Effects of CO2 leakage on soil bacterial communities from simulated CO2-EOR areas

Adding carbon sources to the substrates enhances Cr and Ni removal and mitigates greenhouse gas emissions in constructed wetlands

Constructed wetlands for wastewater treatment pose challenges related to long-term operational efficiency and greenhouse gas emissions on a global scale. This study investigated the impact of adding peat, humic acid, and biochar into the substrates of constructed wetlands and focused on Cr, and Ni removal, greenhouse gas emissions, and microbial communities in constructed wetlands. Biochar addition treatment achieved the highest removal efficiencies for total Cr (99.96%), Cr (VI) (100%), and total Ni (91.04%). Humic acid and biochar addition both significantly increased the heavy metal content in wetland plant Leersia hexandra and substrates of constructed wetlands. Further analysis of microbial community proportions by high-throughput sequencing revealed that biochar and humic acid treatments enhanced Cr and Ni removal efficiency by increasing the abundance of Bacteroidetes, Geobacter and Ascomycota. Humic acid addition treatment reduced CO2 emissions by decreasing the abundance of Bacteroidetes and increasing that of Basidiomycota. Peat treatment decreased CH4 emissions by reducing the abundance of the Bacteroidetes. Biochar treatment increased the abundance of the Firmicutes, Bacteroidetes, Proteobacteria as well as Basidiomycota, resulting in reduced N2O emissions. Biochar and humic acid treatments efficiently removed heavy metals from wastewater and mitigated greenhouse gas emissions in constructed wetlands by modifying the microbial communities.

Enrichment of rare methanogenic Archaea shows their important ecological role in natural high-CO2 terrestrial subsurface environments

Introduction

Long-term stability of underground CO₂ storage is partially affected by microbial activity but our knowledge of these effects is limited, mainly due to a lack of sites. A consistently high flux of mantle-derived CO₂ makes the Eger Rift in the Czech Republic a natural analogue to underground CO₂ storage. The Eger Rift is a seismically active region and H₂ is produced abiotically during earthquakes, providing energy to indigenous microbial communities.

Methods

To investigate the response of a microbial ecosystem to high levels of CO₂ and H₂, we enriched microorganisms from samples from a 239.5 m long drill core from the Eger Rift. Microbial abundance, diversity and community structure were assessed using qPCR and 16S rRNA gene sequencing. Enrichment cultures were set up with minimal mineral media and H₂/CO₂ headspace to simulate a seismically active period with elevated H₂.

Results and discussion

Methane headspace concentrations in the enrichments indicated that active methanogens were almost exclusively restricted to enrichment cultures from Miocene lacustrine deposits (50-60 m), for which we observed the most significant growth. Taxonomic assessment showed microbial communities in these enrichments to be less diverse than those with little or no growth. Active enrichments were especially abundant in methanogens of the taxa Methanobacterium and Methanosphaerula. Concurrent to the emergence of methanogenic archaea, we also observed sulfate reducers with the metabolic ability to utilize H₂ and CO₂, specifically the genus Desulfosporosinus, which were able to outcompete methanogens in several enrichments. Low microbial abundance and a diverse non-CO₂ driven microbial community, similar to that in drill core samples, also reflect the inactivity in these cultures. Significant growth of sulfate reducing and methanogenic microbial taxa, which make up only a small fraction of the total microbial community, emphasize the need to account for rare biosphere taxa when assessing the metabolic potential of microbial subsurface populations. The observation that CO₂ and H₂-utilizing microorganisms could only be enriched from a narrow depth interval suggests that factors such as sediment heterogeneity may also be important. This study provides new insight on subsurface microbes under the influence of high CO₂ concentrations, similar to those found in CCS sites.

