Benzene is an aromatic compound and harmful for the environment. Biodegradation of benzene can reduce the toxicological risk after accidental or controlled release of this chemical in the environment. In this study, we further characterized an anaerobic continuous biofilm culture grown for more than 14 years on benzene with nitrate as electron acceptor. We determined steady state degradation rates, microbial community composition dynamics in the biofilm, and the initial anaerobic benzene degradation reactions. Benzene was degraded at a rate of 0.15 μmol/mg protein/day and a first-order rate constant of 3.04/day which was fourfold higher than rates reported previously. Bacteria belonging to the Peptococcaceae were found to play an important role in this anaerobic benzene-degrading biofilm culture, but also members of the Anaerolineaceae were predicted to be involved in benzene degradation or benzene metabolite degradation based on Illumina MiSeq analysis of 16S ribosomal RNA genes. Biomass retention in the reactor using a filtration finger resulted in reduction of benzene degradation capacity. Detection of the benzene carboxylase encoding gene, abcA, and benzoic acid in the culture vessel indicated that benzene degradation proceeds through an initial carboxylation step.Electronic supplementary materialThe online version of this article (doi:10.1007/s00253-017-8214-8) contains supplementary material, which is available to authorized users.

Halophilic archaea, also referred to as haloarchaea, dominate hypersaline environments. To survive under such extreme conditions, haloarchaea and their enzymes have evolved to function optimally in environments with high salt concentrations and, sometimes, with extreme pH and temperatures. These features make haloarchaea attractive sources of a wide variety of biotechnological products, such as hydrolytic enzymes, with numerous potential applications in biotechnology. The unique trait of haloarchaeal enzymes, haloenzymes, to sustain activity under hypersaline conditions has extended the range of already-available biocatalysts and industrial processes in which high salt concentrations inhibit the activity of regular enzymes. In addition to their halostable properties, haloenzymes can also withstand other conditions such as extreme pH and temperature. In spite of these benefits, the industrial potential of these natural catalysts remains largely unexplored, with only a few characterized extracellular hydrolases. Because of the applied impact of haloarchaea and their specific ability to live in the presence of high salt concentrations, studies on their systematics have intensified in recent years, identifying many new genera and species. This review summarizes the current status of the haloarchaeal genera and species, and discusses the properties of haloenzymes and their potential industrial applications. SYSTEMATICS OF HALOPHILIC ARCHAEAHalophilic micro-organisms are found in the three domains of life, and are extremophiles living in hypersaline environments in the presence of salt (generally NaCl) as a specific requisite for their growth [1]. Hypersaline environments contain higher salt concentrations than seawater (3.5 % total dissolved salts) [2]. According to the optimum NaCl concentration for growth, micro-organisms fall into one of the following categories: extreme halophiles with optimum growth at 15-30 % (2.5-5.2 M) NaCl; moderate halophiles with optimum growth at 3-15 % (0.5-2.5 M) NaCl; slight halophiles with optimum growth at 1-3 % (0.2-0.5 M) NaCl; non-halophiles with optimum growth at less than 1 % (0.2 M) NaCl; and halotolerant micro-organisms which are non-halophiles showing the ability to tolerate high NaCl concentrations [3]. Halophiles can live in hypersaline environments because they are able to sustain osmotic balance. They use two main strategies to withstand the adverse consequences of water loss when exposed to high salt concentrations. The first strategy used by extremely halophilic archaea (haloarchaea) and halophilic anaerobic bacteria such as members of Halanaerobiales is known as the 'salt-in-cytoplasm' strategy, during which salts such as NaCl or KCl accumulate in the cytoplasm at concentrations similar to the extracellular ones [4,5]. Another mechanism to cope with high salt concentrations is the accumulation of organic molecules such as amino acids, sugars and polyols, known as compatible solutes or osmolytes. Compatible solutes are small and highly soluble molecules that can...

Summary Halogenated organic compounds, also termed organohalogens, were initially considered to be of almost exclusively anthropogenic origin. However, over 5000 naturally synthesized organohalogens are known today. This has also fuelled the hypothesis that the natural and ancient origin of organohalogens could have primed development of metabolic machineries for their degradation, especially in microorganisms. Among these, a special group of anaerobic microorganisms was discovered that could conserve energy by reducing organohalogens as terminal electron acceptor in a process termed organohalide respiration. Originally discovered in a quest for biodegradation of anthropogenic organohalogens, these organohalide‐respiring bacteria (OHRB) were soon found to reside in pristine environments, such as the deep subseafloor and Arctic tundra soil with limited/no connections to anthropogenic activities. As such, accumulating evidence suggests an important role of OHRB in local natural halogen cycles, presumably taking advantage of natural organohalogens. In this minireview, we integrate current knowledge regarding the natural origin and occurrence of industrially important organohalogens and the evolution and spread of OHRB, and describe potential implications for natural halogen and carbon cycles.

