Pseudomonas stutzeri Nitrite Reductase Gene Abundance in Environmental Samples Measured by Real-Time PCR

We used real-time PCR to quantify the denitrifying nitrite reductase gene (nirS), a functional gene of biogeochemical significance. The assay was tested in vitro and applied to environmental samples. The primer-probe set selected was specific for nirS sequences that corresponded approximately to the Pseudomonas stutzeri species. The assay was linear from 1 to 10⁶ gene copies (r² = 0.999). Variability at low gene concentrations did not allow detection of twofold differences in gene copy number at less than 100 copies. DNA spiking and cell-addition experiments gave predicted results, suggesting that this assay provides an accurate measure of P. stutzeri nirS abundance in environmental samples. Although P. stutzeri abundance was high in lake sediment and groundwater samples, we detected low or no abundance of this species in marine sediment samples from Puget Sound (Wash.) and from the Washington ocean margin. These results suggest that P. stutzeri may not be a dominant marine denitrifier.     

Nitrite reductase genes (nirK and nirS) as functional markers to investigate diversity of denitrifying bacteria in pacific northwest marine sediment communities

Genetic heterogeneity of denitrifying bacteria in sediment samples from Puget Sound and two sites on the Washington continental margin was studied by PCR approaches amplifying nirK and nirS genes. These structurally different but functionally equivalent single-copy genes coding for nitrite reductases, a key enzyme of the denitrification process, were used as a molecular marker for denitrifying bacteria. nirS sequences could be amplified from samples of both sampling sites, whereas nirK sequences were detected only in samples from the Washington margin. To assess the underlying nir gene structure, PCR products of both genes were cloned and screened by restriction fragment length polymorphism (RFLP). Rarefraction analysis revealed a high level of diversity especially for nirS clones from Puget Sound and a slightly lower level of diversity for nirK and nirS clones from the Washington margin. One group dominated within nirK clones, but no dominance and only a few redundant clones were seen between sediment samples for nirS clones in both habitats. Hybridization and sequencing confirmed that all but one of the 228 putative nirS clones were nirS with levels of nucleotide identities as low as 45.3%. Phylogenetic analysis grouped nirS clones into three distinct subclusters within the nirS gene tree which corresponded to the two habitats from which they were obtained. These sequences had little relationship to any strain with known nirS sequences or to isolates (mostly close relatives of Pseudomonas stutzeri) from the Washington margin sediment samples. nirK clones were more closely related to each other than were the nirS clones, with 78.6% and higher nucleotide identities; clones showing only weak hybridization signals were not related to known nirK sequences. All nirK clones were also grouped into a distinct cluster which could not be placed with any strain with known nirK sequences. These findings show a very high diversity of nir sequences within small samples and that these novel nir clusters, some very divergent from known sequences, are not known in cultivated denitrifiers.     

Presence of two different active nirS nitrite reductase genes in a denitrifying Thauera sp. from a high-nitrate-removal-rate reactor

The nirS nitrite reductase genes were studied in two strains (strains 27 and 28) isolated from two denitrifying reactors and characterized as Thauera according to their 16S rRNA gene sequences. Strain 28 contains a single nirS sequence, which is related to the nirS of Thauera mechernichensis, and strain 27 contains two nirS sequences; one is similar to the nirS sequence from Thauera mechernichensis (gene 2), but the second one (gene 8) is from a separate clade with nirS from Pseudomonas stutzeri, Azoarcus species, Alcaligenes faecalis, and other Thauera species. Both genes were expressed, but gene 8 was constitutively expressed while gene 2 was positively regulated by nitrate.     

