Effect of Elodea nuttallii Roots on Bacterial Communities and MMHg Proportion in a Hg Polluted Sediment

The objective of this study was to assess the effect of a rooted macrophyte Elodea nuttallii on rhizosphere bacterial communities in Hg contaminated sediments. Specimens of E. nuttallii were exposed to sediments from the Hg contaminated Babeni reservoir (Olt River, Romania) in our microcosm. Plants were allowed to grow for two months until they occupied the entirety of the sediments. Total Hg and MMHg were analysed in sediments where an increased MMHg percentage of the total Hg in pore water of rhizosphere sediments was found. E. nuttallii roots also significantly changed the bacterial community structure in rhizosphere sediments compared to bulk sediments. Deltaproteobacteria dominated the rhizosphere bacterial community where members of Geobacteraceae within the Desulfuromonadales and Desulfobacteraceae were identified. Two bacterial operational taxonomic units (OTUs) which were phylogenetically related to sulfate-reducing bacteria (SRB) became abundant in the rhizosphere. We suggest that these phylotypes could be potentially methylating bacteria and might be responsible for the higher MMHg percentage of the total Hg in rhizosphere sediments. However, SRB were not significantly favoured in rhizosphere sediments as shown by qPCR. Our findings support the hypothesis that rooted macrophytes created a microenvironment favorable for Hg methylation. The presence of E. nuttallii in Hg contaminated sediments should therefore not be overlooked.     

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring,Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron‐rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H₂. Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid‐phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens‐like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat‐shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin–cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe‐rich, circumneutral pH geothermal environment.     

High pressure liquid chromatographic analysis of conjugated bile acids in human bile: simultaneous resolution of sulfated and unsulfated lithocholyl amidates and the common conjugated bile acids

A reversed phase high pressure liquid chromatography (HPLC) system capable of simultaneously separating four lithocholyl species (sulfated and unsulfated forms of lithocholylglycine and lithocholyltaurine) as well as the eight other major conjugated bile acids present in human bile is described. The system uses a C18 octadecylsilane column and isocratic elution with methanol phosphate buffer, pH 5.35. Relative bile acid concentration is determined by absorbance at 200 nm. Retention times relative to chenodeoxycholylglycine are reported for the four lithocholic acid forms, the glycine and taurine amidate of the four major bile acids present in human bile (cholic, chenodeoxycholic, ursodeoxycholic, and deoxycholic), and for their corresponding unconjugated forms. Retention times are also reported for the glycine and taurine amidates as well as the unconjugated form of the C23 norderivatives of these bile acids. Maximal absorbance of bile acid amidates is at 200 nm and is very similar for the (unsulfated) glycine and taurine amidates. Sulfated lithocholyl amidates exhibit molar absorptivities at 200 nm which are 1.4 times greater than that of non-sulfated lithocholyl amidates. Unconjugated bile acid absorbance at 200 nm or 210 nm is 20 to 30 times less than that of corresponding peptide conjugates. The method has been applied to samples of gallbladder bile obtained from 14 healthy subjects to define the pattern of conjugated bile acids present in human bile.