To achieve our goal of developing tools to control microbial nitrogen transformations, we need fundamental research on the ecophysiology and regulatory biology of the microbes managing reactive nitrogen in the biosphere.
Our applied research includes collaborative projects with agriculture-, fertilizer-, mining-, and wastewater-industries.
Watch a brief presentation of our research and field robot:
Selected publications from 2018 to 2023 (PDF files are available on request):
Gao, Y., Lycus, P., Arntzen, M.Ø., Shapleigh, J.P., Bakken, LR., Frostegård, Å.
Denitrification by bradyrhizobia under feast and famine and the role of the bc1 complex in securing electrons for N2O reduction. (Applied Environmental Microbiology).
Grosz, B., Matson, A., Butterbach-Bahl, K., Clough, T., Davidson, E.A., Dechow, R., DelGrosso, S., Diamantopoulos, S., Dörsch, P., Haas, E., He, H., Henri, C.V., Hui, D., Kleineidam, K., Kraus, D., Kuhnert, M., Léonard, J., Mueller, C., Petersen, S.O., Sihi, D., Vogeler, I., Well, R., Yeluripati, J.B., Zhang, J., Scheer, C.
Modeling denitrification: can we report what we don’t know? (Earth and Space Science Open Archive)
Song, Y., Wu, D., Ju, X., Dörsch, P., Wang, M., Wang, R., Song, X., Deng, L., Wang, R., Gao, Z., Haider, H., Hou, L., Liu, M., Yu, Y., Yu, H.
Nitrite stimulates HONO and NOx but not N2O emissions in Chinese agricultural soils during nitrification. (Science of the Total Environment)
Yu, H., Duan, Y., Mulder, J., Dörsch, P., Zhu, W., Ri, X., Huang, K., Zheng, Z., Kang, R., Wang, C., Quan, Z., Zhu, F., Liu, D., Peng, S., Han, S., Zhang, Y., Fang, Y.
Universal temperature sensitivity of denitrification nitrogen losses in forest soils. (Nature Climate Change)
Wei, J., Fontaine, L., Valiente, N., Dörsch, P., Hessen, D.O., Eiler, A.
Trajectories of freshwater microbial genomics and greenhouse gas saturation upon glacial retreat. (Nature Communications)
Vekic, T.T., Nadeem, S., Molstad, L., Martinsen, V., Hiis, E.G., Bakken, L.R., Rütting, T., Klemedtsson, L., Dörsch, P.
Effect of calcareous and siliceous amendments on N2O emissions of a grassland soil. (Soil Use and Management)
Jensen, S., Siljanen, H.M.P., Dörsch, P.
Activity and abundance of methanotrophic bacteria in a northern mountainous gradient of wetlands. (Environmental Microbiology Reports)
Highton, M.P., Bakken, L.R., Dörsch, P., Tobias-Hunefeldt, S., Molstad, L., Morales, S.E.
Soil water extract and bacteriome determine N2O emission potential in soils. (Biology and Fertility of Soils)
Hiis, E.G., Vick, S.H.W., Molstad, L., Røsdal, K., Jonassen, K.R., Winiwarter, W., Bakken, L.R.
Effective biotechnology for reducing N2O-emissions from farmland: N2O-respiring bacteria vectored by organic waste. (BioRxiv)
Kellermann, R., Vick, S.H.W., Lindtveit, K., Milligan, D.A., Bergaust, L.L.
Cell density dependent restriction of N2O reductase in denitrifying Pseudomonas spp., a potential driver of N2O emissions in natural systems. (FEMS Microbiology Letters)
Li, H., Song, X., Bakken, L.R., Ju, X.
Reduction of N2O emissions by DMPP depends on the interactions of nitrogen sources (digestate vs. urea) with soil properties. (Journal of Integrative Agriculture)
Lücker, S., Lenferink, W., Bakken, L., Jetten, M., Kessel, M.V.
