Methane and CO2 consumption from a synthetic waste gas by microbial communities in enriched seawater

Abstract

Methane (CH₄) and carbon dioxide (CO₂) are potent greenhouse gases produced as waste in carbon-based fuel processes. This study investigates the use of natural microbial communities to consume CH₄ and CO₂ and convert these gases into biomass. Seawater enriched with nutrients was exposed to a gas stream containing CH₄ and CO₂ under either light or dark conditions. The microbial communities that developed included methanotrophic bacteria consuming CH₄ and cyanobacteria and microalgae consuming CO₂. Chemotaxonomic markers showed that phototrophic growth increased significantly only in the light, with an early dominance by cyanobacteria later overtaken by microalgae, while methanotrophic growth increased significantly only in the dark. Near-full-length 16S and 18S rRNA gene sequencing using Nanopore technology revealed that the microbial diversity in the incubated cultures was significantly reduced compared to the natural communities in the seawater used as inoculum. The most abundant phototrophs in the light-incubated cultures were green algae from the genera Picochlorum, Tetraselmis, Chlamydomonas, and Nannochloris, and a few cyanobacterial genera mostly from Cyanobacteriales and Synechococcales (SILVA taxonomy). Methylomicrobium and Methylobacter were the most abundant methanotrophs in the dark-incubated cultures, whereas Methylomonas methanica was the only methanotroph with notable abundance under light conditions. Methanol-oxidizing Methylophaga were also highly abundant in dark-incubated cultures suggesting that these organisms were also important carbon-oxidizers in the CH₄ consuming microbiomes. We conclude that optimal CH₄ and CO₂ consumption may require separating dark-dependent CH₄ and light-dependent CO₂ consuming microbiomes, or identifying symbiotic co-cultures of methanotrophs that are compatible with the light conditions needed by phototrophs. This research highlights potential microbial candidates for reducing the climate impact of flare gas and other waste gases containing CH₄ and CO₂.