The incidence of nirS and nirK and their genetic heterogeneity in cultivated denitrifiers

Gene sequence analysis of nirS and nirK, both encoding nitrite reductases, was performed on cultivated denitrifiers to assess their incidence in different bacterial taxa and their taxonomical value. Almost half of the 227 investigated denitrifying strains did not render an nir amplicon with any of five previously described primers. NirK and nirS were found to be prevalent in Alphaproteobacteria and Betaproteobacteria, respectively, nirK was detected in the Firmicutes and Bacteroidetes and nirS and nirK with equal frequency in the Gammaproteobacteria. These observations deviated from the hitherto reported incidence of nir genes in bacterial taxa. NirS gene phylogeny was congruent with the 16S rRNA gene phylogeny on family or genus level, although some strains did group within clusters of other bacterial classes. Phylogenetic nirK gene sequence analysis was incongruent with the 16S rRNA gene phylogeny. NirK sequences were also found to be significantly more similar to nirK sequences from the same habitat than to nirK sequences retrieved from highly related taxa. This study supports the hypothesis that horizontal gene transfer events of denitrification genes have occurred and underlines that denitrification genes should not be linked with organism diversity of denitrifiers in cultivation-independent studies.     

Identification and isolation of a Castellaniella species important during biostimulation of an acidic nitrate- and uranium-contaminated aquifer

Immobilization of uranium in groundwater can be achieved through microbial reduction of U(VI) to U(IV) upon electron donor addition. Microbial community structure was analyzed in ethanol-biostimulated and control sediments from a high-nitrate (>130 mM), low-pH, uranium-contaminated site in Oak Ridge, TN. Analysis of small subunit (SSU) rRNA gene clone libraries and polar lipid fatty acids from sediments revealed that biostimulation resulted in a general decrease in bacterial diversity. Specifically, biostimulation resulted in an increase in the proportion of Betaproteobacteria (10% of total clones in the control sediment versus 50 and 79% in biostimulated sediments) and a decrease in the proportion of Gammaproteobacteria and Acidobacteria. Clone libraries derived from dissimilatory nitrite reductase genes (nirK and nirS) were also dominated by clones related to Betaproteobacteria (98% and 85% of total nirK and nirS clones, respectively). Within the nirK libraries, one clone sequence made up 59 and 76% of sequences from biostimulated sediments but only made up 10% of the control nirK library. Phylogenetic analysis of SSU rRNA and nirK gene sequences from denitrifying pure cultures isolated from the site indicate that all belong to a Castellaniella species; nearly identical sequences also constituted the majority of biostimulated SSU rRNA and nirK clone libraries. Thus, by combining culture-independent with culture-dependent techniques, we were able to link SSU rRNA clone library information with nirK sequence data and conclude that a potentially novel Castellaniella species is important for in situ nitrate removal at this site.     

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.     

Diversity of nitrite reductase (nirK and nirS) gene fragments in forested upland and wetland soils

The genetic heterogeneity of nitrite reductase gene (nirK and nirS) fragments from denitrifying prokaryotes in forested upland and marsh soil was investigated using molecular methods. nirK gene fragments could be amplified from both soils, whereas nirS gene fragments could be amplified only from the marsh soil. PCR products were cloned and screened by restriction fragment length polymorphism (RFLP), and representative fragments were sequenced. The diversity of nirK clones was lower than the diversity of nirS clones. Among the 54 distinct nirK RFLP patterns identified in the two soils, only one pattern was found in both soils and in each soil two dominant groups comprised >35% of all clones. No dominance and few redundant patterns were seen among the nirS clones. Phylogenetic analysis of deduced amino acids grouped the nirK sequences into five major clusters, with one cluster encompassing most marsh clones and all upland clones. Only a few of the nirK clone sequences branched with those of known denitrifying bacteria. The nirS clones formed two major clusters with several subclusters, but all nirS clones showed less than 80% identity to nirS sequences from known denitrifying bacteria. Overall, the data indicated that the denitrifying communities in the two soils have many members and that the soils have a high richness of different nir genes, especially of the nirS gene, most of which have not yet been found in cultivated denitrifiers.     