Hydroxylamine production by Alcaligenes faecalis challenges the paradigm of heterotrophic nitrification. (Preprint)
Vick, S.H.W., Jonassen, K.R., Arntzen, M.Ø., Lycus, P., Bakken, L.R.
Meta-omics analyses of dual substrate enrichment culturing of nitrous oxide respiring bacteria suggest that attachment and complex polysaccharide utilisation contributed to the ability of Cloacibacterium strains to reach dominance. (BioRxiv)
Wang, X., Xiang, B., Li, J., Zhang, M., Frostegård, Å., Bakken, L., Zhang, X.
Using adaptive and aggressive N2O-reducing bacteria to augment digestate fertilizer for mitigating N2O emissions from agricultural soils. (Science of the Total Environment)
Yoon, S., Heo, H., Han, H., Song, D.U., Bakken, L.R., Frostegård, Å., Yoon, S.
Suggested role of NosZ in preventing N2O inhibition of dissimilatory nitrite reduction to ammonium. (mBio)
Alvarez, C., Andersen, T.O., Eikanger, K.S., Hamfjord, I.W., Niu, P., Weiby, K.V., Årvik, L., Dörsch, P., Hagen, L.H., Pope, P.B., Forberg, D.K., Hustoft H.K., Schwarm, A., Kidane, A.
Methane inhibition by Asparagopsis taxiformis with rumen fluid collected from ventral and central location – a pilot study. (Acta Agriculturae Scandinavica)
Allesson, L., Valiente, N., Dörsch, P., Anderson, T., Eiler, A., Hessen, D.O.
Drivers and variability of CO2:O2 saturation along a gradient from boreal to Arctic lakes. (Scientific Reports)
Mousavi, H., Cottis, T., Pommeresche, R., Dörsch, P., Solberg, S.Ø.
Plasma-Treated nitrogen-enriched manure does not impose adverse effects on soil fauna feeding activity or springtails and earthworms abundance. (Agronomy)
Duffner, C., Kublik, S., Fösel, B., Frostegård, Å., Schloter, M., Bakken, L.R., Schulz, S.
Genotypic and phenotypic characterization of hydrogenotrophic denitrifiers. (Environmental Microbiology)
Frostegård, Å., Vick, S.H., Lim, N.Y., Bakken, L.R., Shapleigh, J.P.
Linking meta-omics to the kinetics of denitrification intermediates reveals pH-dependent causes of N2O emissions and nitrite accumulation in soil. (The ISME Journal)
Jonassen, K.R., Hagen, L.H., Vick, S.H., Arntzen, M.Ø., Eijsink, V.G., Frostegård, Å., Lycus, P., Molstad, L., Pope, P.B., Bakken, L.R.
Nitrous oxide respiring bacteria in biogas digestates for reduced agricultural emissions. (The ISME Journal)
Jonassen, K.R., Ormåsen, I., Duffner, C., Hvidsten, T.R., Bakken, L.R., Vick, S.H.W.
A Dual Enrichment Strategy Provides Soil- and Digestate-Competent Nitrous Oxide-Respiring Bacteria for Mitigating Climate Forcing in Agriculture. (mBio)
Kellermann, R., Hauge, K., Tjåland, R., Thalmann, S., Bakken, L.R., Bergaust, L.
Preparation for Denitrification and Phenotypic Diversification at the Cusp of Anoxia: a Purpose for N2O Reductase Vis-à-Vis Multiple Roles of O2. (Applied and Environmental Microbiology)
Lekberg, Y., Bååth, E., Frostegård, Å., Hammer, E., Hedlund, K., Jansa, J., Kaiser, C., Ramsey, P.W., Řezanka, T., Rousk, J., Wallander, H., Welc, M., Olsson, P.A.
Fatty acid 16:1ω5 as a proxy for arbuscular mycorrhizal fungal biomass: current challenges and ways forward. (Biology and Fertility of Soils)
Mousavi, S.A., Gao, Y., Penttinen, P., Frostegård, Å., Paulin, L., Lindström, K.
