Ectoine production through a marine methanotroph-microalgae culture allows complete biogas valorization

Methanotrophs have recently emerged as a promising platform for producing bio-based chemicals, like ectoine, from biogas, offering an economical alternative to glucose. However, most studies have focused solely on CH₄ consumption, often overlooking the CO₂, which is both produced by methanotrophs and present in biogas, despite its potential as a carbon source for microorganisms, such as microalgae. In this study, marine methanotrophic-microalgal cultures were enriched from environmental samples collected at the North Sea coast to explore ectoine production from both CH₄ and CO₂ in biogas. The sediment-derived culture exhibited the highest CH₄ removal efficiency and CO₂ uptake, and was selected for further experiments. The culture was primarily composed of Methylobacter marinus, Methylophaga marina, and the microalga Picochlorum oklahomensis. Gas consumption, growth, and ectoine production were evaluated under varying salinity levels and osmotic stress. The NaCl concentrations above 6% negatively impacted CH₄ oxidation and inhibited ectoine synthesis, while osmotic shocks enhanced ectoine accumulation, with a maximum ectoine content of 51.3 mg_ectoine g_VSS^-1 at 4.5% NaCl. This study is the first to report ectoine production from methanotroph-microalgal cultures, showing its potential for biogas valorization into high-value bio-based chemicals, like ectoine, marking a significant step toward sustainable biogas utilization.