The denitrifying betaproteobacterium "" EbN1 regulates the capacity to anaerobically degrade -ethylphenol (via-hydroxyacetophenone) with high substrate specificity. This process is mediated by the σ-dependent transcriptional regulator EtpR, which apparently recognizes both aromatic compounds, yielding congruent expression profiles. The responsiveness of this regulatory system was studied with -hydroxyacetophenone, which is more easily administered to cultures and traced analytically. Cultures of EbN1 were initially cultivated under nitrate-reducing conditions with a growth-limiting supply of benzoate, upon the complete depletion of which -hydroxyacetophenone was added at various concentrations (from 500 μM down to 0.1 nM). Depletion profiles of this aromatic substrate and presumptive effector were determined by highly sensitive micro-high-performance liquid chromatography (microHPLC). Irrespective of the added concentration of-hydroxyacetophenone, depletion commenced after less than 5 min and suggested a response threshold of below 10 nM. This approximation was corroborated by time-resolved transcript profiles (quantitative reverse transcription-PCR) of selected degradation and efflux relevant genes (e.g., , encoding a subunit of predicted-ethylphenol methylenehydroxylase) and narrowed down to a range of 10 to 1 nM. The most pronounced transcriptional response was observed, as expected, for genes located at the beginning of the two operon-like structures, related to catabolism (i.e., ) and potential efflux (i.e.,). Aromatic compounds are widespread microbial growth substrates with natural as well as anthropogenic sources, albeit with their concentrations and their bioavailabilities varying over several orders of magnitude. Even though degradation pathways and underlying regulatory systems have long been studied with aerobic and, to a lesser extent, with anaerobic bacteria, comparatively little is known about the effector concentration-dependent responsiveness. EbN1 is a model organism for the anaerobic degradation of aromatic compounds with the architecture of the catabolic network and its substrate-specific regulation having been intensively studied by means of differential proteogenomics. The present study aims at unraveling the minimal concentration of an aromatic growth substrate (-hydroxyacetophenone here) required to initiate gene expression for its degradation pathway and to learn in principle about the lower limit of catabolic responsiveness of an anaerobic degradation specialist.

The stoichiometric constraints of algal growth are well understood, whereas there is less knowledge for heterotrophic bacterioplankton. Growth of the marine bacterium Phaeobacter inhibens DSM 17395, belonging to the globally distributed Roseobacter group, was studied across a wide concentration range of NH4+ and PO43−. The unique dataset covers 415 different concentration pairs, corresponding to 207 different molar N:P ratios (from 10−2 to 105). Maximal growth (by growth rate and biomass yield) was observed within a restricted concentration range at N:P ratios (∼50−120) markedly above Redfield. Experimentally determined growth parameters deviated to a large part from model predictions based on Liebig's law of the minimum, thus implicating synergistic co-limitation due to biochemical dependence of resources. Internal elemental ratios of P. inhibens varied with external nutrient supply within physiological constraints, thus adding to the growing evidence that aquatic bacteria can be flexible in their internal elemental composition. Taken together, the findings reported here revealed that P. inhibens is well adapted to fluctuating availability of inorganic N and P, expected to occur in its natural habitat (e.g. colonized algae, coastal areas). Moreover, this study suggests that elemental variability in bacterioplankton needs to be considered in the ecological stoichiometry of the oceans.

BackgroundThe denitrifying betaproteobacterium "Aromatoleum aromaticum" EbN1 anaerobically utilizes a multitude of aromatic compounds via specific peripheral degradation routes. Compound-specific formation of these catabolic modules is assumed to be mediated by specific transcriptional activators. In case of the recently elucidated p-ethylphenol/p-hydroxyacetophenone pathway, the highly substrate-specific regulation was implicated to involve the predicted σ54-dependent, NtrC-type regulator EbA324. The latter was suggested to control the expression of the two neighboring gene clusters encoding the catabolic enzymes as well as a corresponding putative solvent efflux system. In the present study, a molecular genetic approach was used to study the predicted function of EbA324.ResultsAn unmarked in frame ΔebA324 (here renamed as ΔetpR; p-ethylphenol regulator) deletion mutation was generated. The ΔetpR mutant was unable to grow anaerobically with either p-ethylphenol or p-hydroxyacetophenone. Growth similar to the wild type was restored in the ΔetpR mutant background by in trans expression of plasmid-born etpR. Furthermore, expression of the "p-ethylphenol" gene clusters as well as corresponding protein formation was shown to depend on the presence of both, EtpR and either p-ethylphenol or p-hydroxyacetophenone. In the wild type, the etpR gene appears to be constitutively expressed and its expression level not to be modulated upon effector presence. Comparison with the regulatory domains of known phenol- and alkylbenzene-responsive NtrC-type regulators of Pseudomonas spp. and Thauera aromatica allowed identifying >60 amino acid residues in the regulatory domain (in particular positions 149 to 192 of EtpR) that may contribute to the effector specificity viz. presumptively restricted effector spectrum of EtpR.ConclusionsThis study provides experimental evidence for the genome predicted σ54-dependent regulator EtpR (formerly EbA324) of "A. aromaticum" EbN1 to be responsive to p-ethylphenol, as well as its degradation intermediate p-hydroxyacetophenone, and to control the expression of genes involved in the anaerobic degradation of these two aromatic growth substrates. Overall, the presented results advance our understanding on the regulation of anaerobic aromatic compound catabolism, foremost based on the sensory discrimination of structurally similar substrates.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0571-9) contains supplementary material, which is available to authorized users.

Designing desirable approach for batch and continuous methods of activity for profoundly effective removal of zinc(II) is of practical importance, given the steady, cost-effective, environmentally friendly attribute. This article offers an answer for foster a continuous stirred tank bioreactor (CSTR) containing single- and two-phase reactors for the viable continuous removal of zinc(II) from aqueous solution by making use of Fusarium sp. collected from soil under growing conditions. The different development stages Fusarium sp. were tried, including initial metal concentration, pH and growth rate in batch bioreactors. The maximum removal of zinc(II) in the batch studies indicated 63.9 mg/g, at controlled pH: 5.0; and at initial 500 mg/L zinc(II) concentration, which is almost of comparable quantity to the most extreme removal of zinc(II) (52.8 mg/g) obtained in the continuous mode of operation. A continuous flow system operated in staged manner was found to be the ideal strategy for obtaining almost complete removal at lower concentration focus up to 100 mg/L in a single stage bioreactor and a multistage reactor at higher zinc(II) concentrations.