Identification and activities in situ of Nitrosospira and Nitrospira spp. as dominant populations in a nitrifying fluidized bed reactor

Stability and microbial community structure of a partial nitrifying fixed-film bioreactor in long run

A partial nitrification system was investigated for 471 days under DO varying concentrations for assessing its stability and population dynamics. Within 130 days of operation at feed DO concentration of 1.0±0.1 mg/L, more than 85% of nitrite was accumulated. Efficiency deteriorated when the feed DO concentration was increased to 4.2±0.3 mg/L. Nitrite accumulation could not be re-established on decreasing feed DO to 1.0±0.1 mg/L. Even at DO concentration of<0.05 mg/L, nitrate production was observed; a condition termed as anoxic nitrification. NOB was detected in the biomass even under this condition by Fluorescence in-situ hybridization (FISH) analysis. Through 16S rRNA gene sequencing a major fraction of unknown bacterial sequences closely resembling haloalkalophilic bacteria of marine origin were detected. The study indicated that these bacterial species might play a role in anoxic nitrification and that NOB could survive extreme low DO condition.

Frankia populations in soil and root nodules of sympatrically grown Alnus taxa

The genetic diversity of Frankia populations in soil and in root nodules of sympatrically grown Alnus taxa was evaluated by rep-polymerase chain reaction (PCR) and nifH gene sequence analyses. Rep-PCR analyses of uncultured Frankia populations in root nodules of 12 Alnus taxa (n=10 nodules each) growing sympatrically in the Morton Arboretum near Chicago revealed identical patterns for nodules from each Alnus taxon, including replicate trees of the same host taxon, and low diversity overall with only three profiles retrieved. One profile was retrieved from all nodules of nine taxa (Alnus incana subsp. incana, Alnus japonica, Alnus glutinosa, Alnus incana subsp. tenuifolia, Alnus incana subsp. rugosa, Alnus rhombifolia, Alnus mandshurica, Alnus maritima, and Alnus serrulata), the second was found in all nodules of two plant taxa (A. incana subsp. hirsuta and A. glutinosa var. pyramidalis), and the third was unique for all Frankia populations in nodules of A. incana subsp. rugosa var. americana. Comparative sequence analyses of nifH gene fragments in nodules representing these three profiles assigned these frankiae to different subgroups within the Alnus host infection group. None of these sequences, however, represented frankiae detectable in soil as determined by sequence analysis of 73 clones from a Frankia-specific nifH gene clone library. Additional analyses of nodule populations from selected alders growing on different soils demonstrated the presence of different Frankia populations in nodules for each soil, with populations showing identical sequences in nodules from the same soil, but differences between plant taxa. These results suggest that soil environmental conditions and host plant genotype both have a role in the selection of Frankia strains by a host plant for root nodule formation, and that this selection is not merely a function of the abundance of a Frankia strain in soil.

Ammonia-oxidizing bacteria in wastewater

Ammonia-oxidizing bacteria (AOB) have a key role in the conversion of ammonia to nitrite in wastewater treatment plants (WWTPs). The characterization of AOB communities in such systems requires the use of genomic methods as AOB are difficult to isolate from environmental samples. Fluorescence in situ hybridization (FISH) using fluorescently labeled probes targeting 16S rRNA molecules provides a robust tool for the detection and quantification of AOB populations in biofilms and activated sludge flocs. The abundance of AOB may be also determined by real-time quantitative polymerase chain reaction (qPCR) using primers that amplify either the 16S rRNA or amoA genes. The evaluation of changes in the AOB community in time and space can be undertaken by PCR amplification of these gene fragments followed by denaturing gradient gel electrophoresis (PCR-DGGE). In this chapter, we summarize the most commonly applied procedures for the analysis of the AOB in wastewater, emphasizing their advantages and limitations.

Using sludge fermentation liquid to improve wastewater short-cut nitrification-denitrification and denitrifying phosphorus removal via nitrite

Wastewater biological nutrient removal (BNR) by short-cut nitrification-denitrification (SCND) and denitrifying phosphorus removal via nitrite (DPRN) has several advantages, such as organic carbon source saving. In this paper, a new method, i.e., by using waste activated sludge alkaline fermentation liquid as BNR carbon source, for simultaneously improving SCND and DPRN was reported. First, the performance of SCND and DPRN with sludge fermentation liquid as carbon source was compared with acetic acid, which was commonly used in literatures. Sludge fermentation liquid showed much higher nitrite accumulation during aerobic nitrification than acetic acid (81.8% versus 40.9%), and the former had significant anoxic denitrification and phosphorus uptake. The soluble phosphorus and total nitrogen removal efficiencies with sludge fermentation liquid were much higher than with acetic acid (97.6% against 73.4% and 98.7% versus 79.2%). Then the mechanisms for sludge fermentation liquid showed higher SCND and DPRN than acetic acid were investigated from the aspects of wastewater composition, microorganisms assayed by 16S rRNA gene clone library, and fluorescence in situ hybridization. More NO(2)(-)-N accumulated by the use of sludge fermentation liquid was attributed to be more humic acids in the influent, which inhibited nitrite oxidizing bacteria (NOB) more serious than ammonia oxidizing bacteria (AOB), and more AOB but less NOB were observed in the BNR system. The reasons for sludge fermentation liquid BNR system exhibiting greater short-cut denitrifying phosphorus removal were that there were less glycogen accumulating organisms and more phosphorus accumulating organisms and anoxic denitrifying phosphorus removal bacteria with higher nitrite reductase activity.

Impact of inocula and growth mode on the molecular microbial ecology of anaerobic ammonia oxidation (anammox) bioreactor communities

The composition of distinctly inoculated granular anammox and biofilm-based completely autotrophic nitrogen removal over nitrite (CANON) bioreactors was investigated from start-up through continuous long-term operation via denaturing gradient gel electrophoresis (DGGE) and sequencing. The granular anammox reactor was seeded with sludge from an operational anammox reactor in Strass, Austria. The CANON reactor was seeded with activated sludge from a local wastewater treatment plant in New York City. The principal anammox bacteria (AMX) shifted from members related to Kuenenia stuttgartiensis present in the initial inoculum to members related to Candidatus Brocadia fulgida during pre-enrichment (before this study) and to members related to Candidatus Brocadia sp. 40 (during this study) in the granular reactor. AMX related to C. Brocadia sp. 40 were also enriched from activated sludge in the CANON reactor. The estimated doubling times of AMX in the granular and CANON reactors were 5.3 and 8.9 days, respectively, which are lower than the value of 11 days, reported previously. Both the granular anammox and CANON reactors also fostered significant amounts of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB). The fractions of AMX and two groups of NOB were generally similar in the granular anammox and CANON reactors. However, the diversity and fractions of AOB in the two reactors was markedly different. Therefore, it is suggested that the composition of the feed and extant substrate concentrations in the reactor likely select for the microbial community composition more than the inocula and reactor configuration. Further, such selection is not equivalent for all resident communities.

