Microbial synthesis and transformation of inorganic and organic chlorine compounds

Organic and inorganic chlorine compounds are formed by a broad range of natural geochemical, photochemical and biological processes. In addition, chlorine compounds are produced in large quantities for industrial, agricultural and pharmaceutical purposes, which has led to widespread environmental pollution. Abiotic transformations and microbial metabolism of inorganic and organic chlorine compounds combined with human activities constitute the chlorine cycle on Earth. Naturally occurring organochlorines compounds are synthesized and transformed by diverse groups of (micro)organisms in the presence or absence of oxygen. In turn, anthropogenic chlorine contaminants may be degraded under natural or stimulated conditions. Here, we review phylogeny, biochemistry and ecology of microorganisms mediating chlorination and dechlorination processes. In addition, the co-occurrence and potential interdependency of catabolic and anabolic transformations of natural and synthetic chlorine compounds are discussed for selected microorganisms and particular ecosystems.The authors thank METAEXPLORE, funded by the European Union Seventh Framework Program (Grant No. 222625), BEBASIC-FES funds from the Dutch Ministry of Economic Affairs (Projects F07.001.05 and F08.004.01), Shell Global Solutions International BV, the ERC Advanced grant “Novel Anaerobes” (Project 323009), the SIAM Gravitation grant “Microbes for Health and the Environment” (Project 024.002.002) of the Netherlands Ministry of Education, Culture and Science, and the Netherlands Science Foundation (NWO) for funding.info:eu-repo/semantics/publishedVersio

Hydrogen producing microbial communities of the biocathode in a microbial electrolysis cell

In the search for alternatives for fossil fuels and the reuse of the energy from waste streams, the microbial electrolysis cell is a promising technique. The microbial electrolysis cell is a two electrode system in which at the anode organic substances, including waste water, are used by microorganisms that release the terminal electrons to the electrode. These electrons are subsequently used at the cathode resulting in the production of a current. By addition of a small voltage, hydrogen gas can be produced by combining electrons and protons at the cathode. To catalyse the hydrogen evolution reaction at the cathode, expensive catalysts such as platinum are required. Recently, the use of biocathodes has shown great potential as an alternative for platinum. The microbial community responsible for the hydrogen evolution in such systems is, however, not well understood. In this study we focused on the characterization of the microbial communities of the microbial electrolysis cell biocathode using molecular techniques. The results show that the microbial community consists of 44% Proteobacteria, 27% Firmicutes, 18% Bacteriodetes and 12% related to other phyla. Within the major phylogenetic groups we found several clusters of uncultured species belonging to novel taxonomic groups at genus level. These novel taxonomic groups developed under environmentally unusual conditions and might have properties that have not been described before. Therefore it is of great interest to study those novel groups further. Within the Proteobacteria a major cluster belonged to the Deltaproteobacteria and based on the known characteristics of the closest related cultured species, we suggest a mechanism for microbial electron transfer for the production of hydrogen at the cathode

The stable isotopic signature of biologically produced molecular hydrogen (H₂)

Biologically produced molecular hydrogen (H₂) is characterised by a very strong depletion in deuterium. Although the biological source to the atmosphere is small compared to photochemical or combustion sources, it makes an important contribution to the global isotope budget of H₂. Large uncertainties exist in the quantification of the individual production and degradation processes that contribute to the atmospheric budget, and isotope measurements are a tool to distinguish the contributions from the different sources. Measurements of &delta; D from the various H₂ sources are scarce and for biologically produced H₂ only very few measurements exist. <br><br> Here the first systematic study of the isotopic composition of biologically produced H₂ is presented. In a first set of experiments, we investigated &delta; D of H₂ produced in a biogas plant, covering different treatments of biogas production. In a second set of experiments, we investigated pure cultures of several H₂ producing microorganisms such as bacteria or green algae. A Keeling plot analysis provides a robust overall source signature of &delta; D = &minus;712&permil; (±13&permil;) for the samples from the biogas reactor (at 38 °C, &delta; D_H2O= +73.4&permil;), with a fractionation constant &varepsilon;_H2-H2O of −689&permil; (±20&permil;) between H₂ and the water. The five experiments using pure culture samples from different microorganisms give a mean source signature of &delta; D = &minus;728&permil; (±28&permil;), and a fractionation constant &varepsilon;_H2-H2O of −711&permil; (±34&permil;) between H₂ and the water. The results confirm the massive deuterium depletion of biologically produced H₂ as was predicted by the calculation of the thermodynamic fractionation factors for hydrogen exchange between H₂ and water vapour. Systematic errors in the isotope scale are difficult to assess in the absence of international standards for &delta; D of H₂. <br><br> As expected for a thermodynamic equilibrium, the fractionation factor is temperature dependent, but largely independent of the substrates used and the H₂ production conditions. The equilibrium fractionation coefficient is positively correlated with temperature and we measured a rate of change of 2.3&permil; / °C between 45 °C and 60 °C, which is in general agreement with the theoretical prediction of 1.4&permil; / °C. Our best experimental estimate for &varepsilon;_H2-H2O at a temperature of 20 °C is −731&permil; (±20&permil;) for biologically produced H₂. This value is close to the predicted value of −722&permil;, and we suggest using these values in future global H₂ isotope budget calculations and models with adjusting to regional temperatures for calculating &delta; D values