Highly different levels of natural transformation are associated with genomic subgroups within a local population of Pseudomonas stutzeri from soil

A highly sensitive and specific PCR-based method of monitoring 16S rRNA genes of Pseudomonas stutzeri was developed for searching P. stutzeri DNA in environmental samples. This monitoring was combined with a reliable and sensitive method for isolating P. stutzeri colony formers from soil and sediment, depending on their utilization of ethylene glycol, starch, and maltose. With these techniques, P. stutzeri populations (n = 2 to 170) were obtained from five of six sites giving positive PCR signals (including three marine sediment and two soil samples). The phylogenetic positions of isolates from the five sites, based on their 16S ribosomal DNA sequences, indicated that the environmental isolates were affiliated with different genomovars of P. stutzeri. Using the broad-host-range plasmid pNS1 with kanamycin and gentamicin resistance determinants as the transforming DNA, naturally transformable strains were identified among the isolates from all sites. For one population from soil, the genetic relationship of the 120 members was determined by randomly amplified polymorphic DNA-PCR with three PCR primers. Among the population members which are taxonomically closely related as determined by 16S sequence comparisons of group representatives, a rather high genetic diversity and a characteristic clustering into subgroups were found. Remarkably, within the population, nontransformability and different levels of transformability (a frequency between about 10(-9) and 10(-4) per cell) were often associated with distinct genetic subgroups. It is concluded that transformability is widespread among environmental P. stutzeri strains and that its specific level is a heritable trait that may vary strongly within a local population.     

Assessment of denitrifying bacterial composition in activated sludge

The abundance and structure of denitrifying bacterial community in different activated sludge samples were assessed, where the abundance of denitrifying functional genes showed nirS in the range of 10(4)-10(5), nosZ with 10(4)-10(6) and 16S rRNA gene in the range 10(9)-10(10) copy number per ml of sludge. The culturable approach revealed Pseudomonas sp. and Alcaligenes sp. to be numerically high, whereas culture independent method showed betaproteobacteria to dominate the sludge samples. Comamonas sp. and Pseudomonas fluorescens isolates showed efficient denitrification, while Pseudomonas mendocina, Pseudomonas stutzeri and Brevundimonas diminuta accumulated nitrite during denitrification. Numerically dominant RFLP OTUs of the nosZ gene from the fertilizer factory sludge samples clustered with the known isolates of betaproteobacteria. The data also suggests the presence of different truncated denitrifiers with high numbers in sludge habitat.     

Comparison of methods for quantification of cytochrome cd(1)-denitrifying bacteria in environmental marine samples

Two PCR primer sets were developed for the detection and quantification of cytochrome cd(1)-denitrifying bacteria in environmental marine samples. The specificity and sensitivity of these primers were tested. Both primer sets were suitable for detection, but only one set, cd3F-cd4R, was suitable for the quantification and enumeration of the functional community using most-probable-number PCR and competitive PCR techniques. Quantification of cytochrome cd(1) denitrifiers taken from marine sediment and water samples was achieved using two different molecular techniques which target the nirS gene, and the results were compared to those obtained by using the classical cultivation method. Enumerations using both molecular techniques yielded similar results in seawater and sediment samples. However, both molecular techniques showed 1,000 or 10 times more cytochrome cd(1) denitrifiers in the sediment or water samples, respectively, than were found by use of the conventional cultivation method for counting.     

Development and evaluation of functional gene arrays for detection of selected genes in the environment.

To determine the potential of DNA array technology for assessing functional gene diversity and distribution, a prototype microarray was constructed with genes involved in nitrogen cycling: nitrite reductase (nirS and nirK) genes, ammonia mono-oxygenase (amoA) genes, and methane mono-oxygenase (pmoA) genes from pure cultures and those cloned from marine sediments. In experiments using glass slide microarrays, genes possessing less than 80 to 85% sequence identity were differentiated under hybridization conditions of high stringency (65 degrees C). The detection limit for nirS genes was approximately 1 ng of pure genomic DNA and 25 ng of soil community DNA using our optimized protocol. A linear quantitative relationship (r(2) = 0.89 to 0.94) was observed between signal intensity and target DNA concentration over a range of 1 to 100 ng for genomic DNA (or genomic DNA equivalent) from both pure cultures and mixed communities. However, the quantitative capacity of microarrays for measuring the relative abundance of targeted genes in complex environmental samples is less clear due to divergent target sequences. Sequence divergence and probe length affected hybridization signal intensity within a certain range of sequence identity and size, respectively. This prototype functional gene array did reveal differences in the apparent distribution of nir and amoA and pmoA gene families in sediment and soil samples. Our results indicate that glass-based microarray hybridization has potential as a tool for revealing functional gene composition in natural microbial communities; however, more work is needed to improve sensitivity and quantitation and to understand the associated issue of specificity.     

