Restoration effects of submerged macrophytes on methane production and oxidation potential of lake sediments

Ecological restoration reduces greenhouse gas emissions by altering planktonic and sedimentary microbial communities in a shallow eutrophic lake

Ecological restoration is a promising approach to alleviate eutrophication. However, its impacts on greenhouse gas (GHG) emissions and the underlying microbial mechanisms in different habitats of lakes remain unclear. To address this knowledge gap, we measured carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes at both water-air and sediment-water interfaces of eutrophic (Caohai) and restored area (Dapokou) of Dianchi Lake, a typical eutrophic lake in China. Meanwhile, we investigated the responses of planktonic and sedimentary bacterial and fungal communities by high-throughput sequencing. Our results indicated that 6 years of ecological restoration significantly reduced CO2 and N2O fluxes by 1.0-3.6 and 2.2-2.8 folds respectively, with more pronounced variations at the water-air interface than the sediment-water interface. Ecological restoration also shifted the structures of planktonic bacterial and fungal communities remarkably, leading to a significant reduction in the relative abundances of Actinobacteriota (by 70.94%), Bacteroidota (by 61.65%), Planctomycetota (by 74.18%) and Chytridiomycota (by 95.44%). Correlation analyses further suggested that GHG fluxes at the water-air interface were significantly correlated with planktonic microbial community composition (P < 0.05), and the significant reduction of CO2 and N2O fluxes under ecological restoration could be attributed to the decreased abundances of organic matter decomposers (such as hgcI_clade, Sporichthyaceae and Acidibacter) and increased abundances of autotrophs (such as Hydrogenophaga and Cyanobium_PCC-6307) in water. Collectively, our findings verify the importance of ecological restoration in reducing GHG emissions in inland lake ecosystems, providing new insights for addressing global climate change and advancing carbon neutrality.

Iron forms regulate methane production and oxidation potentials in paddy soils

Paddy fields serve as significant sources of methane (CH4) emissions. The periodic flooding and draining in paddy soils induce alternating redox processes, leading to iron transformations and further influencing the production and oxidation of CH4. However, the relationships between CH4 production/oxidation and the concentrations/forms of iron oxides in rice paddies across different regions are largely unknown. Here we collected 26 paddy soil samples from various regions spanning from North to South China. We show that the CH4 production potential varies from 0.005 to 0.618 mg kg-1 d-1, which exhibits an overall trend of higher values in the south and lower values in the north. Moreover, the CH4 oxidation potential spans from 0.888 to 57.384 mg kg-1 d-1, showing no significant latitudinal trend. Highly weathered soils exhibit higher CH4 production potentials, mainly due to the high content of free iron oxides and the low reactivity of aged iron minerals. This hinders the protection of organic carbon (OC) by iron minerals, therefore increasing substrate availability for methanogenesis. In addition to the direct effect, iron forms also indirectly influence CH4 production and oxidation potentials by affecting soil pH, OC availability, and CH4-related microbial abundances. The coefficients of the indirect effect of iron forms on CH4 production and oxidation potential are 0.44 and 0.26, respectively, which are larger than that of the direct effects. Our research reveals the pivotal role of various iron forms in controlling CH4 production and oxidation processes in paddy soils, helping to expand the understanding of the effect of iron biogeochemistry on CH4 emissions in paddy soils and offering new perspectives for mitigating agricultural greenhouse gas emissions.

Overlooked drivers of the greenhouse effect: The nutrient-methane nexus mediated by submerged macrophytes

Submerged macrophytes remediation is a commonly used technique for improving water quality and restoring habitat in aquatic ecosystems. However, the drivers of success in the submerged macrophytes assembly process and their specific impacts on methane emissions are poorly understood. Thus, we conducted a mesocosm experiment to test the growth plasticity and carbon fixation of widespread submerged macrophytes (Vallisneria natans) under different nutrient conditions. A refined dynamic chamber method was utilized to concurrently collect and quantify methane emission fluxes arising from ebullition and diffusion processes. Significant correlations were found between methane flux and variations in the physiological activities of V. nantas by the fluorescence imaging system. Our results show that exceeding tolerance thresholds of ammonia in the water significantly interfered with the photosynthetic systems in submerged leaves and the radial oxygen loss in adventitious roots. The recovery process of V. natans accelerated the consumption of dissolved oxygen, leading to increase in the populations of methanogen (153.3 % increase of mcrA genes) and subsequently elevating CH4 emission fluxes (23.7 %) under high nutrient concentrations. Conversely, V. natans increased the available organic carbon under low nutrient conditions by radial oxygen loss, further increasing CH4 emission fluxes (94.7 %). Quantitative genetic and modeling analyses revealed that plant restoration processes drive ecological niche differentiation of methanogenic and methane oxidation microorganisms, affecting methane release fluxes within the restored area. The speciation process of V. natans is incapable of simultaneously meeting improved water purification and reduced methane emissions goals.

