Aerobic denitrification in various heterotrophic nitrifiers

Effect of substrate Henry's constant on biofilter performance

Butanol, ether, toluene, and hexane, which have Henry's constants ranging from 0.0005 to 53, were used to investigate the effects of substrate solubility or availability on the removal of volatile organic compounds (VOCs) in trickle-bed biofilters. Results from this study suggest that, although removal of a VOC generally increases with a decrease in its Henry's constant, an optimal Henry's constant range for biofiltration may exist. For the treatment of VOCs with high Henry's constant values, such as hexane and toluene, the transfer of VOCs between the vapor and liquid phases or between the vapor phase and the biofilm is a rate-determining step. However, oxygen (O2) transfer may become a rate-limiting step in treating VOCs with low Henry's constants, such as butanol, especially at high organic loadings. The results demonstrated that in a gas-phase aerobic biofilter, nitrate can serve both as a growth-controlling nutrient and as an electron acceptor in a biofilm for the respiration of VOCs with low Henry's constants. Microbial communities within the biofilters were examined using denaturing gradient gel electrophoresis to provide a more complete picture of the effect of O2 limitation and denitrification on biofilter performance.

Characterization of the expression and activity of the periplasmic nitrate reductase of Paracoccus pantotrophus in chemostat cultures

The periplasmic nitrate reductase (Nap) from Paracoccus pantotrophus has a role in cellular redox balancing. Previously, transcription from the nap promoter in P. pantotrophus was shown to be responsive to the oxidation state of the carbon substrate. During batch culture, expression was higher during growth on reduced substrates such as butyrate compared to more oxidized substrates such as succinate. In the present study the effect of growth rate on nap expression in succinate-, acetate- and butyrate-limited chemostat cultures was investigated. In all three cases transcription from the nap promoter and Nap enzyme activity showed a strong correlation. At the fastest growth rates tested for the three substrates nap expression and Nap activity were highest when growth occurred on the most reduced substrate (butyrate > acetate > succinate). However, in all three cases a bell-shaped pattern of expression was observed as a function of growth rate, with the highest levels of nap expression and Nap activity being observed at intermediate growth rates. This effect was most pronounced on succinate, where an approximately fivefold variation was observed, and at intermediate dilution rates nap expression and Nap activity were comparable on all three carbon substrates. Analysis of mRNA prepared from the succinate-grown cultures revealed that different transcription initiation start sites for the nap operon were utilized as the growth rate changed. This study establishes a new regulatory feature of nap expression in P. pantotrophus that occurs at the level of transcription in response to growth rate in carbon-limited cultures.

Metabolism of Alcaligenes denitrificans in biofilm vs planktonic cells

AIMS

To compare the effect of phosphorous concentration (200 mg P l(-1) and 20 mg P l(-1)) on the denitrifying efficiency of Alcaligenes denitrificans when in the form of planktonic cells or in the form of a biofilm, and to select the most adequate C/N ratio.

METHODS AND RESULTS

Two types of assays were carried out: with planktonic cells and with cells in biofilm form. Anoxic bottles with the appropriate C/N and phosphorous concentration were incubated at 30 degrees C and submitted to orbital shaking at 150 rev min(-1). The specific activity of cells in biofilm form, in terms of substrate consumption, was significantly higher than cells in planktonic form. With regard to the effect of increasing phosphorous concentration, an increase in specific activity was also only evident when the cells were in biofilm form.

CONCLUSIONS

The two forms showed different performances and phosphorous concentration only markedly affected the biofilm form.

SIGNIFICANCE AND IMPACT OF THE STUDY

The importance of the C/N/P ratio in the denitrification process is demonstrated. As there was no report in the literature about the stoichiometric relationship of heterotrophic denitrification with citrate, its stoichiometry, including the requirement for cell synthesis, was determined.