A species-specific probe and a PCR assay for the marine bacterium,Pseudomonas stutzeri strain Zobell

The cloning, sequencing, and analysis of a Pseudomonas stutzeri Zobell 23S rRNA gene is described. Three variable regions were identified, and oligonucleotides homologous to portions of these regions were synthesized. The oligonucleotides were used as probes to screen DNA from various cultured bacteria to identify a species-specific probe. All probes were found to hybridize strongly with P. stutzeri Zobell DNA under stringent conditions and did not hybridize with other Pseudomonas species. One probe showed slight cross-reactivity with DNA from four other bacteria under the hybridization conditions used. Finally, PCR conditions were optimized for detection of P. stutzeri Zobell in mixed culture with a detection limit of 400 cells. The assay detected P. stutzeri Zobell rDNA in coastal seawater samples sampled over a 20-month period. In the future, these probes could be used to quantify the 23S rRNA and rDNA from P. stutzeri Zobell in mixed culture and in environmental samples.     

Oligonucleotide microarray for the study of functional gene diversity in the nitrogen cycle in the environment

The analysis of functional diversity and its dynamics in the environment is essential for understanding the microbial ecology and biogeochemistry of aquatic systems. Here we describe the development and optimization of a DNA microarray method for the detection and quantification of functional genes in the environment and report on their preliminary application to the study of the denitrification gene nirS in the Choptank River-Chesapeake Bay system. Intergenic and intragenic resolution constraints were determined by an oligonucleotide (70-mer) microarray approach. Complete signal separation was achieved when comparing unrelated genes within the nitrogen cycle (amoA, nifH, nirK, and nirS) and detecting different variants of the same gene, nirK, corresponding to organisms with two different physiological modes, ammonia oxidizers and denitrifying halobenzoate degraders. The limits of intragenic resolution were investigated with a microarray containing 64 nirS sequences comprising 14 cultured organisms and 50 clones obtained from the Choptank River in Maryland. The nirS oligonucleotides covered a range of sequence identities from approximately 40 to 100%. The threshold values for specificity were determined to be 87% sequence identity and a target-to-probe perfect match-to-mismatch binding free-energy ratio of 0.56. The lower detection limit was 10 pg of DNA (equivalent to approximately 10(7) copies) per target per microarray. Hybridization patterns on the microarray differed between sediment samples from two stations in the Choptank River, implying important differences in the composition of the denitirifer community along an environmental gradient of salinity, inorganic nitrogen, and dissolved organic carbon. This work establishes a useful set of design constraints (independent of the target gene) for the implementation of functional gene microarrays for environmental applications.     

Seasonal changes in nitrogen-cycle gene abundances and in bacterial communities in acidic forest soils

The abundance of genes related to the nitrogen biogeochemical cycle and the microbial community in forest soils (bacteria, archaea, fungi) were quantitatively analyzed via real-time PCR using 11 sets of specific primers amplifying nifH, bacterial amoA, archaeal amoA, narG, nirS, nirK, norB, nosZ, bacterial 16S rRNA gene, archaeal 16S rRNA gene, and the ITS sequence of fungi. Soils were sampled from Bukhan Mountain from September of 2010 to July of 2011 (7 times). Bacteria were the predominant microbial community in all samples. However, the abundance of archaeal amoA was greater than bacterial amoA throughout the year. The abundances of nifH, nirS, nirK, and norB genes changed in a similar pattern, while narG and nosZ appeared in sensitive to the environmental changes. Clone libraries of bacterial 16S rRNA genes were constructed from summer and winter soil samples and these revealed that Acidobacteria was the most predominant phylum in acidic forest soil environments in both samples. Although a specific correlation of environmental factor and gene abundance was not verified by principle component analysis, our data suggested that the combination of biological, physical, and chemical characteristics of forest soils created distinct conditions favoring the nitrogen biogeochemical cycle and that bacterial communities in undisturbed acidic forest soils were quite stable during seasonal change.     