A conceptual ecosystem model of microbial communities in enhanced biological phosphorus removal plants

The microbial populations in 25 full-scale activated sludge wastewater treatment plants with enhanced biological phosphorus removal (EBPR plants) have been intensively studied over several years. Most of the important bacterial groups involved in nitrification, denitrification, biological P removal, fermentation, and hydrolysis have been identified and quantified using quantitative culture-independent molecular methods. Surprisingly, a limited number of core species was present in all plants, constituting on average approx. 80% of the entire communities in the plants, showing that the microbial populations in EBPR plants are rather similar and not very diverse, as sometimes suggested. By focusing on these organisms it is possible to make a comprehensive ecosystem model, where many important aspects in relation to microbial ecosystems and wastewater treatment can be investigated. We have reviewed the current knowledge about these microorganisms with focus on key ecophysiological factors and combined this into a conceptual ecosystem model for EBPR plants. It includes the major pathways of carbon flow with specific organic substances, the dominant populations involved in the transformations, interspecies interactions, and the key factors controlling their presence and activity. We believe that the EBPR process is a perfect model system for studies of microbial ecology in water engineering systems and that this conceptual model can be used for proposing and testing theories based on microbial ecosystem theories, for the development of new and improved quantitative ecosystem models and is beneficial for future design and management of wastewater treatment systems.

Microbial community distribution and activity dynamics of granular biomass in a CANON reactor

The application of microelectrodes to measure oxygen and nitrite concentrations inside granules operated at 20 degrees C in a CANON (Complete Autotrophic Nitrogen-removal Over Nitrite) reactor and the application of the FISH (Fluorescent In Situ Hybridization) technique to cryosectioned slices of these granules showed the presence of two differentiated zones inside of them: an external nitrification zone and an internal anammox zone. The FISH analysis of these layers allowed the identification of Nitrosomonas spp. and Candidatus Kuenenia Stutgartiensis as the main populations carrying out aerobic and anaerobic ammonia oxidation, respectively. Concentration microprofiles measured at different oxygen concentrations in the bulk liquid (from 1.5 to 35.2 mg O(2) L(-1)) revealed that oxygen was consumed in a surface layer of 100-350 microm width. The obtained consumption rate of the most active layers was of 80 g O(2) (L(granule))(-1) d(-1). Anammox activity was registered between 400 and 1000 microm depth inside the granules. The nitrogen removal capacity of the studied sequencing batch reactor containing the granular biomass was of 0.5 g N L(-1) d(-1). This value is similar to the mean nitrogen removal rate obtained from calculations based on in- and outflow concentrations. Information obtained in the present work allowed the establishment of a simple control strategy based on the measurements of NH(4)(+) and NO(2)(-) in the bulk liquid and acting over the dissolved oxygen concentration in the bulk liquid and the hydraulic retention time of the reactor.

Influence of physicochemical and operational parameters on Nitrobacter and Nitrospira communities in an aerobic activated sludge bioreactor

To understand how to optimize performance of a partially nitrifying plant, the dynamics of Nitrospira and Nitrobacter abundance were studied over a 1 year period using quantitative polymerase chain reaction (qPCR) and their relative contributions to nitrite oxidation assessed including the affects of temperature and dissolved oxygen (DO). Correlation coefficients linking shifts in the community composition of nitrite-oxidizing bacteria (NOB) to operational or environmental variables indicated Nitrospira was significantly and negatively correlated to nitrite concentrations (r = -0.45, P < 0.01) and DO (r = -0.46, P < 0.01), while temperature showed a strong positive correlation (r = 0.59, P < 0.0001). However, the Nitrobacter portion of the total NOB populations showed a positive correlations with DO (r = 0.38, P < 0.01) and hydraulic retention time (HRT) (r = 0.33, P < 0.05), as well as being negatively correlated with temperature (r = -0.49, P < 0.001) suggesting specific niche adaptations within the NOB community. Nitrospira was dominant being better adapted to the low DO and shorter sludge retention times (SRT) of this plant, while Nitrobacter increased in abundance during the winter months, when temperatures were lower and DO concentrations higher. Principal component analysis (PCA) results supported these findings by the close proximity of Nitrospira and temperature biplots of PC1 and PC2 as well as grouping Nitrobacter, NO(2)(-)-N, HRT, and DO in the loadings together. The clustering of samples from specific dates also exhibited a strong seasonality.

Inoculum effects on community composition and nitritation performance of autotrophic nitrifying biofilm reactors with counter-diffusion geometry

The link between nitritation success in a membrane-aerated biofilm reactor (MABR) and the composition of the initial ammonia- and nitrite-oxidizing bacterial (AOB and NOB) population was investigated. Four identically operated flat-sheet type MABRs were initiated with two different inocula: from an autotrophic nitrifying bioreactor (Inoculum A) or from a municipal wastewater treatment plant (Inoculum B). Higher nitritation efficiencies (NO(2)(-)-N/NH(4)(+)-N) were obtained in the Inoculum B- (55.2-56.4%) versus the Inoculum A- (20.2-22.1%) initiated reactors. The biofilms had similar oxygen penetration depths (100-150 µm), but the AOB profiles [based on 16S rRNA gene targeted real-time quantitative PCR (qPCR)] revealed different peak densities at or distant from the membrane surface in the Inoculum B- versus A-initiated reactors, respectively. Quantitative fluorescence in situ hybridization (FISH) revealed that the predominant AOB in the Inoculum A- and B-initiated reactors were Nitrosospira spp. (48.9-61.2%) versus halophilic and halotolerant Nitrosomonas spp. (54.8-63.7%), respectively. The latter biofilm displayed a higher specific AOB activity than the former biofilm (1.65 fmol cell(-1) h(-1) versus 0.79 fmol cell(-1) h(-1) ). These observations suggest that the AOB and NOB population compositions of the inoculum may determine dominant AOB in the MABR biofilm, which in turn affects the degree of attainable nitritation in an MABR.