Nitrate reduction in the periplasm of gram-negative bacteria

In contrast to the bacterial assimilatory and membrane-associated, respiratory nitrate reductases that have been studied for many years, it is only recently that periplasmic nitrate reductases have attracted growing interest. Recent research has shown that these soluble proteins are widely distributed, but vary greatly between species. All of those so far studied include four essential components: the periplasmic molybdoprotein, NapA, which is associated with a small, di-haem cytochrome, NapB; a putative quinol oxidase, NapC; and a possible pathway-specific chaperone, NapD. At least five other components have been found in different species. Other variations between species include the location of the nap genes on chromosomal or extrachromosomal DNA, and the environmental factors that regulate their expression. Despite the relatively small number of bacteria so far screened, striking correlations are beginning to emerge between the organization of the nap genes, the physiology of the host, the conditions under which the nap genes are expressed, and even the fate of nitrite, the product of Nap activity. Evidence is emerging that Nap fulfills a novel role in nitrate scavenging by some pathogenic bacteria.

Isolation of an aerobic denitrifying bacterial strain NS-2 from the activated sludge of piggery wastewater treatment systems in Taiwan possessing denitrification under 92% oxygen atmosphere

AIMS

To isolate aerobic denitrifying bacteria which will be applied to piggery wastewater treatment facilities for enhanced nitrate and nitrite removal.

METHODS AND RESULTS

Nitrate-supplemented basal medium in airtight, crimp-sealed serum bottles containing an atmosphere of 92% oxygen was inoculated with denitrifiers, strains NS-2 and SM-3, and incubated at 30 degrees C. After 20 h, the concentration of nitrate was decreased rapidly by both NS-2 and SM-3. Nitrite production was almost zero during the whole experimental period for both strains. Nitrogen gas production peaked at the 20 h for both NS-2 (8.20 +/- 1.03 mmol l(-1)) and SM-3 (3.93 +/- 0.16 mmol l(-1)).

CONCLUSIONS

Strain NS-2, which produced the highest N2 concentration in this work, was identified as Pseudomonas stutzeri. This strain is the most capable of aerobic and anaerobic conversion of nitrate to N2 without forming a nitrite intermediate.

SIGNIFICANCE AND IMPACT OF THE STUDY

Strain NS-2 is highly promising for future application in in situ piggery wastewater treatment.

Nitrification and denitrification as a source for NO and NO2 production in high-strength wastewater

Laboratory and half-technical scale experiments were performed to evaluate nitric oxide (NO) and nitrogen dioxide (NO2) production during biological N-elimination from wastewater with high ammonium concentration (about 700 mg N L-1). In a laboratory scale bioreactor with biomass retention, the ammonia oxidizer Nitrosomonas europaea and the denitrifier Paracoccus denitrificans were grown as reference organisms in co-culture in order to simulate the nitrifying and denitrifying community of wastewater treatment plants. Synthetic wastewater and sludge liquor from the municipal wastewater treatment plant in Lueneburg (Germany) were used. In the laboratory scale reactor, during the treatment of synthetic wastewater, 0.28% of the oxidized ammonium-N was released as NO-N by a pure culture of Nitrosomonas. A simultaneously nitrifying and denitrifying co-culture only released 0.04 to 0.2%. NO2 formation was not observed. NO production was much higher in sludge liquor. A pure culture of Nitrosomonas produced 0.52% NO + NO2-N (= NOx-N), a co-culture of Nitrosomonas and Paracoccus even 1.64% NOx-N. The production rate strongly depended on the media and the organisms used. In a co-culture of N. europaea and P denitrificans, Nitrosomonas was shown to be the most efficient NO producer. NO production increased with ammonium oxidation rate and with nitrite concentration of the medium. In synthetic wastewater, NO production was not influenced by reduced oxygen content. However, in sludge liquor NO production rate increased with decreasing O2 concentration. Here, for the first time, the formation of significant amounts of NO2 during simultaneous nitrification/denitrification could be demonstrated. In half-technical scale experiments, only 0.07% of the oxidized ammonium-N was released as NO-N from the nitrification stage. NO2 was not detectable. Release of nitric oxide from the denitrification stage was mainly diffusion limited and the amount produced did not exceed 0.0001%. A calculation on the basis of the results presented, revealed that biological treatment of nitrogen-rich wastewater is not a significant source for pollution of the atmosphere with NOx in industrial areas.

