Microbial Reduction of Iron, Manganese, and other Metals

Recent studies have highlighted the critical role of manganese (Mn) in plant litter decomposition and soil organic carbon (C) cycling in forest ecosystems. Long term nitrogen (N) deposition and N fertilization can increase soil acidity and mobilize bioavailable Mn (Mn²⁺) in soil. However, no studies have examined the interactive effect of N and Mn fertilization on litter decomposition and carbon distribution in agricultural soils, despite agroecosystems being subject to both N and Mn management. We hypothesized that increased soil N and Mn availability would accelerate plant residue decomposition and transfer of its C to mineral-associated organic matter (MAOM), and that the combined effect of Mn and N enrichment would be greater than the individual effect. We conducted a laboratory incubation experiment by adding ¹³C-labeled residue of perennial grass Glyceria striata (Lam.) to agricultural soils that had received 225 kg N ha⁻¹ yr⁻¹ for 27 years (N₁) and comparable soils that received no N (N₀). Before the experiment, these soils also received three levels of dissolved Mn²⁺, designated M₀ (no additional Mn), M₁ (50 mg kg⁻¹), or M₂ (250 mg kg⁻¹). We measured total CO₂ production as well as distribution of ¹³C from the residue into CO₂, particulate organic matter (POM), MAOM, and dissolved organic carbon (DOC) over a 1-year period. Manganese amendments significantly increased CO₂ production from residue decomposition in the N₁ soil, but no such effect was observed in the N₀ soil. Manganese also accelerated the loss of residue-derived C from POM and DOC, but increased its recovery in MAOM. However, the positive effect of added Mn in decomposition and recovery in MAOM in the presence of N fertilization occurred only during the initial 30-day decomposition period, where M₂ showed a 12% increase in cumulative CO₂ production from residue, 8% increase in POM loss, and 43% increase in recovery of residue C in MAOM compared to M₀. For M_1, only CO₂ emission from residue was significantly higher than Mo during this period. At 365 days M₂ showed 8% increase in CO₂ production, 1% increase in POM loss, and 16% increase in recovery of residue C in MAOM compared to M_0, but none of these were statistically significant (p < 0.05). This study adds to the growing evidence that increasing Mn availability enhances plant litter decomposition. However, the occurrence and magnitude of Mn-induced stimulation of decomposition is context specific. Further investigation with greater temporal resolution, involving a multitude of litter and soil types and including microbial compositional and functional characterization, is recommended to fully elucidate the interactive role of Mn and N on C cycling.

Arsenic enrichment in groundwater resources in deltas and floodplains of large sediment-rich rivers is a worldwide natural hazard to human health. High spatial variability of arsenic concentrations in affected river basins limits cost-effective mitigation strategies. Linking the chemical composition of groundwater with the topography and fluvial geomorphology is a promising approach for predicting arsenic pollution on a regional scale. Here we correlate the distribution of arsenic contaminated wells with the fluvial dynamics in the Amazon basin. Groundwater was sampled from tube wells along the Amazon River and its main tributaries in three distinct regions in Peru and Brazil. For each sample, the major and trace element concentrations were analyzed, and the position of the well within the sedimentary structure was determined. The results show that aquifers in poorly weathered sediments deposited by sediment-rich rivers are prone to mobilization and accumulation of aqueous arsenic and manganese, both in sub-Andean foreland basins, and in floodplains downstream. Two zones at risk are distinguished: aquifers in the channel-dominated part of the floodplain (CDF) and aquifers in the overbank deposits on the less-dynamic part of the floodplain (LDF). Some 70 % of the wells located on the CDF and 20 % on the LDF tap groundwater at concentrations exceeding the WHO guideline of 10 μg/L arsenic (max. 430 μg/L), and 70 % (CDF) and 50 % (LDF) exceeded 0.4 mg/L manganese (max. 6.6 mg/L). None of the water samples located outside the actual floodplain of sediment-rich rivers, or on riverbanks of sediment-poor rivers exceed 5 μg/L As, and only 4 % exceeded 0.4 mg/L Mn. The areas of highest risk can be delineated using satellite imagery. We observe similar patterns as in affected river basins in South and Southeast Asia indicating a key role of sedimentation processes and fluvial geomorphology in priming arsenic and manganese contamination in aquifers.

Microbial fuel cells (MFCs), as a promising sustainable bioelectrochemical energy technology for simultaneous wastewater recycling and treatments while generating bioelectricity, have attracted remarkable attention worldwide in the recent decades. Anodic modification materials are remarkably related to the active microbes, electrodes materials and effectual extracellular electron transfer (EET) mechanisms between bacteria and anode, which are crucial for the improvement of EET efficiency, power density, energy conversion efficiency and operational durability in MFCs. This paper summarizes and reviews the currently application of various anodic modification materials in MFCs, simultaneously discusses and looks toward the future development directions of MFCs.

