In petroleum-contaminated aquifers, iron (III) serves as an electron acceptor, enabling microbial degradation of organic matter. While previous studies have focused on iron reduction and organic matter degradation under laboratory conditions, research on iron-associated microorganisms in petroleum-contaminated aquifers is limited. To explore the diversity and distribution of such microorganisms in natural settings, this study used metagenomic analysis of an iron-rich, petroleum-contaminated aquifer. Sixteen groundwater samples from both pollution source and background areas were collected for species annotation and functional gene identification. Results show more than 7000 species were identified as iron-reducing microorganisms (IRMs), including several previously well-characterized iron-reducing species (e.g., Geobacter luticola and Geobacter sulfurreducens). However, the majority of IRMs were not found in existing iron-reducing microbial databases. Some of them, such as Sulfurospirillum sp. and Extensimonas perlucida, could be taxonomically classified at the species level, while most were only annotated as unclassified bacteria. In the contamination source zone, these microorganisms proliferated extensively, which led to hydrocarbon degradation predominantly driven by iron reduction in the aquifer. This study enhances our understanding of hydrocarbon-degrading microorganisms and supports the management of petroleum-contaminated sites.

Iron reduction is one of the most crucial biogeochemical processes in groundwater for organic contaminants biodegradation, especially in the iron-rich aquifers. Previous research has posited that the reduction of iron and the biodegradation of organic substances occur synchronously, with their processes adhering to specific quantitative relationships. However, discrepancies between the observed values of iron reduction and organic compound degradation during the reaction and their theoretical counterparts have been noted. To find out the relationship between organic substance biodegradation and iron reduction, this study conducted batch experiments utilizing toluene as a typical organic compound and electron donor, with various iron minerals serving as electron acceptors. Results indicate that toluene degradation follows first-order kinetic equations with different degradation rate constants under different iron minerals, but the generation of the iron reduction product Fe(II) was not uniform. Based on these dynamic relationships, a conceptual model was developed, which categorizes the reactions into two phases: the transformation of toluene to an intermediate-state dominated phase and the mineralization of the intermediate-state dominated phase. This model revealed the relationships between toluene oxidation and Fe(II) formation in the toluene biodegradation through iron reduction. The coupling mechanism of toluene degradation and iron reduction was revealed, which is expected to improve our ability to accurately assess the attenuation of organic contaminants in groundwater.

The Xiaojiang watershed in Luxi, Yunnan, is a typical rocky desertification area, in which karst groundwater pollution is severe and water resources are scarce. This article takes the watershed as an example and investigates the response mechanisms of surface karst spring water quality to agricultural pollution in rocky desertification areas. Specifically, the study was conducted as follows: (I) A total of 108 water samples from 54 sources were collected during the dry and wet seasons for analysis. (i) Principal component and correlation analyses identified the main pollution indicators in the soil surface karst zone of the area, including total bacterial count, total coliforms, COD, pH, and redox potential. (ii) It was also discovered that surface soil, impacted by agricultural activities, directly contributes to groundwater pollution in the soil surface karst zone. (II) Local soil was used to prepare soil columns under various conditions for simulation. The findings indicate: (i) Temperature significantly affects the surface karst springs, with higher temperatures leading to more pronounced water quality responses, increased enrichment of pathogen-microbiota indicators, and degraded water quality. (ii) Soil porosity substantially influences the water quality of surface karst springs. Increased porosity results in looser soil, more oxidizing conditions in the storage matrix, reduced pathogen-microbiota development, and consequently, less water pollution. This study offers theoretical and technical references for evaluating, monitoring, and issuing early warnings for pathogenic bacteria-microbiota pollution in groundwater in rocky desertification areas.