Prokaryotes in natural environments respond rapidly to high concentrations of chemicals and physical stresses. Exposure to anthropogenic toxic substances-such as oil, chlorinated solvents, or antibiotics-favors the evolution of resistant phenotypes, some of which can use contaminants as an exclusive carbon source or as electron donors and acceptors. Microorganisms similarly adapt to extreme pH, metal, or osmotic stress. The metabolic plasticity of prokaryotes can thus be harnessed for bioremediation and can be exploited in a variety of ways, ranging from stimulated natural attenuation to bioaugmentation and from wastewater treatment to habitat restoration.

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In this study, we report transcription of genes involved in aerobic and anaerobic benzene degradation pathways in a benzene-degrading denitrifying continuous culture. Transcripts associated with the family Peptococcaceae dominated all samples (21–36% relative abundance) indicating their key role in the community. We found a highly transcribed gene cluster encoding a presumed anaerobic benzene carboxylase (AbcA and AbcD) and a benzoate-coenzyme A ligase (BzlA). Predicted gene products showed >96% amino acid identity and similar gene order to the corresponding benzene degradation gene cluster described previously, providing further evidence for anaerobic benzene activation via carboxylation. For subsequent benzoyl-CoA dearomatization, bam-like genes analogous to the ones found in other strict anaerobes were transcribed, whereas gene transcripts involved in downstream benzoyl-CoA degradation were mostly analogous to the ones described in facultative anaerobes. The concurrent transcription of genes encoding enzymes involved in oxygenase-mediated aerobic benzene degradation suggested oxygen presence in the culture, possibly formed via a recently identified nitric oxide dismutase (Nod). Although we were unable to detect transcription of Nod-encoding genes, addition of nitrite and formate to the continuous culture showed indication for oxygen production. Such an oxygen production would enable aerobic microbes to thrive in oxygen-depleted and nitrate-containing subsurface environments contaminated with hydrocarbons.

Surficial riverbed sediments are often characterized by sharp redox gradients between the aerobic benthic sediment and underlying anoxic sediment, potentially representing an ideal niche for aerobic and anaerobic vinyl chloride (VC) degraders. To test this, the fate of VC in aerobic and anaerobic microcosms containing surficial sediment of a riverbed hyporheic zone receiving VC-contaminated groundwater was explored. Quantitative PCR showed that Dehalococcoides 16S rRNA gene and VC reductive dehalogenase-encoding genes (vcrA, bvcA) were highly enriched in anaerobic microcosms, with stoichiometric conversion of VC to ethene. In aerobic microcosms, etnC and etnE involved in aerobic ethene/VC oxidation were enriched with concomitant low or no accumulation of ethene. However, Dehalococcoides 16S rRNA gene, vcrA and bvcA copy numbers were also enriched in oxygen-exposed microcosms containing sediment with high organic carbon and small grain size, whereas they were reduced in oxygen-exposed sediment with low organic carbon and larger grain size in line with extensive oxygen penetration into the sediment. These results suggest the coexistence and coactivity of anaerobic and aerobic VC degraders in the same small volume of surficial sediment and that oxygen distribution, as determined by sediment grain size and organic matter content, affects the local VC-degrading bacterial community and VC biodegradation pathway.

Chlorinated ethenes (CEs) are legacy contaminants whose chemical footprint is expected to persist in aquifers around the world for many decades to come. These organohalides have been reported in river systems with concerning prevalence and are thought to be significant chemical stressors in urban water ecosystems. The aquifer-river interface (known as the hyporheic zone) is a critical pathway for CE discharge to surface water bodies in groundwater baseflow. This pore water system may represent a natural bioreactor where anoxic and oxic biotransformation process act in synergy to reduce or even eliminate contaminant fluxes to surface water. Here, we critically review current process understanding of anaerobic CE respiration in the competitive framework of hyporheic zone biogeochemical cycling fuelled by in-situ fermentation of natural organic matter. We conceptualise anoxic-oxic interface development for metabolic and co-metabolic mineralisation by a range of aerobic bacteria with a focus on vinyl chloride degradation pathways. The superimposition of microbial metabolic processes occurring in sediment biofilms and bulk solute transport delivering reactants produces a scale dependence in contaminant transformation rates. Process interpretation is often confounded by the natural geological heterogeneity typical of most riverbed environments. We discuss insights from recent field experience of CE plumes discharging to surface water and present a range of practical monitoring technologies which address this inherent complexity at different spatial scales. Future research must address key dynamics which link supply of limiting reactants, residence times and microbial ecophysiology to better understand the natural attenuation capacity of hyporheic systems.