Combined phosphate and nitrogen removal in a sequencing batch reactor using the aerobic denitrifier, Microvirgula aerodenitrificans

A phosphate removal sludge was bioaugmented with the aerobic denitrifier, Microvirgula aerodenitrificans in order to reduce the nitrate produced during the aerobic nitrifying-phosphate uptake phase. Fluorescent in situ hybridization (FISH) was used to follow the fate of the added strain. In order to maintain the pure strain in the complex ecosystem, diverse physiological and kinetic based strategies of bioaugmentation were tested under the sequencing batch reactor (SBR) type culture. The nature of the M. aerodenitrificans inoculum (adapted to nitrate-aerobic conditions or to anoxic one) had no influence on the SBR performances and did not enhance aerobic denitrifying performances. The optimum quantity of the added strain (10% of the total biomass) seemed to have much more positive influence on the long term maintenance of the pure strain than on the SBR performances. A small but daily supply of M. aerodenitrificans gave exactly the same result than a massive and 1-day supply, i.e. no enhancement of performances and no amelioration of the length of maintenance. A continuous supply of carbon during the first hour of the aerobic phase combined to a 10% supply of M. aerodenitrificans gave the best compromise in terms of phosphate removal, nitrification and aerobic denitrification performances. It was accompanied too by a decreased number of the ammonia and nitrite-oxidizing bacteria and a modification of the nitrite-oxidizing floc structure. FISH on M. aerodenitrificans revealed that (i) before bioaugmentation, the strain was already present in the phosphate removal sludge and (ii) the added bacteria almost disappeared from the reactor after 16 HRT. In a last experiment, M. aerodenitrificans embedded in alginate beads allowed enhancement of both aerobic denitrifying performances and length of strain maintenance.

Ecological study of a bioaugmentation failure

A nitrifying sequencing batch reactor was inoculated twice with the aerobic denitrifying bacterium Microvirgula aerodenitrificans and fed with acetate. No improvement was obtained on nitrogen removal. The second more massive inoculation was even followed by a nitrification breakdown, while at the same time, nitrification remained stable in a second reactor operated under the same conditions without bioaugmentation. Fluorescent in situ hybridization with rRNA-targeted probes revealed that the added bacteria almost disappeared from the reactor within 2 days, and that digestive vacuoles of protozoa gave strong hybridization signals with the M. aerodenitrificans-specific probe. An overgrowth of protozoa, coincident with the disappearance of free-living bacteria, was monitored by radioactive dot-blot hybridization only in the bioaugmented reactor. Population dynamics were analysed with a newly developed in situ quantification procedure of the probe-targeted bacteria. The nitrifying groups of bacteria decreased in a similar way in the bioaugmented and non-bioaugmented reactors. Other bacterial groups evolved differently. The involvement of different ecological parameters are discussed separately for each reactor. These results underline the importance of predator-prey interaction and illustrate the undesirable effects of massive bioaugmentation.

Closed water recirculating system for fish rearing equipped with bioreactor capable of simultaneous nitrification and denitrification

Five crucian carp, Carassius auratus langsdorfiicarps had been reared in a closed water recirculating system. The system was equipped with the compact bioreactor using the plate gels capable of both nitrification and denitrification in a single unit. Ammonia and nitrite concentrations in the rearing water had been maintained below 0.05 mg-N/L, and nitrate concentration also controlled between 2 and 8 mg-N/L with the bioreactor. As concerns nitrogen budget in the closed system, 95.0% of nitrogen income from feed was lost as nitrogen gas from the closed system. All fish was alive for 91 days without any unusual behavior. Thus, the bioreactor performed both nitrification and denitrification abilities enough to rear the five fish for 91 days. The bioreactor using the plate gels would be effective to simplify the closed system both physically and operationally, since it can remove the ammonia excreted from fish as nitrogen gas by a single step.

