Regulatory mechanisms of CH4 : air volume ratios on metabolic flux partitioning in methane-oxidizing bacteria and their impact on single cell protein biosynthetic efficiency
Year: 2026
Type: article
Abstract: Introduction C1 gas bioconversion for single-cell protein (SCP) production offers dual environmental benefits by mitigating greenhouse gases and generating protein resources. This study systematically determined optimal methane-to-air ratios (CH 4 :air, v/v) for enhancing methane-oxidizing bacteria (MOB) growth and SCP yield under three distinct nitrogen assimilation modes: nitrate-driven pMMO expression, ammonium-driven sMMO expression, and nitrogen-fixing sMMO expression. Methods Experiments were conducted under three nitrogen assimilation regimes: nitrate-fed pMMO expression, ammonium-fed sMMO expression, and nitrogen-fixing sMMO expression systems. By adjusting the volumetric ratio of methane to air, the effects on bacterial growth, biomass accumulation, specific growth rate, and key enzymatic activities were evaluated. Measured parameters included OD 600 , cell dry weight, specific growth rate (μ max ), and nitrogenase activity in the nitrogen-fixing system. Data from repeated measurements were subjected to statistical analysis to clarify the regulatory role of gas ratios on metabolic pathways. Results In the nitrate-fed pMMO expression system, a CH 4 :air ratio of 1:3 yielded optimal growth, with an OD 600 of 1.11, cell dry weight of 0.44 ± 0.023 g/L, and μ max of 0.022 h −1 . Similarly, the ammonium-fed sMMO expression system achieved best performance at the same ratio (OD 600 1.19, biomass 0.56 ± 0.014 g/L, μ max 0.025 h −1 ). In contrast, the nitrogen-fixing sMMO expression system performed better at a lower oxygen ratio (CH 4 :air = 1:2), reaching an OD 600 of 0.62, biomass of 0.28 ±0.008 g/L, nitrogenase activity of 1.09 nmol/(min mg protein), and μ max of 0.016 h −1 ). Discussion The results reveal oxygen's critical dual role: higher O 2 levels enhance methane oxidation by activating the copper-dependent catalytic site of pMMO but simultaneously and irreversibly damage the oxygen-sensitive nitrogenase ssential for N 2 fixation, suppressing its activity. Conversely, lower O 2 protects nitrogenase but limits pMMO efficiency. This creates a fundamental metabolic trade-off where the optimal CH 4 /O 2 ratio balances these opposing effects, strategically partitioning cellular energy either toward efficient methane assimilation (favored by higher O 2 ) or toward the ATP-intensive process of nitrogen fixation (requiring lower O 2 ). These identified gas-ratio thresholds provide actionable parameters for designing scaled SCP bioproduction systems, enabling effective coupling of industrial methane mitigation with sustainable protein synthesis through gas-phase engineering.
Source: Frontiers in Microbiology
Language: English
Subfield: Molecular Biology
Domain: Life Sciences
SDG: Climate action
Open Access status: gold
Estimated APC paid: $3,801
OpenAlex ID: W7125385283