Characterization and quantification of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in a nitrogen-removing reactor using T-RFLP and qPCR

Using ammonia monooxygenase α-subunit (amoA) gene and 16S rRNA gene, the community structure and abundance of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in a nitrogen-removing reactor, which was operated for five phases, were characterized and quantified by cloning, terminal restriction fragment length polymorphism (T-RFLP), and quantitative polymerase chain reaction (qPCR). The results suggested that the dominant AOB in the reactor fell to the genus Nitrosomonas, while the dominant AOA belonged to Crenarchaeotal Group I.1a in phylum Crenarchaeota. Real-time PCR results demonstrated that the levels of AOB amoA varied from 2.9 × 10³ to 2.3 × 10⁵ copies per nanogram DNA, greatly (about 60 times) higher than those of AOA, which ranged from 1.7 × 10² to 3.8 × 10³ copies per nanogram DNA. This indicated the possible leading role of AOB in the nitrification process in this study. T-RFLP results showed that the AOB community structure significantly shifted in different phases while AOA only showed one major peak for all the phases. The analyses also suggested that the AOB community was more sensitive than that of AOA to operational conditions, such as ammonia loading and dissolved oxygen.

Nitrifying microorganisms in fixed-bed biofilm reactors fed with different nitrite and ammonia concentrations

Nitrifying bacteria and archaea were fed in fixed-bed biofilm reactors with different nitrite and ammonia concentrations in synthetic and real wastewater. During high nitrite concentrations (rho(NO(2)(-))=5-10mg/L), an increase in the abundance of Nitrobacter species was detected with fluorescence in situ hybridization (FISH), while Nitrospira species disappeared to a large extent. During high ammonia concentrations (rho(NH(4)(+))=60-80 mg/L), a slight increase in ammonia-oxidizing bacteria was obtained, while the abundance of archaebacteria remained unchanged. Lab-scale reactors showed a similar nitrifying microbial population as reactors fed with real wastewater. However, increased abundances of Nitrospira species as observed in wastewater reactors and in the wastewater trickling filters could not be found in the laboratory reactors.

Phylogenetic characterization and in situ detection of bacterial communities associated with seahorses (Hippocampus guttulatus) in captivity

Although there are several studies describing bacteria associated with marine fish, the bacterial composition associated with seahorses has not been extensively investigated since these studies have been restricted to the identification of bacterial pathogens. In this study, the phylogenetic affiliation of seahorse-associated bacteria was assessed by 16S rRNA gene sequencing of cloned DNA fragments. Fluorescence in situ hybridization (FISH) was used to confirm the presence of the predominant groups indicated by 16S rRNA analysis. Both methods revealed that Vibrionaceae was the dominant population in Artemia sp. (live prey) and intestinal content of the seahorses, while Rhodobacteraceae was dominant in water samples from the aquaculture system and cutaneous mucus of the seahorses. To our knowledge, this is the first time that bacterial communities associated with healthy seahorses in captivity have been described.

The application of molecular techniques to the study of wastewater treatment systems

Wastewater treatment systems tend to be engineered to select for a few functional microbial groups that may be organized in various spatial structures such as activated sludge flocs, biofilm or granules and represented by single coherent phylogenic groups such as ammonia-oxidizing bacteria (AOB) and polyphosphate-accumulating organisms (PAO). In order to monitor and control engineered microbial structure in wastewater treatment systems, it is necessary to understand the relationships between the microbial community structure and the process performance. This review focuses on bacterial communities in wastewater treatment processes, the quantity of microorganisms and structure of microbial consortia in wastewater treatment bioreactors. The review shows that the application of molecular techniques in studies of engineered environmental systems has increased our insight into the vast diversity and interaction of microorganisms present in wastewater treatment systems.

Community analysis of ammonia-oxidizing bacteria in activated sludge of eight wastewater treatment systems

We investigated the communities of ammonia-oxidizing bacteria (AOB) in activated sludge collected from eight wastewater treatment systems using polymerase chain reaction (PCR) followed by terminal restriction fragment length polymorphism (T-RFLP), cloning, and sequencing of the alpha-subunit of the ammonia monooxygenase gene (amoA). The T-RFLP fingerprint analyses showed that different wastewater treatment systems harbored distinct AOB communities. However, there was no remarkable difference among the AOB T-RFLP profiles from different parts of the same system. The T-RFLP fingerprints showed that a full-scale wastewater treatment plant (WWTP) contained a larger number of dominant AOB species than a pilot-scale reactor. The source of influent affected the AOB community, and the WWTPs treating domestic wastewater contained a higher AOB diversity than those receiving mixed domestic and industrial wastewater. However, the AOB community structure was little affected by the treatment process in this study. Phylogenetic analysis of the cloned amoA genes clearly indicated that all the dominant AOB in the systems was closely related to Nitrosomonas spp. not to Nitrosospira spp. Members of the Nitrosomonas oligotropha and Nitrosomonas communis clusters were found in all samples, while members of Nitrosomonas europaea cluster occurred in some systems.

Effect of substrate COD/N ratio on performance and microbial community structure of a membrane aerated biofilm reactor

Two parallel carbon-membrane aerated biofilm reactors were operated at well-defined conditions to investigate the effect of substrate COD/N ratios on the performance and microbial community structure of the bioreactor. Results showed that at substrate COD/N of 5, organic and nitrogen could be eliminated simultaneously, and COD removal degree, nitrification and denitrification efficiency reached 85%, 93% and 92%, respectively. With increasing substrate COD/N ratios, the specific oxygen utilization rates of nitrifying bacteria in biofilm were found to decrease, indicating that nitrifying population became less dominant. At substrate COD/N ratio of 6, excessive heterotrophs inhibited the activity of nitrifying bacteria greatly and thus led to poor nitrification process. With the help of fluorescence in situ hybridization (FISH), Nitrosomonas and Nitrosospira were identified as dominant ammonia-oxidizing bacteria in the biofilm at substrate COD/N of 0, whereas only Nitrosospira were detected in the biofilm at COD/N ratio of 5. Nitrospira were present as dominant nitrite-oxidizing bacteria in our study. Confocal laser scanning microscopy images revealed that at substrate COD/N ratio of 0 nitrifying bacteria existed throughout the biofilm and that at COD/N ratio of 5 they were mainly distributed in the inner layer of biofilm.

