Vanadate reducing bacteria and archaea may use different mechanisms to reduce vanadate in vanadium contaminated riverine ecosystems as revealed by the combination of DNA-SIP and metagenomic-binning

Vanadium Accumulation and Reduction by Vanadium-Accumulating Bacteria Isolated from the Intestinal Contents of Ciona robusta

The sea squirt Ciona robusta (formerly Ciona intestinalis type A) has been the subject of many interdisciplinary studies. Known as a vanadium-rich ascidian, C. robusta is an ideal model for exploring microbes associated with the ascidian and the roles of these microbes in vanadium accumulation and reduction. In this study, we discovered two bacterial strains that accumulate large amounts of vanadium, CD2-88 and CD2-102, which belong to the genera Pseudoalteromonas and Vibrio, respectively. The growth medium composition impacted vanadium uptake. Furthermore, pH was also an important factor in the accumulation and localization of vanadium. Most of the vanadium(V) accumulated by these bacteria was converted to less toxic vanadium(IV). Our results provide insights into vanadium accumulation and reduction by bacteria isolated from the ascidian C. robusta to further study the relations between ascidians and microbes and their possible applications for bioremediation or biomineralization.

The migration and transformation mechanism of vanadium in a soil-pore water-maize system

The migration mechanism of vanadium (V) in the soil-pore water-maize system has not been revealed. This study conducted pot experiments under artificial control conditions to reveal V's distribution and transport mechanism under different growth stages and V content gradient stress. The V content in the soil pore water gradually increased by an order of magnitude. The V content of pore water in the no-plant group was higher than that in the plant group, indicating that the maize roots absorbed V. The V exists in the form of pentavalent oxygen anions, in which H2VO4- occupies the most significant proportion. With increasing V content, the root area, root number, root length, and tip number decreased significantly. The malondialdehyde content in maize leaves showed an increasing trend, indicating the degree of lipid peroxidation was gradually enhanced. The V content was in the order of root > leaf > stem > fruit and maturity stage > flowering stage > jointing stage, respectively. The transfer coefficient reached a maximum under natural conditions, and increased gradually with the growth. The results of synchrotron radiation X-ray absorption near edge structure (XANES) analysis showed that Fe in maize roots mainly comprised of Fe2O3 and Fe3O4. The Fe in the soil is primarily existed in lepidocrocite and Fe2O3. The μ-XRF analysis showed that V and Fe enriched in the roots with a positive relationship, indicating the synergistic absorption of V and Fe by roots. Part of the Fe2+ reduced V5+ to V4+ or V3+ in the forms of VO2+, V(OH)2+, or V(OH)3 (s), and fixed V at the root. Soil weak acid-soluble fraction V and soil total V were vital factors to maize extraction. This study provides new insights into V biogeochemical behavior and a scientific basis for correctly evaluating its ecological and human health risks.

Climate and local environment co-mediate the taxonomic and functional diversity of bacteria and archaea in the Qinghai-Tibet Plateau rivers

Understanding the environmental response patterns of riverine microbiota is essential for predicting the potential impact of future environmental change on river ecosystems. Vulnerable plateau ecosystems are particularly sensitive to climate and local environmental changes, however, the environmental response patterns of the taxonomic and functional diversity of riverine microbiota remain unclear. Here, we conducted a systematic investigation of the taxonomic and functional diversity of bacteria and archaea from riparian soils, sediments, and water across the elevation of 1800- 4800 m in the Qinghai-Tibet Plateau rivers. We found that within the elevation range of 1800 to 3800 m, riparian soils and sediments exhibited similarities and stabilities in microbial taxonomic and functional diversity, and water microbiomes were more sensitive with great fluctuations in microbial diversity. Beyond the elevation of 3800 m, microbial diversity declined across all riverine matrixes. Local environmental conditions can influence the sensitivity of microbiomes to climate change. The combination of critical climate and local environmental factors, including total nitrogen, total organic carbon, as well as climate variables associated with temperature and precipitation, provided better explanations for microbial diversity than single-factor analyses. Under the extremely adverse scenario of high greenhouse gas emission concentrations (SSP585), we anticipate that by the end of this century, the bacterial, archaeal, and microbial functional diversity across the river network of the Yangtze and Yellow source basin would potentially change by -16.9- 5.2 %, -16.1- 5.7 %, and -9.3- 6.4 %, respectively. Overall, climate and local environments jointly shaped the microbial diversity in plateau river ecosystems, and water microbiomes would provide early signs of environmental changes. Our study provides effective theoretical foundations for the conservation of river biodiversity and functional stability under environmental changes.