Nitrite-dependent anaerobic methane oxidation (N-DAMO) in global aquatic environments: A review

The vast majority of processes in the carbon and nitrogen cycles are driven by microorganisms. The nitrite-dependent anaerobic oxidation of methane (N-DAMO) process links carbon and nitrogen cycles, offering a novel approach for the simultaneous reduction of methane emissions and nitrite pollution. However, there is currently no comprehensive summary of the current status of the N-DAMO process in natural aquatic environments. Therefore, our study aims to fill this knowledge gap by conducting a comprehensive review of the global research trends in N-DAMO processes in various aquatic environments (excluding artificial bioreactors). Our review mainly focused on molecular identification, global study sites, and their interactions with other elemental cycling processes. Furthermore, we performed a data integration analysis to unveil the effects of key environmental factors on the abundance of N-DAMO bacteria and the rate of N-DAMO process. By combining the findings from the literature review and data integration analysis, we proposed future research perspectives on N-DAMO processes in global aquatic environments. Our overarching goal is to advance the understanding of the N-DAMO process and its role in synergistically reducing carbon emissions and removing nitrogen. By doing so, we aim to make a significant contribution to the timely achievement of China's carbon peak and carbon neutrality targets.

Microbial communities drive flux of acid orange 7 and crystal violet dyes in water-sediment system

Unchecked dye effluent discharge poses escalating environmental and economic concerns, especially in developing nations. While dyes are well-recognized water pollutants, the mechanisms of their environmental spread are least understood. Therefore, the present study examines the partitioning of Acid Orange 7 (AO7) and Crystal Violet (CV) dyes using water-sediment microcosms and reports that native microbes significantly affect AO7 decolorization and transfer. Both dyes transition from infused to pristine matrices, reaching equilibrium in a fortnight. While microbes influence CV partitioning, their role in decolorization is minimal, emphasizing their varied impact on the environmental fate of dyes. Metagenomic analyses reveal contrasting microbial composition between control and AO7-infused samples. Control water samples displayed a dominance of Proteobacteria (62%), Firmicutes (24%), and Bacteroidetes (9%). However, AO7 exposure led to Proteobacteria reducing to 57% and Bacteroidetes to 3%, with Firmicutes increasing to 34%. Sediment samples, primarily comprising Firmicutes (47%) and Proteobacteria (39%), shifted post-AO7 exposure: Proteobacteria increased to 53%, and Firmicutes dropped to 38%. At the genus level, water samples dominated by Niveispirillum (34%) declined after AO7 exposure, while Bacillus and Pseudomonas increased. Notably, Serratia and Sphingomonas, known for azo dye degradation, rose post-exposure, hinting at their role in AO7 decolorization. Conversely, sediment samples showed a decrease in the growth of Bacillus and an increase in that of Pseudomonas and Serratia. These findings emphasize the significant role of microbial communities in determining the environmental fate of dyes, providing insights on its environmental implications and management.

Variable promotion of algae and macrophyte organic matter on methanogenesis in anaerobic lake sediment

Shallow lakes are an important natural source of atmospheric methane (CH4), and the input of autochthonous organic matter (OM) into their sediments encourages methanogenesis. Although algal- and macrophytic-originated OM in these lakes are expected to have different impacts on methanogenesis and methanogenic archaeal communities in lake sediments owing to their various properties, their specific influence and role in sediment remain unclear. In this study, a 148-day incubation was carried out by adding algal- and macrophytic-OM to the sediments of shallow eutrophic Lake Chaohu and Lake Taihu in China. CH4 was periodically monitored, while the methanogens were examined via qPCR and high-throughput sequencing at the end of incubation. Algal-OM stimulated CH4 production more than macrophytic-OM in both sediments, with the rates initially increasing and then decreasing before reaching a relative constant. Macrophytic-OM promoted CH4 production to a comparable extent in both lakes, while algal-OM promoted greater CH4 in Lake Chaohu than in Lake Taihu. However, algal-OM did not significantly increase mcrA gene copies, while macrophytic-OM did by 17.0-20.1-fold. Algal-OM potentially promoted the methylotrophic pathway in Lake Taihu but did not change the methanogenic structure in Lake Chaohu. Comparatively, macrophytic-OM promoted CH4 production mainly by acetoclastic methanogen proliferation in both lakes. More CH4 release with algal-OM compared to macrophytic-OM deserves further attention owing to the prevailing increasing algal blooms and the declining macrophyte population in lakes.

Role of hydrophytes in constructed wetlands for nitrogen removal and greenhouse gases reduction

With the prominence of global climate change and proposal of carbon reduction concept, how to maximize the comprehensive effect of nitrogen removal and greenhouse gases (GHGs) reduction in constructed wetlands (CWs) has become crucial. As indispensable biological component of CWs, hydrophytes have received extensive attention owing to their application potential. This review comprehensively evaluates the functions of hydrophytes in nitrogen removal and GHGs reduction in CWs in terms of plants themselves, plant-mediated microbes and plant residues (hydrophyte carbon sources and hydrophyte-derived biochars). On this basis, the strategies for constructing an ideal CW system are put forward from the perspective of full life-cycle utilization of hydrophytes. Finally, considering the variability of plant species composition in CWs, outlooks for future research are specifically proposed. This review provides guidance and novel perspectives for the full life-cycle utilization of hydrophytes in CWs, as well as for the construction of an ideal CW system.