Jelly-like Microbial Mats over Subsurface Fields of Gas Hydrates at the St. Petersburg Methane Seep (Central Baikal)

<div><p>Jelly-like microbial mat samples were collected from benthic surfaces at the St. Petersburg methane seep located in Central Baikal. The concentrations of certain ions, specifically chloride, bromide, sulphate, acetate, iron, calcium, and magnesium, were 2–40 times higher in the microbial mats than those in the pore and bottom water. A large number of diatom valves, cyanobacteria, and filamentous, rod-shaped and coccal microorganisms were found in the samples of bacterial mats using light, epifluorescence and scanning microscopy.Comparative analysis of a 16S rRNA gene fragment demonstrated the presence of bacteria and archaea belonging to the following classes and phyla: Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, Verrucomicrobia, Cytophaga-Flavobacteria-Bacteroidetes, Cyanobacteria, and Euryarchaeota. The chemical composition and phylogenetic structure of the microbial community showed that the life activity of the mat occurs due to methane and its derivatives involved. Values of δ¹³C for the microbial mats varied from −73.6‰ to −65.8‰ and for animals from −68.9‰ to −36.6‰. Functional genes of the sequential methane oxidation (<i>pmoA</i> and <i>mxaF</i>) and different species of methanotrophic bacteria inhabiting cold ecosystems were recorded in the total DNA. Like in other psychroactive communities, the destruction of organic substances forming formed as a result of methanotrophy, terminates at the stage of acetate formation in the microbial mats of Lake Baikal (1,400 m depth). Its further transformation is limited by hydrogen content and carried out in the subsurface layers of sediments.</p></div

Solubility of Nanocrystalline Cerium Dioxide: Experimental Data and Thermodynamic Modeling

Ultrafine 5 nm ceria isotropic nanoparticles were prepared using the rapid chemical precipitation approach from cerium­(III) nitrate and ammonium hydroxide aqueous solutions. The as-prepared nanoparticles were shown to contain predominantly Ce­(IV) species. The solubility of nanocrystalline CeO₂ at several pH values was determined using ICP-MS and radioactive tracer methods. Phase composition of the ceria samples remained unchanged upon partial dissolution, while the shape of the particles changed dramatically, yielding nanorods under neutral pH conditions. According to X-ray absorption spectroscopy investigation of the supernatant, Ce­(III) was the main cerium species in solution at pH < 4. Based on the results obtained, a reductive dissolution model was used for data interpretation. According to this model, the solubility product for ceria nanoparticles was determined to be log <i>K</i>_sp = −59.3 ± 0.3 in 0.01 M NaClO₄. Taken together, our results show that the pH dependence of ceria anti- and pro-oxidant activity can be related to the dissolution of CeO₂ in aqueous media