convert H2 and CO2 into methane. Methanococcus maripaludis is a methanogen that has shown significant potential for BM due to its ability to achieve high gas conversion efficiency, its adaptability to different reactor configurations and simple cultivation conditions. There are several challenges that hinder the achievement of a stable performance of BM, including low biomass production of the species, gas-liquid mass transfer, nutrient availability and operational strategies. Therefore, the main goal of our study was to investigate the performance of M. maripaludis in continuous stirred-tank reactors (CSTRs) to optimize biomass growth and BM process with a focus on developing a scalable strategy for industrial applications. We evaluated different operational conditions such as nutrient supply frequency, H2/CO2 gas inflow rates, gas recirculation methods, agitation speeds, and H2/CO2 ratio to determine their impacts on critical BM performance indicators such as biomass growth, methane evolution rate (MER), methane yield, and gas conversion efficiency. Our results showed that at 500 rpm, biomass growth reached an optical density of 578 nm (OD578nm) of 1.6 and methane yield of 33 %, whereas at 400 rpm, OD578nm decreased to 1 and methane yield dropped to 14 %. This indicates that higher mixing speed supported better bio-mass growth and methane. Additionally, gas inflow rate had an inverse relationship with gas conversion efficiency. The highest H2 conversion efficiency (30 % ± 1.97) with methane purity of 9 ± 0.97 % was observed at the lowest gas inflow rate of 5 mL/min. Although higher gas inflow rates increased MER up to 7.40 ± 3.02 mmol CH4 per gram dry cell weight (gDCW) per hour, it reduced H2 conversion efficiency from 27 to 12 % ± 4.93. Moreover, increasing the ratio of CO2 in H2/CO2 gas inflow had negative impacts on biomass growth and MER, while bioreactors connected in series method increased gas conversion efficiency. The results of nutrient supple-mentation have shown that regular medium replacement (120 mL per week) is needed to achieve a stable MER, though higher media replenishment (360 mL per week) did not improve BM performance. All in all, our findings have shown that reactor configuration, agitation speed, gas inflow optimization and providing stable nutrient supplementation are key parameters for M. maripaludis to achieve efficient biomass growth and BM. Further research is required to explore the impacts of other parameters such as selective nutrient supplementation, lower gas inflow rates, higher mixing speed and having pressure within the system on biomass growth and BM efficiency.