Molecular Ecological Network Complexity Drives Stand Resilience of Soil Bacteria to Mining Disturbances among Typical Damaged Ecosystems in China

Understanding the interactions of soil microbial species and how they responded to disturbances are essential to ecological restoration and resilience in the semihumid and semiarid damaged mining areas. Information on this, however, remains unobvious and deficiently comprehended. In this study, based on the high throughput sequence and molecular ecology network analysis, we have investigated the bacterial distribution in disturbed mining areas across three provinces in China, and constructed molecular ecological networks to reveal the interactions of soil bacterial communities in diverse locations. Bacterial community diversity and composition were classified measurably between semihumid and semiarid damaged mining sites. Additionally, we distinguished key microbial populations across these mining areas, which belonged to Proteobacteria, Acidobacteria, Actinobacteria, and Chloroflexi. Moreover, the network modules were significantly associated with some environmental factors (e.g., annual average temperature, electrical conductivity value, and available phosphorus value). The study showed that network interactions were completely different across the different mining areas. The keystone species in different mining areas suggested that selected microbial communities, through natural successional processes, were able to resist the corresponding environment. Moreover, the results of trait-based module significances showed that several environmental factors were significantly correlated with some keystone species, such as OTU_8126 (Acidobacteria), OTU_8175 (Burkholderiales), and OTU_129 (Chloroflexi). Our study also implied that the complex network of microbial interaction might drive the stand resilience of soil bacteria in the semihumid and semiarid disturbed mining areas.

Potential CO2 intrusion in near-surface environments: a review of current research approaches to geochemical processes

Carbon dioxide (CO₂) capture and storage (CCS) plays a crucial role in reducing carbon emissions to the atmosphere. However, gas leakage from deep storage reservoirs, which may flow back into near-surface and eventually to the atmosphere, is a major concern associated with this technology. Despite an increase in research focusing on potential CO₂ leakage into deep surface features and aquifers, a significant knowledge gap remains in the geochemical changes associated with near-surface. This study reviews the geochemical processes related to the intrusion of CO₂ into near-surface environments with an emphasis on metal mobilization and discusses about the geochemical research approaches, recent findings, and current knowledge gaps. It is found that the intrusion of CO₂(g) into near-surface likely induces changes in pH, dissolution of minerals, and potential degradation of surrounding environments. The development of adequate geochemical research approaches for assessing CO₂ leakage in near-surface environments, using field studies, laboratory experiments, and/or geochemical modeling combined with isotopic tracers, has promoted extensive surveys of CO₂-induced reactions. However, addressing knowledge gaps in geochemical changes in near-surface environments is fundamental to advance current knowledge on how CO₂ leaks from storage sites and the consequences of this process on soil and water chemistry. For reliable detection and risk management of the potential impact of CO₂ leakage from storage sites on the environmental chemistry, currently available geochemical research approaches should be either combined or used independently (albeit in a manner complementarily to one another), and the results should be jointly interpreted.

Impacts of Elevated CO2 Levels on the Soil Bacterial Community in a Natural CO2-Enhanced Oil Recovery Area

Knowledge of the interactions among different microorganisms is important to understand how ecological function transformation is affected by elevated CO2 levels in CO2-enhanced oil recovery (CO2-EOR) sites. Molecular ecological networks were established to reveal the interactions among different microbes of the soil bacterial community with the high-throughput sequencing data of 16S rRNA genes. The results showed that these networks are a powerful tool to identify and explain the interactions and keystone species in the communities under elevated CO2 pressure. The structures of networks under different CO2 leakage concentrations were different as a result of the networks’ topology properties, such as node numbers, topological roles of individual nodes, and network hubs. These indicators imply that the interactions among different groups were obviously changed. Moreover, changes in the network structure were significantly correlated with soil pH value, which might suggest that the large CO2 leakage affected the soil ecosystem functions by changing the network interactions. Additionally, the key microbial populations such as Bacteroidetes and Proteobacteria were distinguished based on network topology to reveal community structure and ecosystem functioning. The work developed in this study could help microbiologists to address some research questions that could not be approached previously, and, hence, might represent a new area of research for microbial ecology.