Autotrophic ammonia oxidation in a deep-sea hydrothermal plume

Direct evidence for autotrophic ammonia oxidation is documented for the first time in a deep-sea hydrothermal plume. Elevated NH(4) (+) concentrations of up to 341+/-136 nM were recorded in the plume core at Main Endeavour Field, Juan de Fuca Ridge. This fueled autotrophic ammonia oxidation rates as high as 91 nM day(-1), or 92% of the total net NH(4) (+) removal. High abundance of ammonia-oxidizing bacteria was detected using fluorescence in situ hybridization. Ammonia-oxidizing bacteria within the plume core (1.0-1.4x10(4) cells ml(-1)) accounted for 7.0-7.5% of the total microbial community, and were at least as abundant as methanotrophs. Ammonia-oxidizing bacteria were a substantial component of the particle-associated communities (up to 51%), with a predominance of the r-strategist Nitrosomonas-like cells. In situ chemolithoautotrophic organic carbon production via ammonia oxidation may yield 3.9-18 mg C m(-2) day(-1) within the plume directly over Main Endeavour Field. This rate was comparable to that determined for methane oxidation in a previous study, or at least four-fold greater than the flux of photosynthetic carbon reaching plume depths measured in another study. Hence, autotrophic ammonia oxidation in the neutrally buoyant hydrothermal plume is significant to both carbon and nitrogen cycling in the deep-sea water column at Endeavour, and represents another important link between seafloor hydrothermal systems and deep-sea biogeochemistry.

The inoculum effect on the ammonia-oxidizing bacterial communities in parallel sequential batch reactors

Three identical sequential batch reactors (SBRs) were each inoculated with sludge from a full-scale wastewater treatment plant (WWTP) treating a waste stream of different origin, i.e. a hospital, a meat processing company, and a municipal WWTP. The SBRs were run in parallel for 84 consecutive days to investigate whether the reactors would become more phylogenetically similar or stay separated concerning their functionality and microbial communities. Overall, the nitrification functionality was high throughout the experiment, and the size and structure of the sludge flocs were very similar. The total bacterial and ammonia-oxidizing bacterial (AOB) communities were analyzed by PCR-DGGE. Cluster analysis demonstrated very distinct bacterial communities in the three SBRs, not showing any trend becoming more similar. The carrying capacity, dynamics and functional organization of the communities were assessed by DGGE analysis and based on these patterns the range-weighted richness, moving window analysis, and constructing Pareto-Lorenz evenness distribution curves were calculated. Between the SBRs, highly comparable internal structure and dynamics of the AOB communities were observed, although they had only one AOB DGGE band in common. These observations indicate that community characteristics such as the extent of biodiversity and dynamics are more important indicators of good microbial functionality than the presence of certain specific species.

Diversity of frankiae in soils from five continents

Clone libraries of nifH gene fragments specific for the nitrogen-fixing actinomycete Frankia were generated from six soils obtained from five continents using a nested PCR. Comparative sequence analyses of all libraries (n=247 clones) using 96 to 97% similarity thresholds revealed the presence of three and four clusters of frankiae representing the Elaeagnus and the Alnus host infection groups, respectively. Diversity of frankiae was represented by fewer clusters (i.e., up to four in total) within individual libraries, with one cluster generally harboring the vast majority of sequences. Meta-analysis including sequences previously published for cultures (n=48) and for uncultured frankiae in root nodules of Morella pensylvanica formed in bioassays with the respective soils (n=121) revealed a higher overall diversity with four and six clusters of frankiae representing the Elaeagnus and the Alnus host infection groups, respectively, and displayed large differences in cluster assignments between sequences retrieved from clone libraries and those obtained from nodules, with assignments to the same cluster only rarely encountered for individual soils. These results demonstrate large differences between detectable Frankia populations in soil and those in root nodules indicating the inadequacy of bioassays for the analysis of frankiae in soil and the role of plants in the selection of frankiae from soil for root nodule formation.

Temperature influences the population structure of nitrite-oxidizing bacteria in activated sludge

Activated sludge from the municipal waste water treatment plant in Hamburg was seeded with mineral nitrite medium and incubated at 10°C, 17°C and 28°C. Dominant lithoautotrophic nitrite-oxidizing bacteria have been identified by electron microscopy, denaturing and temperature gradient gel electrophoresis and PCR with genus-specific primer pairs. The results have revealed the existence of three different genera of nitrite-oxidizing bacteria, namely Nitrospira, Nitrobacter and a novel cold-adapted nitrite oxidizer. As shown by electron microscopy members of the novel genus coexisted in activated sludge together with Nitrospira. A temperature-dependent shift in the population structure was demonstrated by cultivation-based approaches. The novel nitrite oxidizer was enriched at temperatures of 10°C and 17°C. Representatives of Nitrospira were able to grow in a broad temperature range between 10°C and 28°C and members of Nitrobacter were enriched during incubations at 17°C and 28°C. By subsequent 16S rDNA sequencing, the cold-adapted nitrite oxidizer was shown to be closely related to the betaproteobacterium 'Candidatus Nitrotoga arctica'. These findings demonstrated that the population structure of nitrite-oxidizing bacteria in activated sludge is more complex than previously thought and responds strongly to long-term temperature changes.

Distribution and rate of microbial processes in an ammonia-loaded air filter biofilm

The in situ activity and distribution of heterotrophic and nitrifying bacteria and their potential interactions were investigated in a full-scale, two-section, trickling filter designed for biological degradation of volatile organics and NH(3) in ventilation air from pig farms. The filter biofilm was investigated by microsensor analysis, fluorescence in situ hybridization, quantitative PCR, and batch incubation activity measurements. In situ aerobic activity showed a significant decrease through the filter, while the distribution of ammonia-oxidizing bacteria (AOB) was highly skewed toward the filter outlet. Nitrite oxidation was not detected during most of the experimental period, and the AOB activity therefore resulted in NO(2)(-), accumulation, with concentrations often exceeding 100 mM at the filter inlet. The restriction of AOB to the outlet section of the filter was explained by both competition with heterotrophic bacteria for O(2) and inhibition by the protonated form of NO(2)(-), HNO(2). Product inhibition of AOB growth could explain why this type of filter tends to emit air with a rather constant NH(3) concentration irrespective of variations in inlet concentration and airflow.