On the occurrence of anoxic microniches, denitrification, and sulfate reduction in aerated activated sludge

A combination of different methods was applied to investigate the occurrence of anaerobic processes in aerated activated sludge. Microsensor measurements (O(2), NO(2)(-), NO(3)(-), and H(2)S) were performed on single sludge flocs to detect anoxic niches, nitrate reduction, or sulfate reduction on a microscale. Incubations of activated sludge with (15)NO(3)(-) and (35)SO(4)(2-) were used to determine denitrification and sulfate reduction rates on a batch scale. In four of six investigated sludges, no anoxic zones developed during aeration, and consequently denitrification rates were very low. However, in two sludges anoxia in flocs coincided with significant denitrification rates. Sulfate reduction could not be detected in any sludge in either the microsensor or the batch investigation, not even under short-term anoxic conditions. In contrast, the presence of sulfate-reducing bacteria was shown by fluorescence in situ hybridization with 16S rRNA-targeted oligonucleotide probes and by PCR-based detection of genes coding for the dissimilatory sulfite reductase. A possible explanation for the absence of anoxia even in most of the larger flocs might be that oxygen transport is not only diffusional but enhanced by advection, i.e., facilitated by flow through pores and channels. This possibility is suggested by the irregularity of some oxygen profiles and by confocal laser scanning microscopy of the three-dimensional floc structures, which showed that flocs from the two sludges in which anoxic zones were found were apparently denser than flocs from the other sludges.

Molecular genetics of the genus Paracoccus: metabolically versatile bacteria with bioenergetic flexibility

Paracoccus denitrificans and its near relative Paracoccus versutus (formerly known as Thiobacilllus versutus) have been attracting increasing attention because the aerobic respiratory system of P. denitrificans has long been regarded as a model for that of the mitochondrion, with which there are many components (e.g., cytochrome aa3 oxidase) in common. Members of the genus exhibit a great range of metabolic flexibility, particularly with respect to processes involving respiration. Prominent examples of flexibility are the use in denitrification of nitrate, nitrite, nitrous oxide, and nitric oxide as alternative electron acceptors to oxygen and the ability to use C1 compounds (e.g., methanol and methylamine) as electron donors to the respiratory chains. The proteins required for these respiratory processes are not constitutive, and the underlying complex regulatory systems that regulate their expression are beginning to be unraveled. There has been uncertainty about whether transcription in a member of the alpha-3 Proteobacteria such as P. denitrificans involves a conventional sigma70-type RNA polymerase, especially since canonical -35 and -10 DNA binding sites have not been readily identified. In this review, we argue that many genes, in particular those encoding constitutive proteins, may be under the control of a sigma70 RNA polymerase very closely related to that of Rhodobacter capsulatus. While the main focus is on the structure and regulation of genes coding for products involved in respiratory processes in Paracoccus, the current state of knowledge of the components of such respiratory pathways, and their biogenesis, is also reviewed.

Stable-Isotope Analysis of a Combined Nitrification-Denitrification Sustained by Thermophilic Methanotrophs under Low-Oxygen Conditions

To simulate growth conditions experienced by microbiota at O(inf2)-limited interfaces of organic matter in compost, an experimental system capable of maintaining dual limitations of oxygen and carbon for extended periods, i.e., a pO(inf2)-auxostat, has been used. (sup15)N tracer studies on thermophilic (53(deg)C) decomposition processes occurring in manure-straw aggregates showed the emission of dinitrogen gas from the reactor as a result of simultaneous nitrification and denitrification at low pO(inf2) values (0.1 to 2.0%, vol/vol). The N loss was confirmed by nitrogen budget studies of the system. Depending on the imposed pO(inf2), 0.6 to 1.4 mmol of N/day (i.e., 20 to 40% of input N) was emitted as N(inf2). When the pO(inf2) was raised, the rates of both nitrification and denitrification increased instantaneously, indicating that ammonia oxidation was limited by oxygen. In auxostats permanently running at pO(inf2) >= 2% (vol/vol), the free ammonium pool was almost completely oxidized and was converted to nitrite plus nitrate and N(inf2) gas. Labelling of the auxostat with [(sup13)C]carbonate was conducted to reveal whether nitrification was of autotrophic or heterotrophic origin. Incorporation of (sup13)CO(inf2) into population-specific cellular compounds was evaluated by profiling the saponifiable phospholipid fatty acids (FAs) by using capillary gas chromatography and subsequently analyzing the (sup13)C/(sup12)C ratios of the individual FAs, after their combustion to CO(inf2), by isotope ratio mass spectrometry. Apart from the observed label incorporation into FAs originating from a microflora belonging to the genus Methylococcus (type X group), supporting nitrification of a methylotrophic nature, this analysis also corroborated the absence of truly autotrophic nitrifying populations. Nevertheless, the extent to which ammonia oxidation continued to exist in this thermophilic community suggested that a major energy gain could be associated with it.