Effects of elevated carbon dioxide on environmental microbes and its mechanisms: A review

Before the industrial revolution, the atmospheric CO₂ concentration was 180-330 ppm; however, fossil-fuel combustion and forest destruction have led to increased atmospheric CO₂ concentration. CO₂ capture and storage is regarded as a promising strategy to prevent global warming and ocean acidification and to alleviate elevated atmospheric CO₂ concentration, but the leakage of CO₂ from storage system can lead to rapid acidification of the surrounding circumstance, which might cause negative influence on environmental microbes. The effects of elevated CO₂ on microbes have been reported extensively, but the review regarding CO₂ affecting different environmental microorganisms has never been done previously. Also, the mechanisms of CO₂ affecting environmental microorganisms are usually contributed to the change of pH values, while the direct influences of CO₂ on microorganisms were often neglected. This paper aimed to provide a systematic review of elevated CO₂ affecting environmental microbes and its mechanisms. Firstly, the influences of elevated CO₂ and potential leakage of CO₂ from storage sites on community structures and diversity of different surrounding environmental microbes were assessed and compared. Secondly, the adverse impacts of CO₂ on microbial growth, cell morphology and membranes, bacterial spores, and microbial metabolism were introduced. Then, based on biochemical principles and knowledge of microbiology and molecular biology, the fundamental mechanisms of the influences of carbon dioxide on environmental microbes were discussed from the aspects of enzyme activity, electron generation and transfer, and key gene and protein expressions. Finally, key questions relevant to the environmental effect of CO₂ that need to be answered in the future were addressed.

Short-term effects of CO2 leakage on the soil bacterial community in a simulated gas leakage scenario

The technology of carbon dioxide (CO₂) capture and storage (CCS) has provided a new option for mitigating global anthropogenic emissions with unique advantages. However, the potential risk of gas leakage from CO₂ sequestration and utilization processes has attracted considerable attention. Moreover, leakage might threaten soil ecosystems and thus cannot be ignored. In this study, a simulation experiment of leakage from CO₂ geological storage was designed to investigate the short-term effects of different CO₂ leakage concentration (from 400 g m⁻² day⁻¹ to 2,000 g m⁻² day⁻¹) on soil bacterial communities. A shunt device and adjustable flow meter were used to control the amount of CO₂ injected into the soil. Comparisons were made between soil physicochemical properties, soil enzyme activities, and microbial community diversity before and after injecting different CO₂ concentrations. Increasing CO₂ concentration decreased the soil pH, and the largest variation ranged from 8.15 to 7.29 (p < 0.05). Nitrate nitrogen content varied from 1.01 to 4.03 mg/Kg, while Olsen-phosphorus and total phosphorus demonstrated less regular downtrends. The fluorescein diacetate (FDA) hydrolytic enzyme activity was inhibited by the increasing CO₂ flux, with the average content varying from 22.69 to 11.25 mg/(Kg h) (p < 0.05). However, the increasing activity amplitude of the polyphenol oxidase enzyme approached 230%, while the urease activity presented a similar rising trend. Alpha diversity results showed that the Shannon index decreased from 7.66 ± 0.13 to 5.23 ± 0.35 as the soil CO₂ concentration increased. The dominant phylum in the soil samples was Proteobacteria, whose proportion rose rapidly from 28.85% to 67.93%. In addition, the proportion of Acidobacteria decreased from 19.64% to 9.29% (p < 0.01). Moreover, the abundances of genera Methylophilus, Methylobacillus, and Methylovorus increased, while GP4, GP6 and GP7 decreased. Canonical correlation analysis results suggested that there was a correlation between the abundance variation of Proteobacteria, Acidobacteria, and the increasing nitrate nitrogen, urease and polyphenol oxidase enzyme activities, as well as the decreasing FDA hydrolytic enzyme activity, Olsen-phosphorus and total phosphorus contents. These results might be useful for evaluating the risk of potential CO₂ leakages on soil ecosystems.