In situ characterization of nitrifying biofilm: minimizing biomass loss and preserving perspective

Methods for characterizing nitrifying bacteria within biofilms are of key importance to understand and optimize the nitrification kinetics of attached growth treatment facilities. In this work, we propose an analytical protocol based upon environmental scanning electron microscopy (ESEM) and confocal laser scanning microscopy (CSLM) in combination with fluorescent in situ hybridization (FISH) to characterize the structure of nitrifying biofilm as it remains attached to the original reactor substratum. This protocol minimizes the loss of mass and distortion of in situ perspective commonly associated with traditionally applied microscopic techniques and thereby enables a more accurate estimation of the nitrifying biomass within biofilm attached to the substratum. The use of ESEM eliminates the destructive preparatory procedures associated with traditional scanning electron microscopy and thus the loss of mass and shrinking of the samples. ESEM is used in this study to evaluate the percent coverage of the substratum with biofilm and the biofilm thickness. CLSM-FISH is used to determine cell counts in the biofilm and to characterize the undisturbed substratum/biofilm interface. By hybridizing and analyzing the nitrifying biofilm using CLSM as it remains attached to the substratum, the loss of material and distortion of in situ perspective associated with the biofilm detachment process is minimized. Moreover, by conducting the CLSM analysis directly on the nitrifying biofilm as it remains attached to the substratum it is shown that cell counts at the substratum/biofilm interface differ significantly from that located above the interface.

Nitrifying community structures and nitrification performance of full-scale municipal and swine wastewater treatment plants

This study evaluated nitrification performance and microbial ecology of nitrifying sludge in two full-scale wastewater treatment plants (WWTPs) including a municipal WWTP treating 20mgNL(-1) of ammonium and a swine WWTP treating 220mgNL(-1) of ammonium. These two plants differed in both wastewater characteristics and operating parameters, such as influent COD, TKN, ammonium, hydraulic retention time, and solids retention time, even though both plants achieve >85% nitrification efficiency. By employing molecular techniques, including terminal restriction fragment length polymorphism, cloning-sequencing and phylogenetic analyses targeting the 16S ribosomal RNA and group specific ammonia-monooxygenase functional gene (amoA), microbial community structures of nitrifying sludge and their significance to nitrification performance were evaluated. The results reveal that for the municipal WWTP Nitrosomonas marina-like AOB (ammonia-oxidizing bacteria) and Nitrospira-like NOB (nitrite-oxidizing bacteria) were the ubiquitously dominant nitrifiers, while Nitrosomonas europaea-, Nitrosomonas oligotropha-, and Nitrosospira-like AOB and Nitrobacter- and Nitrospira-like NOB were the major nitrifying populations found in the swine WWTP. The observed dissimilar nitrifying populations prevailing in these two plants may be related to niche differentiation concerning ammonium concentrations, system operation, and salinity. Moreover, our results suggest that the swine nitrifying sludge, involving relatively diverse AOB and NOB populations that perform the same task but with distinct growth and survival characters, may allow communities to maintain nitrifying capabilities when conditions change such as sudden increases in ammonium concentrations as examined with nitrification kinetic batch tests.

Performance of a pilot-scale submerged membrane bioreactor (MBR) in treating bathing wastewater

Bathing wastewater was treated by a pilot-scale submerged membrane bioreactor (MBR) for more than 60 days. The results showed that the removal rates of main pollutants of wastewater such as COD(Cr), LAS, NH(4)(+)-N and total nitrogen (TN) were above 93%, 99%, 99%, and 90%, respectively. The results of denaturing gel gradient electrophoresis (DGGE) and fluorescent in situ hybridization (FISH) indicated that the bacteria were stable. The abundant nitrobacteria intercepted by the membrane led to the high removal rate of ammonia and TN. FISH and 16S rDNA gene sequence analysis revealed that some specific phylogenetic group of bacteria, the Pseudomonas sp. Ochrobactrum anthropi sp. and Enterobacter sp. probably played a major role in the development of the mature biofilms, which led to the severe irreversible membrane biofouling.

Elevated lytic phage production as a consequence of particle colonization by a marine Flavobacterium (Cellulophaga sp.)

Bacteria growing on marine particles generally have higher densities and cell-specific activities than free-living bacteria. Since rapidity of phage adsorption is dependent on host density, while infection productivity is a function of host physiological status, we hypothesized that marine particles are sites of elevated phage production. In the present study, organic-matter-rich agarose beads and a marine phage-host pair (Cellulophaga sp., PhiS(M)) were used as a model system to examine whether bacterial colonization of particles increases phage production. While no production of phages was observed in plain seawater, the presence of beads enhanced attachment and growth of bacteria, as well as phage production. This was observed because of extensive lysis of bacteria in the presence of beads and a subsequent increase in phage abundance both on beads and in the surrounding water. After 12 h, extensive phage lysis reduced the density of attached bacteria; however, after 32 h, bacterial abundance increased again. Reexposure to phages and analyses of bacterial isolates suggested that this regrowth on particles was by phage-resistant clones. The present demonstration of elevated lytic phage production associated with model particles illustrates not only that a marine phage has the ability to successfully infect and lyse surface-attached bacteria but also that acquisition of resistance may affect temporal phage-host dynamics on particles. These findings from a model system may have relevance to the distribution of phage production in environments rich in particulate matter (e.g., in coastal areas or during phytoplankton blooms) where a significant part of phage production may be directly linked to these nutrient-rich "hot spots."

Presence of previously undescribed bacterial taxa in non-axenic Chlorella cultures

We determined the bacterial community profile in non-axenic cultures of Chlorella (Chlorophyceae, Chlorophyta) isolated from soil. The bacterial composition at the phylum level was different from that of whole soil bacteria, but it was similar to that reported for non-axenic cultures of marine microalgae such as diatoms (Bacillariophyceae, Heterokontophyta). Expected novel bacteria, i.e. those which do not have close relatives among described species, were maintained in the cultures, and these bacteria were chiefly composed of members of the phylum Bacteroidetes. They may have been 'as-yet-uncultured' but in practice unintentionally been cultured in microalgal cultures. They could serve as good bioresources in various fields of biological and ecological studies.