Using amplicon sequencing of rpoB for identification of inoculant rhizobia from peanut nodules. (Applied Microbiology)
Dong, Y., Yang, J.L., Zhao, X.R., Yang, S.H., Mulder, J., Dörsch, P., Peng, X.H., Zhang, G.L.
Soil acidification and loss of base cations in a subtropical agricultural watershed. (Science of the Total Environment)
Munera-Echeverri, J.L., Martinsen, V., Dörsch, P., Obia, A., Mulder, J.
Pigeon pea biochar addition in tropical Arenosol under maize increases gross nitrification rate without an effect on nitrous oxide emission. (Plant and Soil)
Dong, Y., Yang, J.L., Zhao, X.R., Yang, S.H., Mulder, J., Dörsch, P., Peng, X.H., Zhang, G.L.
Seasonal dynamics of soil pH and N transformation as affected by N fertilization in subtropical China: An in situ 15N labeling study. (Science of the Total Environment)
Dong, Y., Yang, J.L., Zhao, X.R., Yang, S.H., Mulder, J., Dörsch, P., Zhang, G.L.
Nitrate runoff loss and source apportionment in a typical subtropical agricultural watershed. (Environmental Science and Pollution Research)
Dong, Y., Yang, J.L., Zhao, X.R., Yang, S.H., Mulder, J., Dörsch, P., Zhang, G.L.
Nitrate leaching and N accumulation in a typical subtropical red soil with N fertilization. (Geoderma)
Kang, R., Behrendt, T., Mulder, J., Dörsch, P.
Soil Moisture Control of NO Turnover and N2O Release in Nitrogen-Saturated Subtropical Forest Soils. (Forests)
Biasi, C., Jokinen, S., Prommer, J., Ambus, P., Dörsch, P., Yu, L., Halas, S., Granger, S., Boeckx, P., Van Nieuland, K., Brüggemann, N., Wissel, H., Voropaev, A., Zilberman, T., Jäntti, H., Trubnikova, T., Welti, N., Voigt, C., Gebus, B., Czupyt, Z., Wanek, W.
Challenges in measuring δ15N in inorganic nitrogen forms: An interlaboratory comparison of three common measurement approaches. (Rapid Communications in Mass Spectrometry)
Valiente, N., Eiler, A., Allesson, L., Andersen, T., Clayer, F., Crapart, C., Dörsch, P., Fontaine, L., Heuschele, J., Vogt, R.D., Wei, J., de Wit, H.A., Hessen, D.O.
Catchment properties as predictors of greenhouse gas concentrations across a gradient of boreal lakes. (Frontiers in Environmental Science)
Clayer, F., Thrane, J.E., Brandt, U., Dörsch, P., De Wit, H.A.
Boreal Headwater Catchment as Hot Spot of Carbon Processing From Headwater to Fjord. (Journal of Geophysical Research: Biogeosciences)
Sturite, I., Rivedal, S., Dörsch, P.
Clover increases N2O emissions in boreal leys during winter. (Soil Biology and Biochemistry)
Øvsthus, I., Thorup-Kristensen, K., Seljåsen, R., Riley, H., Dörsch, P., Breland, T.A.
Calibration and validation of the EU-rotate_N model using data from incubation of waste-derived organic materials and their use in a field vegetable experiment in Norway. (European Journal of Agronomy)
Zhu, J., Jansen-Willems, A., Müller, C., Dörsch, P.
Topographic differences in nitrogen cycling mediate nitrogen retention in a subtropical, N-saturated forest catchment. (Soil Biology and Biochemistry)
Gao, Y., Mania, D., Mousavi, S.A., Lycus, P., Arntzen, M.Ø., Woliy, K., Lindström, K., Shapleigh, J.P., Bakken, L.R., Frostegård, Å.
Competition for electrons favours N 2 O reduction in denitrifying Bradyrhizobium isolates. (Environmental Microbiology)
Ruiz, B., Frostegård, Å., Bruand, C., Meilhoc, E.