Immunofluorescence detection of the denitrifying strain Pseudomonas stutzeri (ATCC 14405) in seawater and intertidal sediment environments

A strain-specific immunofluorescence assay for enumeration of a marine denitrifying bacterium was developed and applied in the marine environment. The polyclonal antiserum for Pseudomonas stutzeri (ATCC 14405) did not react with other pseudomonads, other heterotrophs, or autotrophic nitrifying strains. The abundance of P. stutzeri in the shallow water column of Monterey Bay was less than 0.1% of the total bacterial abundance and decreased with depth, whereas the total bacterial abundance was variable and nearly constant with depth. P. stutzeri was also detected in the sediments of a microbial mat from Tomales Bay. The relatively low contribution of P. stutzeri to the total bacterial abundance in both environments implies that it is not a major component of the heterotrophic assemblage. This conclusion appears to hold for most other strains for which specific assays have been applied in the marine environment. The isolation of several different denitrifying strains from local marine environments implies that the culturable population is quite diverse, even in the absence of different selective enrichment media. Thus, strain specific immunofluorescence is of limited use in quantifying functional groups of bacteria. Conversely, they provide specific information on the diversity of natural populations and their relation to culturable strains.Offprint requests to: B. Ward     

福建敖江口表层沉积硅藻空间分布特征

为探讨福建敖江口表层沉积硅藻空间分布特征,2019年7月对敖江口进行表层沉积硅藻的采样调查,并研究了硅藻与环境因子的关系。结果表明,从13个站点共检出硅藻114种,隶属于39属。硅藻丰度具有从河口向外海先减少后增加的变化特征。聚类分析表明,主要硅藻属种可划分为淡水种组合带和半咸水-咸水种组合带。典型对应分析（CCA）表明,表层海水温度和盐度是影响硅藻分布的主要环境变量。河口区盐度相对较低,主要分布淡水种双面曲壳藻、优美曲壳藻和颗粒沟链藻;外海区北部盐度相对较高,主要分布咸水种流水双菱藻;外海区南部盐度相对较低,主要分布淡水种优美曲壳藻和颗粒沟链藻;滩涂的沉积物较粗,主要分布优美曲壳藻。此外,河口区和外海区南部可能存在一定的水体污染。     

Quantifying functional gene populations: comparing gene abundance and corresponding enzymatic activity using denitrification and nitrogen fixation in pulp and paper mill effluent treatment systems.

The relationship between the abundance of three functional genes and their corresponding biochemical reaction rates was investigated in several activated sludge and mill effluent microbial communities. Gene probes were prepared for two key denitrification genes (nirS and nirK) and for one nitrogen-fixation gene (nifH) and were validated using a variety of strains of known nir and nif genotype. ATP-based measures of viable cell numbers were used to provide total population sizes. In certain microbial communities (activated sludge enrichment cultures and multiple samples taken from the same mill primary clarifier), a strong correlation was observed between gene abundance and biochemical activity rates. However, when comparing several different nonenriched activated sludge bioreactors and separate primary clarifier microbial communities, the ratio of specific gene abundance to biochemical activity rates varied widely. These results suggest that in cases where a microbial community is not fully induced for a given biochemical activity or when very different communities are compared, quantitative gene probing can give a better measure of a community's potential to carry out the encoded function than can the relevant biochemical assay. However, the gene quantitation method employed here probably underestimated the true number of probed genes present in the microbial communities due to nirS and nifH genes in the communities having reduced DNA sequence similarity with the probes used.     

Molecular quantification of environmental DNA using microfluidics and digital PCR