Analysis of the nitrifying bacterial community in BioCube sponge media using fluorescent in situ hybridization (FISH) and microelectrodes

There is growing interest in the development of more cost-effective and retrofit technologies for the upgrade and expansion of existing wastewater treatment plants with extreme space constraints. A free-floating sponge media (BioCube) process, using a 24 L lab scale reactor, was operated to study the nitrification profiles and microbial community. The COD removal efficiencies were maintained, at an average of 95%, with the mixed liquor suspended solids (MLSS) inside the BioCube sponge media maintained at 12,688 mg/L. The nitrification removal efficiencies were between 92% and 100%, with an average value of 99%. From the results of microelectrode measurements, the ammonium ion concentration was found to rapidly decrease from the surface of the BioCube sponge media to a depth of 2mm due to chemical reactions carried out by ammonia oxidizing bacteria (AOB) species. Multi-fluorescence in situ hybridization (FISH) has been used to investigate the spatial distributions of various microbial activities within reactors. Microbial communities were targeted using different oligonucleotide probes specific to AOB and nitrite oxidizing bacteria (NOB). There were a large number of AOB populations, but these were not uniformly distributed in the biofilm compared to the NOB populations.

Use of quantitative real-time PCR to monitor population dynamics of ammonia-oxidizing bacteria in batch process

A quantitative real-time PCR (QPCR) assay with the TaqMan system was used to quantify 16S rRNA genes of beta-proteobacterial ammonia-oxidizing bacteria (AOB) in a batch nitrification bioreactor. Five different sets of primers, together with a TaqMan probe, were used to quantify the 16S rRNA genes of beta-proteobacterial AOB belonging to the Nitrosomonas europaea, Nitrosococcus mobilis, Nitrosomonas nitrosa, and Nitrosomonas cryotolerans clusters, and the genus Nitrosospira. We also used PCR followed by denaturing gradient gel electrophoresis (DGGE), cloning, and sequencing of their 16S rRNA genes to identify the AOB species. Seed sludge from an industrial wastewater treatment process controlling high-strength nitrogen wastewater (500 mg/L NH4+-N) was used as the inoculum for subsequent batch experiment. The Nitrosomonas nitrosa cluster was the predominant AOB (2.3x10(5) copies/mL) in the start-up period of the batch experiment. However, from the exponential growth period, the Nitrosomonas europaea cluster was the most abundant AOB, and its 16S rRNA gene copy number increased to 8.9x10(6) copies/mL. The competitive dominance between the two AOB clusters is consistent with observed differences in ammonia tolerance and substrate affinity. Analysis of the DGGE results indicated the presence of Nitrosomonas europaea ATCC19718 and Nitrosomonas nitrosa Nm90, consistent with the QPCR results.

Rapid and reliable quantification of biofilm weight and nitrogen content of biofilm attached to polystyrene beads

Increased popularity of attached-growth wastewater treatment systems (e.g. biological aerated filtration processes-BAF) has created the need for a rapid and reliable method of characterizing biofilms. In addition to the mass of the biofilm that may serve as a control parameter for attached-growth treatment systems, the nitrogen content of the biofilm is also of great interest with increasingly strict nitrogen removal guidelines. Existing methods that may be used to analyse biofilms in such processes involve complex sample preparation and microbiological expertise that limit their application in many biofilm wastewater treatment studies and at existing treatment facilities as a feasible method of monitoring the biofilm. This paper describes a simple technical procedure that enables biofilm samples attached to polystyrene beads to be characterized in terms of the biofilm mass and the nitrogen content of the biofilm. The proposed protocol incorporates an agitation procedure that demonstrates 99.9% removal of the biofilm from polystyrene beads; a modified TSS procedure that measures the removed biofilm mass; and subsequently a modified total Kjeldahl nitrogen (TKN) procedure that enables the nitrogen content of the biofilm to be measured directly on the filter. Moreover, this protocol allows numerous beads to be analysed with limited manipulation and without the loss of critical mass.

Estimating nitrifying biomass in drinking water filters for surface water treatment

The objective of this work is to estimate active nitrifying biomass and its main influencing factors in low-loaded biofilters based on operational data. An analytical approach based on balancing growth, decay and biomass removed by backwashing is proposed. The method is developed and applied in pilot-scale rapid sand filters for drinking water treatment. Decay rate was measured directly in the filter for different temperatures. To assess the amount of active biomass in backwash water, a technique based on respiration measurements was used. Backwash losses increased overproportional with balanced biomass in the filter. The impact of both parameters on active biomass is quantified exemplarily for a given constant nitrification rate.

Primer and probe sets for group-specific quantification of the genera Nitrosomonas and Nitrosospira using real-time PCR

Use of quantitative real-time PCR (QPCR) with TaqMan probes is increasingly popular in various environmental works to detect and quantify a specific microorganism or a group of target microorganism. Although many aspects of conducting a QPCR assay have become very easy to perform, a proper design of oligonucleotide sequences comprising primers and a probe is still considered as one of the most important aspects of a QPCR application. This work was conducted to design group specific primer and probe sets for the detection of ammonia oxidizing bacteria (AOB) using a real-time PCR with a TaqMan system. The genera Nitrosomonas and Nitrosospira were grouped into five clusters based on similarity of their 16S rRNA gene sequences. Five group-specific AOB primer and probe sets were designed. These sets separately detect four subgroups of Nitrosomonas (Nitrosomonas europaea-, Nitrosococcus mobilis-, Nitrosomonas nitrosa-, and Nitrosomonas cryotolerans-clusters) along with the genus Nitrosospira. Target-group specificity of each primer and probe set was initially investigated by analyzing potential false results in silico, followed by a series of experimental tests for QPCR efficiency and detection limit. In general, each primer and probe set was very specific to the target group and sensitive to detect target DNA as low as two 16S rRNA gene copies per reaction mixture. QPCR efficiency, higher than 93.5%, could be achieved for all primer and probe sets. The primer and probe sets designed in this study can be used to detect and quantify the beta-proteobacterial AOB in biological nitrification processes and various environments.

