Effects of bioelectricity generation processes on methane emission and bacterial community in wetland and carbon fate analysis

Abstract

Wetlands are an important carbon sink for greenhouse gases (GHGs), and embedding microbial fuel cell (MFC) into constructed wetland (CW) has become a new technology to control methane (CH4) emission. Rhizosphere anode CW–MFC was constructed by selecting rhizome-type wetland plants with strong hypoxia tolerance, which could provide photosynthetic organics as alternative fuel. Compared with non-planted system, CH4 emission flux and power output from the planted CW–MFC increased by approximately 0.48 ± 0.02 mg/(m2·h) and 1.07 W/m3, respectively. The CH4 emission flux of the CW–MFC operated under open-circuit condition was approximately 0.46 ± 0.02 mg/(m2·h) higher than that under closed-circuit condition. The results indicated that plants contributed to the CH4 emission from the CW–MFC, especially under open-circuit mode conditions. The CH4 emission from the CW–MFC was proportional to external resistance, and it increased by 0.67 ± 0.01 mg/(m2·h) when the external resistance was adjusted from 100 to 1000 Ω. High throughput sequencing further showed that there was a competitive relationship between electrogenic bacteria and methanogens. The flora abundance of electrogenic bacteria was high, while methanogens mainly consisted of Methanothrix, Methanobacterium and Methanolinea. The form and content of element C were analysed from solid phase, liquid phase and gas phase. It was found that a large amount of carbon source (TC = 254.70 mg/L) was consumed mostly through microbial migration and conversion, and carbon storage and GHGs emission accounted for 60.38% and 35.80%, respectively. In conclusion, carbon transformation in the CW–MFC can be properly regulated via competition of microorganisms driven by environmental factors, which provides a new direction and idea for the control of CH4 emission from wetlands. Graphical Abstract

Recent advances in constructed wetlands methane reduction: Mechanisms and methods

“…MFC technology is a promising approach to the control of CH 4 emissions from CWs. For example, Liu et al (2022) reported that microbial competition in a CW-MFC system can convert unstable carbon sources to CO 2 rather than CH 4 , which can considerably reduce the contribution of CWs to global CH 4 emissions.…”

“…Anaerobic microorganisms play the major role in CH 4 production. Under anaerobic conditions, anaerobic hydrolytic microbes, fermentative microbes, and hydrogen-producing acetogens can decompose organic matter from wastewater, substrate materials, and plant biomass (Liu et al, 2022), forming simple inorganic (e.g., CO 2 and H 2 ) and organic compounds (e.g., acetate; López et al, 2019), that are subsequently converted to CH 4 by methanogens (specialized anaerobic archaea; Malyan et al, 2016). Anaerobic environments occur widely in the substrate layer of CWs.…”

“…The relationships between CH 4 emissions and plant occurrence and diversity in CWs remain unknown (Han et al, 2019), as plants are able to both produce CH 4 independently, and mediate or influence CH 4 emission pathways. For example, organic matter and root exudates (such as sugars and amino acids) synthesized by plants via photosynthesis, can provide electron donors (Chen et al, 2020c;Liu et al, 2022), while root exudates also release degradable carbon which increases the number of methanogens and methanotrophs (Zhang et al, 2018b). Furthermore, plant root-secreted O 2 can provide electron acceptors (Kasak et al, 2020), with the intensity of O 2 secretion varying depending on the plant species, biomass, temperature, O 2 concentration, and photointensity conditions (Feng et al, 2022).…”

“…Disruption to the CH 4 source-sink balance of CWs is a direct driver of dramatic increases in atmospheric CH 4 . In CW ecosystems, plants use assimilation to fix inorganic carbon from both the air and the water column (converting it into organic carbon), while also fixing organic carbon in the water column through substrate sequestration and uptake, resulting in wetlands serving as a carbon sink (Liu et al, 2022). When wetlands are subjected to long-term anaerobic conditions, plant debris, litter and the substrate gradually convert macromolecular organic matter into CH 4 and CO 2 , which is released into the atmosphere through anaerobic microbial metabolic activities, resulting in wetlands also serving as a carbon source (Yan et al, 2012).…”

“…Carbon uptake in vegetated wetlands decreases with increasing levels of salinity, mainly due to the inhibition of plant productivity (Sheng et al, 2015). Microbial transport and transformation are the main reason for the high carbon source consumption of CW-coupled microbial fuel cell (MFC) systems, with these processes regulated by microbial competition driven by environmental aspects, providing a novel approach to controlling CH 4 emissions from wetland systems (Liu et al, 2022). The contributions of carbon "source" and "sink" functions in CWs, as well as their relationship and interactions, are crucial to the material and energy cycles within CW ecosystems, global carbon dynamics and global climate change trends (Dreyfus et al, 2022).…”