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.     

Molecular diversity of denitrifying genes in continental margin sediments within the oxygen-deficient zone off the Pacific coast of Mexico

To understand the composition and structure of denitrifying communities in the oxygen-deficient zone off the Pacific coast of Mexico, the molecular diversity of nir genes from sediments obtained at four stations was examined by using a PCR-based cloning approach. A total of 50 operational taxonomic units (OTUs) for nirK and 82 OTUs for nirS were obtained from all samples. Forty-four of the nirS clones and 31 of the nirK clones were sequenced; the levels of similarity of the nirS clones were 52 to 92%, and the levels of similarity of the nirS clones were 50 to 99%. The percentages of overlapping OTUs between stations were 18 to 30% for nirS and 5 to 8% for nirK. Sequence analysis revealed that 26% of the nirS clones were related to the nirS genes of Alcaligenes faecalis (80 to 94% similar) and Pseudomonas stutzeri (80 to 99%), whereas 3 to 31% of the nirK clones were closely related to the nirK genes of Pseudomonas sp. strain G-179 (98 to 99%), Bradyrhizobium japonicum (91%), Blastobacter denitrificans (83%), and Alcaligenes xylosoxidans (96%). The rest of the clones, however, were less than 80% similar to nirS and nirK sequences available in sequence databases. The results of a principal-component analysis (PCA) based on the percentage of OTUs and biogeochemical data indicated that the nitrate concentration and oxygen have an effect on the denitrifying communities. The communities at the stations in oxygen-deficient zones were more similar than the communities at the stations in the oxygenated zone. The denitrifying communities were more similar at the stations that were closer together and had similar nitrate levels. Also, the results of PCA based on biogeochemical properties suggest that geographic location and biogeochemical conditions, especially the nitrate and oxygen levels, appear to be the key factors that control the structure of denitrifying communities.     

Free-living and aggregate-associated Planctomycetes in the Black Sea

We examined the distribution of uncultured Planctomycetes phylotypes along depth profiles spanning the redox gradient of the Black Sea suboxic zone to gain insight into their respective ecological niches. Planctomycetes phylogeny correlated with depth and chemical profiles, implying similar metabolisms within phylogenetic groups. A suboxic zone sample was split into >?30 and ?30?μm fraction except for members of the Scalindua genus, which were apparently free-living. Sequences from Candidatus Scalindua, known to carry out the anammox process, formed two distinct clusters with nonoverlapping depth ranges. One cluster, only 97.1% similar to the named species, was present at high nitrite/nitrate and low ammonium concentrations in the upper suboxic zone. We propose this sequence type be named 'Candidatus Scalindua richardsii'. A second cluster, containing sequences more similar to 'Candidatus Scalindua sorokinii', was present at high ammonium and low nitrite conditions in the lower suboxic zone. Sequences obtained from the sulfidic zone (1000?m depth) yielded Planctomycetes from two uncharacterized Planctomycetacia clusters and three potentially new genera as well as sequences from the uncultured OP3 phylum.     

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.     

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.     

Protist diversity in suboxic and sulfidic waters of the Black Sea

The oxic-anoxic transition zone of the Black Sea comprises a large suboxic zone as well as anoxic and sulfidic waters. While the prokaryotes and biogeochemical cycles that characterize this zone have been frequently studied, little is known about the diversity or ecology of its microbial eukaryotes. Here, we present the first broad qualitative report of the protist species composition in the Black Sea redoxcline using molecular tools. Fingerprint analysis from the whole redoxcline revealed a complex community structure of metabolically active protists with distinct shifts along the redox gradient. Additionally, 18S rRNA gene clone libraries were used to compare protist species composition of suboxic and sulfidic water layers. Among the ciliates, sequences related to Pleuronema and Strombidium were dominant in both water layers whereas sequences affiliated with anaerobic plagiopylids and Cyclidium were detected only in the sulfidic zone. Among the flagellates, mainly stramenopiles (mostly bicosoecids and chrysophytes) occurred throughout the redoxcline. In the sulfidic zone we found stramenopile sequences but also euglenozoans, jakobids and choanoflagellates that were related to clonal sequences from other anoxic marine habitats, thus indicating the existence of globally distributed groups of anoxic flagellates. Higher species diversity in the sulfidic zone and about twice as many novel sequence types of ciliates and stramenopiles compared with the suboxic layer emphasizes the importance of anoxic, sulfidic waters as habitat for high protist diversity although the function of these organisms is yet unknown.     