Zinc deprivation of methanol fed anaerobic granular sludge bioreactors

The effect of omitting zinc from the influent of mesophilic (30 °C) methanol fed upflow anaerobic sludge bed (UASB) reactors, and latter zinc supplementation to the influent to counteract the deprivation, was investigated by coupling the UASB reactor performance to the microbial ecology of the bioreactor sludge. Limitation of the specific methanogenic activity (SMA) on methanol due to the absence of zinc from the influent developed after 137 days of operation. At that day, the SMA in medium with a complete trace metal solution except Zn was 3.4 g CH₄-COD g VSS⁻¹ day⁻¹, compared to 4.2 g CH₄-COD g VSS⁻¹ day⁻¹ in a medium with a complete (including zinc) trace metal solution. The methanol removal capacity during these 137 days was 99% and no volatile fatty acids accumulated. Two UASB reactors, inoculated with the zinc-deprived sludge, were operated to study restoration of the zinc limitation by zinc supplementation to the bioreactor influent. In a first reactor, no changes to the operational conditions were made. This resulted in methanol accumulation in the reactor effluent after 12 days of operation, which subsequently induced acetogenic activity 5 days after the methanol accumulation started. Methanogenesis could not be recovered by the continuous addition of 0.5 μM ZnCl₂ to the reactor for 13 days. In the second reactor, 0.5 μM ZnCl₂ was added from its start-up. Although the reactor stayed 10 days longer methanogenically than the reactor operated without zinc, methanol accumulation was observed in this reactor (up to 1.1 g COD-MeOH L⁻¹) as well. This study shows that zinc limitation can induce failure of methanol fed UASB reactors due to acidification, which cannot be restored by resuming the continuous supply of the deprived metal.

Role of nickel in high rate methanol degradation in anaerobic granular sludge bioreactors

The effect of nickel deprivation from the influent of a mesophilic (30 degrees C) methanol fed upflow anaerobic sludge bed (UASB) reactor was investigated by coupling the reactor performance to the evolution of the Methanosarcina population of the bioreactor sludge. The reactor was operated at pH 7.0 and an organic loading rate (OLR) of 5-15 g COD l(-1) day(-1) for 191 days. A clear limitation of the specific methanogenic activity (SMA) on methanol due to the absence of nickel was observed after 129 days of bioreactor operation: the SMA of the sludge in medium with the complete trace metal solution except nickel amounted to 1.164 (+/-0.167) g CH(4)-COD g VSS(-1) day(-1) compared to 2.027 (+/-0.111) g CH(4)-COD g VSS(-1) day(-1) in a medium with the complete (including nickel) trace metal solution. The methanol removal efficiency during these 129 days was 99%, no volatile fatty acid (VFA) accumulation was observed and the size of the Methanosarcina population increased compared to the seed sludge. Continuation of the UASB reactor operation with the nickel limited sludge lead to incomplete methanol removal, and thus methanol accumulation in the reactor effluent from day 142 onwards. This methanol accumulation subsequently induced an increase of the acetogenic activity in the UASB reactor on day 160. On day 165, 77% of the methanol fed to the system was converted to acetate and the Methanosarcina population size had substantially decreased. Inclusion of 0.5 muM Ni (dosed as NiCl(2)) to the influent from day 165 onwards lead to the recovery of the methanol removal efficiency to 99% without VFA accumulation within 2 days of bioreactor operation.

Physiological state, growth mode, and oxidative stress play a role in Cd(II)-mediated inhibition of Nitrosomonas europaea 19718

The goal of this study was to determine the impact of physiological growth states (batch exponential and batch stationary growth) and growth modes (substrate-limited chemostat, substrate-sufficient exponential batch, and substrate-depleted stationary batch growth) on several measures of growth and responses to Cd(II)-mediated inhibition of Nitrosomonas europaea strain 19718. The specific oxygen uptake rate (sOUR) was the most sensitive indicator of inhibition among the different responses analyzed, including total cell abundance, membrane integrity, intracellular 16S rRNA/DNA ratio, and amoA expression. This observation remained true irrespective of the physiological state, the growth mode, or the mode of Cd(II) exposure. Based on the sOUR, a strong time-dependent exacerbation of inhibition (in terms of an inhibition coefficient [K(i)]) in exponential batch cultures was observed. Long-term inhibition levels (based on K(i) estimates) in metabolically active chemostat and exponential batch cultures were also especially severe and comparable. In contrast, the inhibition level in stationary-phase cultures was 10-fold lower and invariable with exposure time. Different strategies for surviving substrate limitation (a 10-fold increase in amoA expression) and starvation (the retention of 16S rRNA levels) in N. europaea cultures were observed. amoA expression was most negatively impacted by Cd(II) exposure in the chemostat cultures, was less impacted in exponential batch cultures, and was least impacted in stationary batch cultures. Although the amoA response was consistent with that of the sOUR, the amoA response was not as strong. The intracellular 16S rRNA/DNA ratio, as determined by fluorescence in situ hybridization, also did not uniformly correlate with the sOUR under conditions of inhibition or no inhibition. Finally, Cd(II)-mediated inhibition of N. europaea was attributed partially to oxidative stress.

Biological removal of nitrogen from wastewater

This comprehensive review discusses diverse conventional and novel technologies for nitrogen removal from wastewater. Novel technologies have distinct advantages in terms of saving configuration, aeration, and carbon sources. Each novel technology possesses promising features and potential problems. For instance, SND and OLAND processes can achieve 100% total nitrogen removal, but the low oxygen concentration required by these two processes substantially reduces the nitrification rate, which limits their application. On the other hand, denitrification can still be carried out by aerobic denitrifiers at high DO levels in activated sludge process, but it is difficult to cultivate this type of bacteria. The SHARON process is most commonly used for shortcut nitrification and denitrification because of its low requirements for retention time, oxygen concentration, and carbon source. However, its high operational temperature (about 35 degrees C) limits the application. Several real-time control strategies (DO, pH, and ORP) have been developed to achieve a stable nitrite accumulation in SHARON. The ANAMMOX process can sustain at high total-N loadings and has been employed in full-scale treatment plants, but the problem of nitrite supply has not been solved, and the treated wastewater still contains nitrate. In addition, the inoculation and enrichment of ANAMMOX bacteria (i.e., anaerobic AOB) is difficult. The problem of nitrite supply has been solved by combining partial nitrification with ANAMMOX, which provides abundant nitrite for anaerobic AOB. ANAMMOX is currently used for treating sludge digestion supernatant. Aerobic dammonitrification is a process combining partial nitrification and ANAMMOX at different layers of biofilm. Although the technology has been tested in pilot- and full-scale experiments, the mechanism is still unclear. CANON and OLAND are one-step ammonium removal processes that possess distinct advantages of saving carbon sources and aeration costs. The major challenge is the enrichment of anaerobic microorganisms capable of oxidizing ammonia with nitrite as the electron acceptor. Molecular biology and environmental biotechnology can help identify functional microorganisms, characterize microbial communities, and develop new nitrogen removal processes. Extensive research should be conducted to apply and optimize these novel processes in wastewater treatment plants. More effort should be invested to combine these novel processes (e.g., partial nitrification, ANAMMOX) to enhance nitrogen removal efficiency.