Real-time PCR has been widely used to evaluate gene abundance in natural microbial habitats. However, PCR-inhibitory substances often reduce the efficiency of PCR, leading to the underestimation of target gene copy numbers. Digital PCR using microfluidics is a new approach that allows absolute quantification of DNA molecules. In this study, digital PCR was applied to environmental samples, and the effect of PCR inhibitors on DNA quantification was tested. In the control experiment using λ DNA and humic acids, underestimation of λ DNA at 1/4400 of the theoretical value was observed with 6.58ngμL(-1) humic acids. In contrast, digital PCR provided accurate quantification data with a concentration of humic acids up to 9.34ngμL(-1). The inhibitory effect of paddy field soil extract on quantification of the archaeal 16S rRNA gene was also tested. By diluting the DNA extract, quantified copy numbers from real-time PCR and digital PCR became similar, indicating that dilution was a useful way to remedy PCR inhibition. The dilution strategy was, however, not applicable to all natural environmental samples. For example, when marine subsurface sediment samples were tested the copy number of archaeal 16S rRNA genes was 1.04×10(3)copies/g-sediment by digital PCR, whereas real-time PCR only resulted in 4.64×10(2)copies/g-sediment, which was most likely due to an inhibitory effect. The data from this study demonstrated that inhibitory substances had little effect on DNA quantification using microfluidics and digital PCR, and showed the great advantages of digital PCR in accurate quantifications of DNA extracted from various microbial habitats.     

Molecular diversity and characterization of nitrite reductase gene fragments (nirK and nirS) from nitrate- and uranium-contaminated groundwater

Nitrate-contaminated groundwater samples were analysed for nirK and nirS gene diversity. The samples differed with respect to nitrate, uranium, heavy metals, organic carbon content, pH and dissolved oxygen levels. A total of 958 nirK and 1162 nirS clones were screened by restriction fragment length polymorphism (RFLP) analysis: 48 and 143 distinct nirK and nirS clones, respectively, were obtained. A single dominant nirK restriction pattern was observed for all six samples and was 83% identical to the Hyphomicrobium zavarzinii nirK gene. A dominant nirS pattern was observed for four of the samples, including the background sample, and was 95% identical to the nirS of Alcaligenes faecalis. Diversity indices for nirK and nirS sequences were not related to any single geochemical characteristic, but results suggested that the diversity of nirK genes was inversely proportional to the diversity of nirS. Principal component analysis (PCA) of the sites based on geochemistry grouped the samples by low, moderate and high nitrate but PCA of the unique operational taxonomic units (OTUs) distributions grouped the samples differently. Many of the sequences were not closely related to previously observed genes and some phylogenetically related sequences were obtained from similar samples. The results indicated that the contaminated groundwater contained novel nirK and nirS sequences, functional diversity of both genes changed in relation to the contaminant gradient, but the nirK and nirS functional diversity was affected differently.     

黄海海域海洋沉积物细菌多样性分析

【背景】海洋独特的环境造就了海洋生物的多样性,海洋沉积物中细菌对海洋环境具有至关重要的作用。【目的】研究陆地土壤和海洋沉积物间细菌群落相似性和差异性,以便更好地认识海洋细菌多样性,深入了解沉积物细菌在海洋环境中的潜在作用。【方法】从中国黄海海域及大连市大黑山脚下分别采集样品,以陆地土壤为对照,采用16SrRNA基因高通量测序技术分析海洋沉积物的细菌群落结构。【结果】海洋沉积物样品中芽孢杆菌纲(Bacilli)、鞘氨醇单胞菌属(Sphingomonas)和芽孢杆菌属(Bacillus)丰度高于陆地土壤样品;海洋沉积物中亚硝化单胞菌(unculturedbacterium f. Nitrosomonadaceae)和厌氧绳菌(uncultured bacterium f. Anaerolineaceae)丰度虽低于陆地土壤,但丰度值也均高于1%;样品分类学统计显示酸杆菌门(Acidobacteria)在海洋沉积物和陆地土壤样品中的序列丰度比例都较大,鞘氨醇单胞菌属(Sphingomonas)在海洋沉积物样品中的序列丰度大于陆地土壤样品。【结论】海洋沉积物细菌多样性可作为海洋环境恢复情况的重...     