Anaerobic selenite-reducing bacteria and their metabolic potentials in Se-rich sediment revealed by the combination of DNA-stable isotope probing, metagenomic binning, and metatranscriptomics

Microorganisms play a critical role in the biogeochemical cycling of selenium (Se) in aquatic environments, particularly in reducing the toxicity and bioavailability of selenite (Se(IV)). This study aimed to identify putative Se(IV)-reducing bacteria (Se^IVRB) and investigate the genetic mechanisms underlying Se(IV) reduction in anoxic Se-rich sediment. Initial microcosm incubation confirmed that Se(IV) reduction was driven by heterotrophic microorganisms. DNA stable-isotope probing (DNA-SIP) analysis identified Pseudomonas, Geobacter, Comamonas, and Anaeromyxobacter as putative Se^IVRB. High-quality metagenome-assembled genomes (MAGs) affiliated with these four putative Se^IVRB were retrieved. Annotation of functional gene indicated that these MAGs contained putative Se(IV)-reducing genes such as DMSO reductase family, fumarate and sulfite reductases. Metatranscriptomic analysis of active Se(IV)-reducing cultures revealed significantly higher transcriptional levels of genes associated with DMSO reductase (serA/PHGDH), fumarate reductase (sdhCD/frdCD), and sulfite reductase (cysDIH) compared to those in cultures not amended with Se(IV), suggesting that these genes played important roles in Se(IV) reduction. The current study expands our knowledge of the genetic mechanisms involved in less-understood anaerobic Se(IV) bio-reduction. Additinally, the complementary abilities of DNA-SIP, metagenomics, and metatranscriptomics analyses are demonstrated in elucidating the microbial mechanisms of biogeochemical processes in anoxic sediment.

Methylmercury formation in biofilms of Geobacter sulfurreducens

Introduction

Mercury (Hg) is a major environmental pollutant that accumulates in biota predominantly in the form of methylmercury (MeHg). Surface-associated microbial communities (biofilms) represent an important source of MeHg in natural aquatic systems. In this work, we report MeHg formation in biofilms of the iron-reducing bacterium Geobacter sulfurreducens.

Methods

Biofilms were prepared in media with varied nutrient load for 3, 5, or 7 days, and their structural properties were characterized using confocal laser scanning microscopy, cryo-scanning electron microscopy and Fourier-transform infrared spectroscopy.

Results

Biofilms cultivated for 3 days with vitamins in the medium had the highest surface coverage, and they also contained abundant extracellular matrix. Using 3 and 7-days-old biofilms, we demonstrate that G. sulfurreducens biofilms prepared in media with various nutrient load produce MeHg, of which a significant portion is released to the surrounding medium. The Hg methylation rate constant determined in 6-h assays in a low-nutrient assay medium with 3-days-old biofilms was 3.9 ± 2.0 ∙ 10^-14  L ∙ cell^-1 ∙ h^-1, which is three to five times lower than the rates found in assays with planktonic cultures of G. sulfurreducens in this and previous studies. The fraction of MeHg of total Hg within the biofilms was, however, remarkably high (close to 50%), and medium/biofilm partitioning of inorganic Hg (Hg(II)) indicated low accumulation of Hg(II) in biofilms.

Discussion

These findings suggest a high Hg(II) methylation capacity of G. sulfurreducens biofilms and that Hg(II) transfer to the biofilm is the rate-limiting step for MeHg formation in this systems.