Redox control bioreactor: A unique biological water processor

The redox control bioreactor (RCB) is a new hollow fiber membrane bioreactor (HFMBR) design in which oxygen and hydrogen gases are provided simultaneously through separate arrays of juxtaposed hollow fiber (HF) membranes. This study applied the RCB for completely autotrophic conversion of ammonia to N(2) through nitrification with O(2) and denitrification using hydrogen as an electron donor (i.e., autohydrogentrophic denitrification). The hypothesis of this research was that efficient biofilm utilization of O(2) and H(2) at respective HFs would limit transport of these gases to bulk fluid, thereby enabling completely autotrophic ammonia conversion to N(2) through the co-occurrence of ammonia oxidation (O(2)-HF biofilms) and autohydrogenotrophic denitrification (H(2)-HF biofilms). A prototype RCB was fabricated and operated for 215 days on a synthetic, organic-free feedstream containing 217 mg L(-1) NH(4)(+)-N. When O(2) and H(2) were simultaneously supplied, the RCB achieved a steady NH(4)(+)-N removal flux of 5.8 g m(-2) day(-1) normalized to O(2)-HF surface area with a concomitant removal flux of 4.4 g m(-2) day(-1) (NO(3)(-))+NO(2)(-))-N based on H(2)-HF surface area. The significance of H(2) supply was confirmed by an increase in effluent NO(3)(-)-N when H(2) supply was discontinued and a decline in NO(3)(-)-N when H(2) supply was restarted. Increases in H(2) pressure caused decreased ammonia utilization, suggesting that excess H(2) interfered with nitrification. Microprobe profiling across radial transects revealed significant gradients in dissolved O(2) on spatial scales of 1 mm or less. Physiological and molecular analysis of biofilms confirmed that structurally and functionally distinct biofilms developed on adjacent, juxtaposed fibers.

Nitrosomonas Nm143-like ammonia oxidizers and Nitrospira marina-like nitrite oxidizers dominate the nitrifier community in a marine aquaculture biofilm

Zero-discharge marine aquaculture systems are an environmentally friendly alternative to conventional aquaculture. In these systems, water is purified and recycled via microbial biofilters. Here, quantitative data on nitrifier community structure of a trickling filter biofilm associated with a recirculating marine aquaculture system are presented. Repeated rounds of the full-cycle rRNA approach were necessary to optimize DNA extraction and the probe set for FISH to obtain a reliable and comprehensive picture of the ammonia-oxidizing community. Analysis of the ammonia monooxygenase gene (amoA) confirmed the results. The most abundant ammonia-oxidizing bacteria (AOB) were members of the Nitrosomonas sp. Nm143-lineage (6.7% of the bacterial biovolume), followed by Nitrosomonas marina-like AOB (2.2% of the bacterial biovolume). Both were outnumbered by nitrite-oxidizing bacteria of the Nitrospira marina-lineage (15.7% of the bacterial biovolume). Although more than eight other nitrifying populations were detected, including Crenarchaeota closely related to the ammonia-oxidizer 'Nitrosopumilus maritimus', their collective abundance was below 1% of the total biofilm volume; their contribution to nitrification in the biofilter is therefore likely to be negligible.

Nitrifying community analysis in a single submerged attached-growth bioreactor for treatment of high-ammonia waste stream

This study investigated the nitrifying community structure in a single-stage submerged attached-growth bioreactor (SAGB) that successfully achieved stable nitrogen removal over nitrite of a high-strength ammonia wastewater. The reactor was operated with intermittent aeration and external carbon addition (methanol). With influent ammonia and total Kjeldahl nitrogen ranging from 537 to 968 mg/L and 643 to 1510 mg/L, respectively, 85% nitrogen removal was obtained, and effluent was dominated by nitrite (NO2-/NOx > 0.95). Nitrifying community analysis using fluorescence in situ hybridization (FISH), with a hierarchical set of probes targeting known ammonia-oxidizing bacteria (AOB) within beta-proteobacteria, showed that the AOB community of the biofilter consists almost entirely of members of the Nitrosomonas europaea/eutropha and the Nitrosococcus mobilis lineages. Image analysis of FISH pictures was used to quantify the identified AOB, and it was estimated that Nitrosomonas europaea/eutropha-like AOB accounted for 4.3% of the total volume of the biofilm, while Nitrosococcus mobilis-like AOB made up 1.2%; these numbers summed up to a total AOB fraction of 5.5% of the total volume on the biofilm. Nitrite-oxidizing bacteria (NOB) were not detectable in the biofilm samples with probes for either Nitrospira sp. or Nitrobacter sp., which indicated that NOB were either absent from the biofilters or present in numbers below the detection limit of FISH (< 0.1% of the total biofilm). Nitrite oxidizers were likely outcompeted from the system because of the free ammonia inhibition and the possibility that the aeration period (from intermittent aeration) was not sufficiently long for the NOB to be released from the competition for oxygen with heterotrophs and AOB. The nitrogen removal via nitrite in a SAGB reactor described in this study is applicable for high-ammonia-strength wastewater treatment, such as centrate or industrial wastes.

Effects of environmental conditions on the nitrifying population dynamics in a pilot wastewater treatment plant

The effect of environmental conditions, especially ammonium concentration, on community composition and nitrification activity of nitrifying bacterial biofilms in a pilot wastewater treatment plant was examined. A decreasing ammonium gradient was created when four aerated tanks with suspended carrier material were serially fed with wastewater. Community composition was analysed using fluorescence in situ hybridization (FISH) probes as well as partial 16S rRNA and amoA gene analysis using polymerase chain reaction-denaturating gradient gel electrophoresis (PCR-DGGE) and sequencing. Fluorescence in situ hybridization probes identified at least five ammonia-oxidizing bacterial (AOB) and two nitrite-oxidizing bacterial (NOB) populations. A change in nitrifying community was detected in the tanks, indicating that ammonium was an important structuring factor. Further, we found support for different autoecology within the Nitrosomonas oligotropha lineage, as at least one population within this lineage increased in relative abundance with ammonium concentration while another population decreased. Absolute numbers of AOB and NOB growing in biofilms on the carriers were determined and the cell specific nitrification rates calculated seemed strongly correlated to ammonium concentration. Oxygen could also be limiting in the biofilms of the first tank with high ammonium concentrations. The response of the nitrifying community to increased ammonium concentrations differed between the tanks, indicating that activity correlates with community structure.