Nitrous oxide production by Alcaligenes faecalis under transient and dynamic aerobic and anaerobic conditions

Nitrous oxide can be a harmful by-product in nitrogen removal from wastewater. Since wastewater treatment systems operate under different aeration regimens, the influence of different oxygen concentrations and oxygen fluctuations on denitrification was studied. Continuous cultures of Alcaligenes faecalis TUD produced N2O under anaerobic as well as aerobic conditions. Below a dissolved oxygen concentration of 5% air saturation, the relatively highest N2O production was observed. Under these conditions, significant activities of nitrite reductase could be measured. After transition from aerobic to anaerobic conditions, there was insufficient nitrite reductase present to sustain growth and the culture began to wash out. After 20 h, nitrite reductase became detectable and the culture started to recover. Nitrous oxide reductase became measurable only after 27 h, suggesting sequential induction of the denitrification reductases, causing the transient accumulation of N2O. After transition from anaerobic conditions to aerobic conditions, nitrite reduction continued (at a lower rate) for several hours. N2O reduction appeared to stop immediately after the switch, indicating inhibition of nitrous oxide reductase, resulting in high N2O emissions (maximum, 1.4 mmol liter-1 h-1). The nitrite reductase was not inactivated by oxygen, but its synthesis was repressed. A half-life of 16 to 22 h for nitrite reductase under these conditions was calculated. In a dynamic aerobic-anaerobic culture of A. faecalis, a semisteady state in which most of the N2O production took place after the transition from anaerobic to aerobic conditions was obtained. The nitrite consumption rate in this culture was equal to that in an anaerobic culture (0.95 and 0.92 mmol liter-1 h-1, respectively), but the production of N2O was higher in the dynamic culture (28 and 26% of nitrite consumption, respectively).

Distribution, activity, and diversity of heterotrophic nitrifiers originating from East Pacific deep-sea hydrothermal vents

Nitrifiers (bacteria, n = 160) were enriched and isolated from samples of hydrothermal waters, sediments, invertebrate tissues, and chimney rocks collected from two East Pacific deep-sea hydrothermal vents (2000 m): the 13°N site and the Guaymas Basin. They were nitrite producers and seemed be widely and uniformly distributed in various parts of hydrothermal ecosystem. These bacteria grew and nitrified better heterotrophically than autotrophically and they possessed characteristics of heterotrophic nitrifiers. All were aerobic, mesophilic gram-negative rods with a unfermentative metabolism and 88% were nitrate reducers or denitrifiers. They were characterized by a high physiological and nutritional diversity, and because of their ability to ammonify, nitrify, and reduce nitrate, they could largely contribute to the nitrogen cycle in hydrothermal sites.Key words: hydrothermal vents, heterotrophic bacteria, nitrifying activity.

