Sulfide restrains the growth of Methylocapsa acidiphila converting renewable biogas to single cell protein

Abstract

Methane-oxidizing bacteria (MOB) that can use biogas and recycled nitrogen from wastewater as a sustainable feedstock for single cell protein (SCP) synthesis are receiving increasing attention. Though promising, limited knowledge is available on the alternative strains especially the ones that can tolerant to strict environments such as acidic conditions. Furthermore, how would the hydrogen sulfide affect the MOB (especially the alternative strains) for SCP synthesis when crude biogas is used as feedstock is still unknown. In this study, the capability of an acidic-tolerant methanotrophic bacterium Methylocapsa acidiphila for SCP production using raw biogas and the associated inhibitory effect of sulfide on the bioconversion was for the first time investigated. Results showed that the inhibitory effect of sulfide on the growth of M. acidiphila was observed starting from 8.13 mg L−1 Na2S (equivalent to approximately 1000 ppm of H2S in crude biogas). The total amino acid content in the dry biomass decreased more than two times due to sulfide inhibition compared with the control samples without the presence of sulfide (585.96 mg/g dry biomass), while the proportion of essential amino acids in the total amino acid was not affected when the concentration of Na2S was lower than 5.73 mg L−1. The performance of M. acidiphila in a sulfide-rich environment was further studied at different operational conditions. The feeding gas with a CH4/O2 ratio of 6:4 could help to alleviate the sulfide inhibition, compared with other ratios (4:6 and 8:2). Moreover, the sequential supply of the feed gas could also alleviate sulfide inhibition. In addition, the MOB’s growth rate was higher when applying a higher mixing rate of 120 rpm, compared with 70 rpm and 0, due to a better gas-liquid mass transfer. The inoculum size of 20% and 10% resulted in a faster growth rate compared with the 5%. Furthermore, M. acidiphila could assimilate either NH4+ or NO3− as nitrogen source with a similar growth rate, giving it the potential to recycle nitrogen from a wide range of wastewaters. The results will not only create new knowledge for better understanding the role of hydrogen sulfide in the assimilation of raw biogas by acid-tolerant M. acidiphila but also provide technical insights into the development of an efficient and robust process for the waste-to-protein conversion.