Effects of abrupt and gradual increase of atmospheric CO2 concentration on methanotrophs in paddy fields

Elevated CO2 and goethite inhibited anaerobic oxidation of methane in paddy soils

Microbially mediated anaerobic oxidation of methane (AOM) regulates methane (CH4) fluxes. Increases in the global atmospheric carbon dioxide (CO2) concentration and iron oxide rich in paddy soils influence AOM. However, the response and mechanisms between these two processes and AOM remain unclear. Here, we investigated the coupling of elevated atmospheric CO2 concentrations (ambient CO2 + 200 ppm) and goethite with AOM via 13CH4 isotope tracer techniques and explored the dynamics of bacterial and archaeal communities by high-throughput sequencing. The coupling of 13CH4 with electron acceptors generates 13CO2, and its enrichment was used to evaluate CH4 oxidation. The results showed that elevated atmospheric CO2 and the addition of goethite resulted in a significant decrease of 13CO2 value produced from 13CH4 oxidation, thereby inhibiting CH4 oxidation. In addition, both elevated atmospheric CO2 and goethite addition increased the Richness, Shannon, and ACE indices of bacteria to varying degrees, whereas the diversity of archaea exhibited the opposite pattern. Additionally, the microbial community composition was significantly altered. Overall, the negative response of elevated atmospheric CO2 and goethite to AOM may guide CH4 emissions reduction from paddy soils under global warming and climate change, as well as the formulation of environmental policies such as carbon budgets for farmland.

Recent findings in methanotrophs: genetics, molecular ecology, and biopotential

The potential consequences for mankind could be disastrous due to global warming, which arises from an increase in the average temperature on Earth. The elevation in temperature primarily stems from the escalation in the concentration of greenhouse gases (GHG) such as CO2, CH4, and N2O within the atmosphere. Among these gases, methane (CH4) is particularly significant in driving alterations to the worldwide climate. Methanotrophic bacteria possess the distinctive ability to employ methane as both as source of carbon and energy. These bacteria show great potential as exceptional biocatalysts in advancing C1 bioconversion technology. The present review describes recent findings in methanotrophs including aerobic and anaerobic methanotroph bacteria, phenotypic characteristics, biotechnological potential, their physiology, ecology, and native multi-carbon utilizing pathways, and their molecular biology. The existing understanding of methanogenesis and methanotrophy in soil, as well as anaerobic methane oxidation and methanotrophy in temperate and extreme environments, is also covered in this discussion. New types of methanogens and communities of methanotrophic bacteria have been identified from various ecosystems and thoroughly examined for a range of biotechnological uses. Grasping the processes of methanogenesis and methanotrophy holds significant importance in the development of innovative agricultural techniques and industrial procedures that contribute to a more favorable equilibrium of GHG. This current review centers on the diversity of emerging methanogen and methanotroph species and their effects on the environment. By amalgamating advanced genetic analysis with ecological insights, this study pioneers a holistic approach to unraveling the biopotential of methanotrophs, offering unprecedented avenues for biotechnological applications. KEY POINTS: • The physiology of methanotrophic bacteria is fundamentally determined. • Native multi-carbon utilizing pathways in methanotrophic bacteria are summarized. • The genes responsible for encoding methane monooxygenase are discussed.

Methanogenic and methanotrophic communities determine lower CH4 fluxes in a subtropical paddy field under long-term elevated CO2

Clarifying the effects of elevated CO2 concentration (e[CO2]) on CH4 emissions from paddy fields and its mechanisms is a crucial part of the research on agricultural systems in response to global climate change. However, the response of CH4 fluxes from rice fields to long-term e[CO2] (e[CO2] duration >10 years) and its microbial mechanism is still lacking. In this study, we used a long-term free-air CO2 enrichment experiment to examine the relationship between CH4 fluxes and the methanogenic and methanotrophic consortia under long- and short-term e[CO2]. We demonstrated that contrary to the effect of short-term e[CO2], long-term e[CO2] decreased CH4 fluxes. This may be associated with the reduction of methanogenic abundance and the increase of methanotrophic abundance under long-term e[CO2]. In addition, long-term e[CO2] also changed the community structure and composition of methanogens and methanotrophs compared with short-term e[CO2]. Partial least squares path modeling analysis showed that long-term e[CO2] also could affect the abundance and composition of methanogens and methanotrophs indirectly by influencing soil physical and chemical properties, thereby ultimately altering CH4 fluxes in paddy soils. These findings suggest that current estimates of short-term e[CO2]-induced CH4 fluxes from paddy fields may be overestimated. Therefore, a comprehensive assessment of climate‑carbon cycle feedbacks may need to consider the microbial regulation of CH4 production and oxidation processes in paddy ecosystems under long-term e[CO2].

Comparing the variations and influencing factors of CH4 emissions from paddies and wetlands under CO2 enrichment: A data synthesis in the last three decades

Understanding and quantifying the impact of elevated tropospheric carbon dioxide concentration (e [CO₂]) on methane (CH₄) globally is important for effectively assessing and mitigating climate warming. Paddies and wetlands are the two important sources of CH₄ emissions. Yet, a quantitative synthetic investigation of the effects of e [CO₂] on CH₄ emissions from paddies and wetlands on a global scale has not been conducted. Here, we conducted a meta-analysis of 488 observation cases from 40 studies to assess the long-term effects of e [CO₂] (ambient [CO₂]+ 53-400 μmol mol^-1) on CH₄ emissions and to identify the relevant key drivers. On aggregate, e [CO₂] increased CH₄ emissions by 25.7% (p < 0.05) from paddies but did not affect CH₄ emissions from wetlands (-3.29%; p > 0.05). The e [CO₂] effects on paddy CH₄ emissions were positively related to that on belowground biomass and soil-dissolved CH₄ content. However, these factors under e [CO₂] resulted in no significant change in CH₄ emissions in wetlands. Particularly, the e [CO₂]-induced abundance of methanogens increased in paddies but decreased in wetlands. In addition, tillering number of rice and water table levels affected e [CO₂]-induced CH₄ emissions in paddies and wetlands, respectively. On a global scale, CH₄ emissions changed from an increase (+0.13 and + 0.86 Pg CO₂-eq yr^-1) under short-term e [CO₂] into a decrease and no changes (-0.22 and + 0.03 Pg CO₂-eq yr^-1) under long-term e [CO₂] in paddies and wetlands, respectively. This suggested that e [CO₂]-induced CH₄ emissions from paddies and wetlands changed over time. Our results not only shed light on the different stimulative responses of CH₄ emissions to e [CO₂] from paddy and wetland ecosystems but also suggest that estimates of e [CO₂]-induced CH₄ emissions from global paddies and wetlands need to account for long-term changes in various regions.