Iron cycling, including reduction of Fe(III) and oxidation of Fe(II), involves the formation, transformation, and dissolution of minerals and dissolved iron-organic matter compounds. It has been shown previously that Fe can be cycled so rapidly that no measurable changes in Fe(II) and Fe(III) concentrations occur, leading to a so-called cryptic cycle. Cryptic Fe cycles have been shown to be driven either abiotically by a combination of photochemical reduction of Fe(III)-OM complexes and reoxidation of Fe(II) by O ₂ , or microbially by a combination of Fe(III)-reducing and Fe(II)-oxidizing bacteria. Our study demonstrates a new type of light-driven cryptic Fe cycle that is relevant for the photic zone of aquatic habitats involving abiotic photochemical reduction of Fe(III)-OM complexes and microbial phototrophic Fe(II) oxidation. This new type of cryptic Fe cycle has important implications for biogeochemical cycling of iron, carbon, nutrients, and heavy metals and can also influence the composition and activity of microbial communities.

Urban wetlands, although often overlooked, are hotspots for CH ₄ cycling. However, the understanding of anaerobic CH ₄ oxidation and microbial responses to different electron acceptors in urban wetlands remains limited. Here, we employed DNA-stable isotope probing (SIP) and metagenomic techniques to identify the core CH ₄ -oxidizing microbial groups in the presence of NO ₃ ^- , SO ₄ ^2- , or ferrihydrite (Fe(III)) in anoxic urban-lake sediments. The addition of NO ₃ ^- to sediment microcosms promoted CH ₄ oxidation, whereas SO ₄ ^2- and Fe(III) had minimal or inhibitory effects on CH ₄ oxidation rates in anoxic microcosms. The SIP-identified methanotrophs in response to the addition of CH ₄ comprised the Marine Benthic Group D (MBG-D), “ Candidatus Methylomirabilis Sh765B-TzT-35 and Z114MB74,” and the putatively aerobic methanotrophs Methylobacter and Methylocystis . The presence of NO ₃ ^- further enriched Methylocaldum , Methylomonas , ‘ Ca . Methylomirabilis Sh765B-TzT-35 and Z114MB74’, Methylococcaceae , and Methylospira . In contrast, SO ₄ ^2- and Fe(III) amendments notably shifted the dominant methanotrophs through the significant labeling of Methylocystis . Furthermore, metagenomic analysis of ¹³ C-labeled DNA showed enrichment of genes for oxidation of CH ₄ and its intermediates, as well as the reduction of NO ₃ ^- , SO ₄ ^2- , Fe(III), and fermentation. These results underscore the activity of NO ₃ ^- -dependent CH ₄ -oxidizing microorganisms and highlight their potential metabolic versatility in highly anthropogenically influenced urban wetlands. IMPORTANCE: Urban wetlands are critical ecosystems for CH ₄ cycling but are increasingly impacted by complex pollutants from urban development, such as nitrates, sulfates, and Fe(III) from industrial runoff, atmospheric deposition, and wastewater discharge. This study reveals how these pollutants act as electron acceptors, modulating microbial metabolic pathways and reshaping methanotrophic communities under anoxic conditions. By uncovering the microbial mechanisms driving CH₄ oxidation in urban wetland sediments, our findings provide a deeper understanding of how anthropogenic pollution alters biogeochemical cycles. These insights are crucial for developing targeted strategies to mitigate CH₄ emissions and improve greenhouse gas control in rapidly urbanizing regions.

Diesel spills in freshwater systems have adverse impacts on the water quality and the shore wetland. Microbial degradation is the major and ultimate natural mechanism that can clean the diesel from the environment. However, which, and how fast, diesel-degrading microorganisms could degrade spilled diesel has not been well-documented in river water. Using a combination of ¹⁴C-/³H--based radiotracer assays, analytical chemistry, MiSeq sequencing, and simulation-based microcosm incubation approaches, we demonstrated succession patterns of microbial diesel-degrading activities, and bacterial and fungal community compositions. The biodegradation activities of alkanes and polycyclic aromatic hydrocarbons (PAHs) were induced within 24 h after diesel addition, and reached their maximum after incubation for 7 days. Potential diesel-degrading bacteria Perlucidibaca, Acinetobacter, Pseudomonas, Acidovorax, and Aquabacterium dominated the community initially (day 3 and day 7), but later community structure (day 21) was dominated by bacteria Ralstonia and Planctomyces. The key early fungi responders were Aspergillus, Mortierella, and Phaeoacremonium by day 7, whereas Bullera and Basidiobolus dominated the fungal community at day 21. These results directly characterize the rapid response of microbial community to diesel spills, and suggest that the progression of diesel microbial degradation is performed by the cooperative system of the versatile obligate diesel-degrading and some general heterotrophic microorganisms in river diesel spills.

Microbial Fe(III) reduction significantly influences the fate of various elements and contaminants. Previous research has employed different Fe(III)-OM complexes and ferrihydrite to study Fe(III)-reduction-related biogeochemistry processes. However, the effects of adding specific Fe(III)-OM complexes and ferrihydrite on the Fe(III)-reducing bacterial community, Fe(III)-reducing kinetics, and Fe(III)-related functional genes remain largely unexplored. This study applied microcosm experiments and metagenomic analysis of lake sediments with and without amendments of ferrihydrite, Fe(III)-citrate, or Fe(III)-EDTA. Results showed that sediments amended with Fe(III)-citrate and Fe(III)-EDTA exhibited faster Fe(III) reduction rates and more significant changes in bacterial community structures compared to those amended with ferrihydrite. Geobacter and Clostridium were enriched in the sediments amended with Fe(III)-EDTA and Fe(III)-citrate, respectively. Despite a slower reduction rate and lack of enrichment of specific Fe(III)-reducing bacteria, ferrihydrite still led to an increase in the copy numbers of genes related to Fe(III) reduction and iron assimilation in the metagenomes, suggesting an increase in these capacities. These results suggest that introducing various Fe(III)-OM complexes and ferrihydrite into the environment would result in differences in not only Fe(III) reduction rates and Fe(III)-reducing bacterial communities but also in iron-related functional genes. Meanwhile, variations in Fe(III) reduction rates and Fe(III)-reducing bacterial communities do not necessarily correlate with changes in the abundances of functional genes relevant to Fe(III) reduction and iron assimilation in the metagenomes. These results provide a better understanding of the adaptive mechanisms of Fe(III)-reducing bacteria in different environmental systems.