Limitation of rice growth by low phosphorus (P) availability is a widespread problem in tropical and subtropical soils because of the high content of iron (Fe) (oxyhydr)oxides. Ferric iron-bound P (Fe(III)-P) can serve as a P source in paddies after Fe(III) reduction to Fe(II) and corresponding H₂PO₄⁻ release. However, the relevance of reductive dissolution of Fe(III)-P for plant and microbial P uptake is still an open question. To quantify this, ³²P-labeled ferrihydrite (30.8 mg P kg⁻¹) was added to paddy soil mesocosms with rice to trace the P uptake by microorganisms and plants after Fe(III) reduction. Nearly 2% of ³²P was recovered in rice plants, contributing 12% of the total P content in rice shoots and roots after 33 days. In contrast, ³²P recovery in microbial biomass decreased from 0.5% to 0.08% of ³²P between 10 and 33 days after rice transplantation. Microbial biomass carbon (MBC) and dissolved organic C content decreased from day 10 to 33 by 8–54% and 68–77%, respectively, suggesting that the microbial-mediated Fe(III) reduction was C-limited. The much faster decrease of MBC in rooted (by 54%) vs. bulk soil (8–36%) reflects very fast microbial turnover in the rice rhizosphere (high C and oxygen inputs) resulting in the mineralization of the microbial necromass. In conclusion, Fe(III)-P can serve as small but a relevant P source for rice production and could partly compensate plant P demand. Therefore, the P fertilization strategies should consider the P mobilization from Fe (oxyhydr)oxides in flooded paddy soils during rice growth. An increase in C availability for microorganisms in the rhizosphere intensifies P mobilization, which is especially critical at early stages of rice growth.

Extremely high phosphorus (P) concentrations can be found in eutrophic freshwater sediments during algal blooms (ABs). However, few investigations have revealed the mechanism of labile P production in anoxic sediments following ABs decomposition. This limits our understanding of P cycling and mitigation of ABs in aquatic ecosystems. To identify such a mechanism, we conducted a microcosm experiment to identify how ABs decomposition enhances endogenous P release, using the combined techniques of diffusive gradients in thin films, high-resolution dialysis, and 16S rRNA amplicon sequencing. We show the concentrations of labile iron, manganese, sulfide, and P can be well predicted by quality and quantity of algal biomass. The relative abundance of iron reduction bacteria positively correlated with the decrease of pH induced by ABs decomposition, suggesting that this decomposition facilitates microbial iron and manganese reduction. In addition, the reductive dissolution of iron and manganese oxides leads to the labile P release, resulting in higher concentrations of labile P in those sediments affected by ABs compared with those not affected. The P fluxes in the algae-dominated regions exhibited higher values in the algae group than in the control group, with gains of 14.07–100.04%. Furthermore, endogenous P release is strongly controlled by Mn when the Fe(II):Mn(II) ratio is low (below 0.47), and by both Fe and Mn when the Fe(II):Mn(II) ratio is high (above 0.63). Our results quantify the endogenous P diffusion fluxes across the sediment–water interface can be attributed to ABs decomposition, and are therefore useful for further understanding of P cycling in freshwater.

Urge of development through industrialization has led to disturbance/misbalance of the ecosystem by the release of various contaminants (heavy metals, metalloids, organic pollutants, dyes, etc.). Unlike organic contaminants, heavy metals and metalloids pose a serious threat to the flora and fauna of the surroundings due to their immutable nature. The high cost and nonecofriendly nature of physicochemical methods used for heavy metal removal lead to the innovation of the biological technique “bioremediation.” Phytoremediation is one of the bioremediation methods which use accumulator/hyperaccumulator plants for heavy metal removal from soil, sediments, or water. The phytoremediation process by using plants only is time-consuming and may result in reduced metal uptake in high levels of pollutants. High pollutant concentration may result in toxicity to the plants used for remediation purposes. This situation may be overcome by the plant-microbe association, which will result in improved plant growth and heavy metal sequestration. Various rhizospheric processes are responsible for heavy metal removal by secretion of root exudates (siderophores, carboxylic acid ions) and phytohormones, which affect the mobile and bioavailable form of heavy metals. The plant-microbe association may help in enhancing or reducing the mobility and bioavailability of heavy metal, as well as result in improved plant growth, which could result in a significant speedup of the phytoremediation process. This review enlightens the role of plant growth-promoting bacteria (PGPB) in the acceleration of phytoremediation. The metal uptake mechanisms are also discussed.