Impacts of nitrogen application rates on the activity and diversity of denitrifying bacteria in the Broadbalk Wheat Experiment

Bacterial denitrification results in the loss of fertilizer nitrogen and greenhouse gas emissions as nitrous oxides, but ecological factors in soil influencing denitrifier communities are not well understood, impeding the potential for mitigation by land management. Communities vary in the relative abundance of the alternative dissimilatory nitrite reductase genes nirK and nirS, and the nitrous oxide reductase gene nosZ; however, the significance for nitrous oxide emissions is unclear. We assessed the influence of different long-term fertilization and cultivation treatments in a 160-year-old field experiment, comparing the potential for denitrification by soil samples with the size and diversity of their denitrifier communities. Denitrification potential was much higher in soil from an area left to develop from arable into woodland than from a farmyard manure-fertilized arable treatment, which in turn was significantly higher than inorganic nitrogen-fertilized and unfertilized arable plots. This correlated with abundance of nirK but not nirS, the least abundant of the genes tested in all soils, showing an inverse relationship with nirK. Most genetic variation was seen in nirK, where sequences resolved into separate groups according to soil treatment. We conclude that bacteria containing nirK are most probably responsible for the increased denitrification potential associated with nitrogen and organic carbon in this soil.     

Metabolic profiles and genetic diversity of denitrifying communities in activated sludge after addition of methanol or ethanol

External carbon sources can enhance denitrification rates and thus improve nitrogen removal in wastewater treatment plants. The effects of adding methanol and ethanol on the genetic and metabolic diversity of denitrifying communities in activated sludge were compared using a pilot-scale plant with two parallel lines. A full-scale plant receiving the same municipal wastewater, but without external carbon source addition, was the reference. Metabolic profiles obtained from potential denitrification rates with 10 electron donors showed that the denitrifying communities altered their preferences for certain compounds after supplementation with methanol or ethanol and that methanol had the greater impact. Clone libraries of nirK and nirS genes, encoding the two different nitrite reductases in denitrifiers, revealed that methanol also increased the diversity of denitrifiers of the nirS type, which indicates that denitrifiers favored by methanol were on the rise in the community. This suggests that there might be a niche differentiation between nirS and nirK genotypes during activated sludge processes. The composition of nirS genotypes also varied greatly among all samples, whereas the nirK communities were more stable. The latter was confirmed by denaturing gradient gel electrophoresis of nirK communities on all sampling occasions. Our results support earlier hypotheses that the compositions of denitrifier communities change during predenitrification processes when external carbon sources are added, although no severe effect could be observed from an operational point of view.     

Diversity of nitrite reductase genes in "Candidatus Accumulibacter phosphatis"-dominated cultures enriched by flow-cytometric sorting

"Candidatus Accumulibacter phosphatis" is considered a polyphosphate-accumulating organism (PAO) though it has not been isolated yet. To reveal the denitrification ability of this organism, we first concentrated this organism by flow cytometric sorting following fluorescence in situ hybridization (FISH) using specific probes for this organism. The purity of the target cells was about 97% of total cell count in the sorted sample. The PCR amplification of the nitrite reductase genes (nirK and nirS) from unsorted and sorted cells was performed. Although nirK and nirS were amplified from unsorted cells, only nirS was detected from sorted cells, indicating that "Ca. Accumulibacter phosphatis" has nirS. Furthermore, nirS fragments were cloned from unsorted (Ba clone library) and sorted (Bd clone library) cells and classified by restriction fragment length polymorphism analysis. The most dominant clone in clone library Ba, which represented 62% of the total number of clones, was not found in clone library Bd. In contrast, the most dominant clone in clone library Bd, which represented 59% of the total number of clones, represented only 2% of the total number of clones in clone library Ba, indicating that this clone could be that of "Ca. Accumulibacter phosphatis." The sequence of this nirS clone exhibited less than 90% similarity to the sequences of known denitrifying bacteria in the database. The recovery of the nirS genes makes it likely that "Ca. Accumulibacter phosphatis" behaves as a denitrifying PAO capable of utilizing nitrite instead of oxygen as an electron acceptor for phosphorus uptake.     