A comparison of NO and N2O production by the autotrophic nitrifier Nitrosomonas europaea and the heterotrophic nitrifier Alcaligenes faecalis

Soil microorganisms are important sources of the nitrogen trace gases NO and N2O for the atmosphere. Present evidence suggests that autotrophic nitrifiers such as Nitrosomonas europaea are the primary producers of NO and N2O in aerobic soils, whereas denitrifiers such as Pseudomonas spp. or Alcaligenes spp. are responsible for most of the NO and N2O emissions from anaerobic soils. It has been shown that Alcaligenes faecalis, a bacterium common in both soil and water, is capable of concomitant heterotrophic nitrification and denitrification. This study was undertaken to determine whether heterotrophic nitrification might be as important a source of NO and N2O as autotrophic nitrification. We compared the responses of N. europaea and A. faecalis to changes in partial O2 pressure (pO2) and to the presence of typical nitrification inhibitors. Maximal production of NO and N2O occurred at low pO2 values in cultures of both N. europaea (pO2, 0.3 kPa) and A. faecalis (pO2, 2 to 4 kPa). With N. europaea most of the NH4+ oxidized was converted to NO2-, with NO and N2O accounting for 2.6 and 1% of the end product, respectively. With A. faecalis maximal production of NO occurred at a pO2 of 2 kPa, and maximal production of N2O occurred at a pO2 of 4 kPa. At these low pO2 values there was net nitrite consumption. Aerobically, A. faecalis produced approximately the same amount of NO but 10-fold more N2O per cell than N. europaea did. Typical nitrification inhibitors were far less effective for reducing emissions of NO and N2O by A. faecalis than for reducing emissions of NO and N2O by N. europaea.(ABSTRACT TRUNCATED AT 250 WORDS)

The purification of a cd1-type nitrite reductase from, and the absence of a copper-type nitrite reductase from, the aerobic denitrifier Thiosphaera pantotropha; the role of pseudoazurin as an electron donor

Thiosphaera pantotropha has been reported to contain a copper-type nitrite reductase on the basis that the copper chelator diethyldithiocarbamate inhibited the overall process of denitrification. It is now shown that nitrous oxide reduction is 100% inhibited by 10 mM diethyldithiocarbamate or 100 microM azide. We also found that both these inhibitors partially inhibited nitrite reduction in this organism. We purified the nitrite reductase of T. pantotropha and found that it was of the cytochrome cd1 type, contrary to the published report of it being a copper-type nitrite reductase. This is of importance since T. pantotropha is capable of aerobic nitrite reduction. The only detectable nitrite reductase in anaerobically or aerobically grown cells is the cd1 type. We also purified a small copper-containing protein, pseudoazurin. Pseudoazurin was found to be capable of donating electrons to the cd1-type nitrite reductase in vitro, and its copper centre was chelated by diethyldithiocarbamate. Since nitrite reduction is partially inhibited by diethyldithiocarbamate, it is thought that pseudoazurin is an electron donor to nitrite reductase in vivo.

Heterotrophic nitrification and aerobic denitrification in Alcaligenes faecalis strain TUD

Heterotrophic nitrification and aerobic and anaerobic denitrification by Alcaligenes faecalis strain TUD were studied in continuous cultures under various environmental conditions. Both nitrification and denitrification activities increased with the dilution rate. At dissolved oxygen concentrations above 46% air saturation, hydroxylamine, nitrite and nitrate accumulated, indicating that both the nitrification and denitrification were less efficient. The overall nitrification activity was, however, essentially unaffected by the oxygen concentration. The nitrification rate increased with increasing ammonia concentration, but was lower in the presence of nitrate or nitrite. When present, hydroxylamine, was nitrified preferentially. Relatively low concentrations of acetate caused substrate inhibition (KI = 109 microM acetate). Denitrifying or assimilatory nitrate reductase were not detected, and the copper nitrite reductase, rather than cytochrome cd, was present. Thiosulphate (a potential inhibitor of heterotrophic nitrification) was oxidized by A. faecalis strain TUD, with a maximum oxygen uptake rate of 140-170 nmol O2.min-1.mg prot-1. Comparison of the behaviour of A. faecalis TUD with that of other bacteria capable of heterotrophic nitrification and aerobic denitrification established that the response of these organisms to environmental parameters is not uniform. Similarities were found in their responses to dissolved oxygen concentrations, growth rate and ammonia concentration. However, they differed in their responses to externally supplied nitrite and nitrate.