Real-time quantitative PCR for assessment of abundance of Pseudoalteromonas species in marine samples

A real-time quantitative PCR (RTQ-PCR) method for measuring the abundance of Pseudoalteromonas species in marine samples is presented. PCR primers targeting a Pseudoalteromonas-specific region of the 16S rRNA gene were tested at three different levels using database searches (in silico), a selection of pure cultures (in vitro), and a combined denaturing gradient gel electrophoresis and cloning approach on environmental DNA (in situ). The RTQ-PCR method allowed for the detection of SYBR Green fluorescence from double-stranded DNA over a linear range spanning six orders of magnitude. The detection limit was determined as 1.4 fg of target DNA (1,000 gene copies) measured in the presence of 20 ng of nontarget DNA from salmon testes. In this study, we discuss the importance of robust post-PCR analyses to overcome pitfalls in RTQ-PCR when samples from different complex marine habitats are analyzed and compared on a nonroutine basis. Representatives of the genus Pseudoalteromonas were detected in samples from all investigated habitats, suggesting a widespread distribution of this genus across many marine habitats (e.g., seawater, rocks, macroalgae, and marine animals). Three sample types were analyzed by RTQ-PCR to determine the relative abundance of Pseudoalteromonas ribosomal DNA (rDNA) compared to the total abundance of eubacterial rDNA. The rDNA fractions of Pseudoalteromonas compared to all Eubacteria were 1.55% on the green alga Ulva lactuca, 0.10% on the tunicate Ciona intestinalis, and 0.06% on the green alga Ulvaria fusca.     

Diversity of the nitrite reductase gene nirS in the sediment of a free-water surface constructed wetland.

The diversity of the nitrite reductase gene nirS was studied in the bulk sediment of a free-water surface constructed wetland (FWS-CW) located next to the Empuriabrava wastewater treatment plant (WWTP), in Castelló d'Empúries (Girona, NE Spain). The study period extended from the inception of the treatment wetland, in June 1998, until March 1999 and comprised periods of relatively high nitrate and ammonium concentrations at the influent and low nitrate-removal efficiencies. To evaluate nirS diversity, partial gene sequences were obtained by cloning of the respective PCR products. Rarefaction curves based on DOTUR analyses of the deduced amino-acid sequences predicted a greater diversity of nirS genes in samples containing higher ammonium concentrations. Estimated Shannon-Weaver indices of the four cloned samples showed a positive relationship with the N-NH4 +/N-NO3 - ratios measured at the FWS-CW inlet. Identities between the deduced amino-acid sequences and those previously deposited in public databases ranged from 72 to 97%. Phylogenetic analysis based on these deduced sequences grouped 165 nirS clones in seven main clusters according to high similarity indices. Up to 60% of the clones clustered together in a highly homogeneous group with little homologies to any sequence retrieved from cultured representatives. Moreover, prevailing environmental conditions appeared to select for particular denitrifying populations, e.g., with respect to ammonium load and nitrogen removal efficiencies. This observation is of particular interest for the management of treatment wetlands, in which only slight variations in the theoretical denitrification potential of the system can occur.     

Characterization of the rRNA locus of Pfiesteria piscicida and development of standard and quantitative PCR-based detection assays targeted to the nontranscribed spacer

Pfiesteria piscicida is a heterotrophic dinoflagellate widely distributed along the middle Atlantic shore of the United States and associated with fish kills in the Neuse River (North Carolina) and the Chesapeake Bay (Maryland and Virginia). We constructed a genomic DNA library from clonally cultured P. piscicida and characterized the nontranscribed spacer (NTS), small subunit, internal transcribed spacer 1 (ITS1), 5.8S region, ITS2, and large subunit of the rRNA gene cluster. Based on the P. piscicida ribosomal DNA sequence, we developed a PCR-based detection assay that targets the NTS. The assay specificity was assessed by testing clonal P. piscicida and Pfiesteria shumwayae, 35 additional dinoflagellate species, and algal prey (Rhodomonas sp.). Only P. piscicida and nine presumptive P. piscicida isolates tested positive. All PCR-positive products yielded identical sequences for P. piscicida, suggesting that the PCR-based assay is species specific. The assay can detect a single P. piscicida zoospore in 1 ml of water, 10 resting cysts in 1 g of sediment, or 10 fg of P. piscicida DNA in 1 micro g of heterologous DNA. An internal standard for the PCR assay was constructed to identify potential false-negative results in testing of environmental sediment and water samples and as a competitor for the development of a quantitative competitive PCR assay format. The specificities of both qualitative and quantitative PCR assay formats were validated with >200 environmental samples, and the assays provide simple, rapid, and accurate methods for the assessment of P. piscicida in water and sediments.