Genetic characterization of denitrifier communities with contrasting intrinsic functional traits

Microorganisms capable of denitrification are polyphyletic and exhibit distinct denitrification regulatory phenotypes (DRP), and thus, denitrification in soils could be controlled by community composition. In a companion study (D?rsch et al., 2012) and preceding work, ex situ denitrification assays of three organic soils demonstrated profoundly different functional traits including N(2) O/N(2) ratios. Here, we explored the composition of the underlying denitrifier communities by analyzing the abundance and structure of denitrification genes (nirK, nirS, and nosZ). The relative abundance of nosZ (vs. nirK + nirS) was similar for all communities, and hence, the low N(2) O reductase activity in one of the soils was not because of the lack of organisms with this gene. Similarity in community composition between the soils was generally low for nirK and nirS, but not for nosZ. The community with the most robust denitrification (consistently low N(2) O/N(2) ) had the highest diversity/richness of nosZ and nirK, but not of nirS. Contrary results found for a second soil agreed with impaired denitrification (low overall denitrification activity, high N(2) O/N(2) ). In conclusion, differences in community composition and in the absolute abundance of denitrification genes clearly reflected the functional differences observed in laboratory studies and may shed light on differences in in situ N(2) O emission of the soils.     

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.     

Sediment denitrifier community composition and nirS gene expression investigated with functional gene microarrays

A functional gene microarray was used to investigate denitrifier community composition and nitrite reductase (nirS) gene expression in sediments along the estuarine gradient in Chesapeake Bay, USA. The nirS oligonucleotide probe set was designed to represent a sequence database containing 539 Chesapeake Bay clones, as well as sequences from many other environments. Greatest nirS diversity was detected at the freshwater station at the head of the bay and least diversity at the higher salinity station near the mouth of the Bay. The most common OTUs from the sequence database were detected on the array with high signal strength in most samples. One of the most abundant OTUs, CB2-S-138, was identified as dominant at the mid-bay site by both microarray and quantitative PCR assays, but it comprised a much smaller fraction of the assemblage in the north and south bay samples. cDNA (transcribed from total RNA extracts) targets were hybridized to the same array to compare the profiles of community composition at the DNA (relative abundance) and mRNA (gene expression) levels. Only the three dominant denitrifying groups (in terms of relative strength of DNA hybridization signal) were detected at the mRNA level. These results suggest that the most actively denitrifying groups are responsible for most nirS expression as well.     

Putative ammonia-oxidizing Crenarchaeota in suboxic waters of the Black Sea: a basin-wide ecological study using 16S ribosomal and functional genes and membrane lipids

Within the upper 400 m at western, central and eastern stations in the world's largest stratified basin, the Black Sea, we studied the qualitative and quantitative distribution of putative nitrifying Archaea based on their genetic markers (16S rDNA, amoA encoding for the alpha-subunit of archaeal ammonia monooxygenase), and crenarchaeol, the specific glycerol diphytanyl glycerol tetraether of pelagic Crenarchaeota within the Group I.1a. Marine Crenarchaeota were the most abundant Archaea (up to 98% of the total archaeal 16S rDNA copies) in the suboxic layers with oxygen levels as low as 1 microM including layers where previously anammox bacteria were described. Different marine crenarchaeotal phylotypes (both 16S rDNA and amoA) were found at the upper part of the suboxic zone as compared with the base of the suboxic zone and the upper 15-30 m of the anoxic waters with prevailing sulfide concentrations of up to 30 microM. Crenarchaeol concentrations were higher in the sulfidic chemocline as compared with the suboxic zone. These results indicate an abundance of putative nitrifying Archaea at very low oxygen levels within the Black Sea and might form an important source of nitrite for the anammox reaction.