Metabolic pathways in Paracoccus denitrificans and closely related bacteria in relation to the phylogeny of prokaryotes

Denitrification and methylotrophy in Paracoccus denitrificans are discussed. The properties of the enzymes of denitrification: the nitrate-nitrite antiporter, nitrate reductase, nitrite reductase, nitric oxide reductase and nitrous oxide reductase are described. The genes for none of these proteins have yet been cloned and sequenced from P. denitrificans. A number of sequences are available for enzymes from Escherichia coli, Pseudomonas stutzeri and Pseudomonas aeruginosa. It is concluded that pathway specific c-type cytochromes are involved in denitrification. At least 40 genes are involved in denitrification. In methanol oxidation at least 20 genes are involved. In this case too pathway specific c-type cytochromes are involved. The sequence homology between the quinoproteins methanol dehydrogenase, alcoholde-hydrogenase and glucose dehydrogenase is discussed. This superfamily of proteins is believed to be derived from a common ancestor. The moxFJGI operon determines the structural components of methanol dehydrogenase and the associated c-type cytochrome. Upstream of this operon 3 regulatory proteins were found. The moxY protein shows the general features of a sensor protein and the moxX protein those of a regulatory protein. Thus a two component regulatory system is involved in both denitrification and methylotrophy. The phylogeny of prokaryotes based on 16S rRNA sequence is discussed. It is remarkable that the 16S rRNA of Thiosphaera pantotropha is identical to that of P. denitrificans. Still these bacteria show a number of differences. T. pantotropha is able to denitrify under aerobic circumstances and it shows heterotrophic nitrification. Nitrification and heterotrophic nitrification are found in species belonging to the beta-and gamma-subdivisions of purple non-sulfur bacteria. Thus the occurrence of heterotrophic nitrification in T. pantotropha, which belongs to the alpha-subdivision of purple non-sulfur bacteria is a remarkable property. Furthermore T. pantotropha contains two nitrate reductases of which the periplasmic one is supposed to be involved in aerobic denitrification. The nitrite reductase is of the Cu-type and not of the cytochrome cd1 type as in P. denitrificans. Also the cytochrome b of the Qbc complex of T. pantotropha is highly similar to its counterpart in P. denitrificans. It is hypothesized that the differences between these two organisms which both contain large megaplasmids is due to a combination of loss of genetic information and plasmid-coded properties. The distribution of a number of complex metabolic systems in eubacteria and in a number of species belonging to the alpha-group of purple non sulphur bacteria is reviewed.(ABSTRACT TRUNCATED AT 400 WORDS)

Combined heterotrophic nitrification and aerobic denitrification in Thiosphaera pantotropha and other bacteria

Reports of the simultaneous use of oxygen and denitrification by different species of bacteria have become more common over the past few years. Research with some strains (e.g. Thiosphaera pantotropha) has indicated that there might be a link between this 'aerobic denitrification' and a form of nitrification which requires rather than generates energy and is therefore known as heterotrophic nitrification. This paper reviews recent research into heterotrophic nitrification and aerobic denitrification, and presents a preliminary model which, if verified, will provide at least a partial explanation for the simultaneous occurrence of nitrification and denitrification in some bacteria.

The effect of thiosulphate and other inhibitors of autotrophic nitrification on heterotrophic nitrifiers

It has been found that heterotrophic nitrification by Thiosphaera pantotropha can be inhibited by thiosulphate in batch and chemostat cultures. Allythiourea and nitrapyrin, both classically considered to be specific inhibitors of autotrophic nitrification, inhibited nitrification by Tsa. pantotropha in short-term experiments with resting cell suspensions. Hydroxylamine inhibited ammonia oxidation in chemostat cultures, but was itself fully oxidized. Thus the total nitrification rate for the culture remained the same. Heterotrophic nitrification by another organism, a strain of "Pseudomonas denitrificans" has also been shown to be inhibited by thiosulphate in short term experiments and in the chemostat. During these experiments it became evident that this strain is able to grow mixotrophically (with acetate) and autotrophically in a chemostat with thiosulphate as the energy source.