Rhizobia: highways to NO. (Biochemical Society Transactions)
Svennevik, O.K., Jonassen, K.R., Svensson, K., Hagen, L.H., Westereng, B., Solheim, O.E., Nilsen, P.J., Horn, S.J., Bakken, L.
Protecting Thermally Hydrolyzed Biosolids from Pathogenic Bacterial Growth by Addition of Compost. (Waste and Biomass Valorization)
Kelova, M., Ali, A.M., Eich-Greatorex, S., Dörsch, P., Kallenborn, R., Jenssen, P.D.
Small-scale on-site treatment of fecal matter: comparison of treatments for resource recovery and sanitization. (Environmental Science and Pollution Research)
Yu, L., Zhu, J., Ji, H., Bai, X., Lin, Y., Zhang, Y., Sha, L., Liu, Y., Song, Q., Dörsch, P., Mulder, J., Zhou, J.
Topography-related controls on N2O emission and CH4 uptake in a tropical rainforest catchment. (Science of the Total Environment)
Byers, E., Azzaroli-Bleken, M., Dörsch, P.
Winter N2O accumulation and emission in sub-boreal grassland soil depends on clover proportion and soil pH. (Environmental Research Communications)
Allesson, L., Andersen, T., Dörsch, P., Eiler, A., Wie, J., Hessen, D.O.
Phosphorus Availability Promotes Bacterial DOC-Mineralization, but not Cumulative CO2-Production. (Frontiers in Microbiology)
Abalos, D., Liang, Z., Dörsch, P., Elsgaard, L.
Trade-offs in greenhouse gas emissions across a liming-induced gradient of soil pH: role of microbial structure and functioning. (Soil Biology and Biochemistry)
Angell, I.L., Bergaust, L., Hanssen, J.F., Aasen, E.M., Rudi, K.
Ecological Processes Affecting Long-Term Eukaryote and Prokaryote Biofilm Persistence in Nitrogen Removal from Sewage. (Genes)
Bakken, L.R., Frostegård, Å.
Emerging options for mitigating N2O emissions from food production by manipulating the soil microbiota. (Current Opinion in Environmental Sustainability)
Highton, M.P., Bakken, L.R., Dörsch, P., Wakelin, S., de Klein, C.A., Molstad, L., Morales, S.E.
Soil N2O emission potential falls along a denitrification phenotype gradient linked to differences in microbiome, rainfall and carbon availability. (Soil Biology and Biochemistry)
Keuschnig, C., Gorfer, M., Li, G., Mania, D., Frostegård, Å., Bakken, L., Larose, C.
NO and N2O transformations of diverse fungi in hypoxia: evidence for anaerobic respiration only in Fusarium strains. (Environmental Microbiology)
Mania, D., Woliy, K., Degefu, T., Frostegård, Å.
A common mechanism for efficient N2O reduction in diverse isolates of nodule‐forming bradyrhizobia. (Environmental Microbiology)
Nadeem, S., Bakken, L.R., Frostegård, Å., Gaby, J.C., Dörsch, P.
Contingent effects of liming on N2O-emissions driven by autotrophic nitrification. (Frontiers in Environmental Science)
Torregrosa-Crespo, J., Pire, C., Bergaust, L., Martínez-Espinosa, R.M.
Haloferax mediterranei, an Archaeal Model for Denitrification in Saline Systems, Characterized Through Integrated Physiological and Transcriptional Analyses. (Frontiers in Microbiology)
Chen, J., Harstad, O.M., McAllister, T., Dörsch, P., Holo, H.
Propionic acid bacteria enhance ruminal feed degradation and reduce methane production in vitro. (Acta Agriculturae Scandinavica)
Thers, H., Abalos, D., Dörsch, P., Elsgaard, L.
Soil N2O emissions from oilseed rape cultivation show insignificant response to biochar: an analysis of biochar ageing, rate and feedstock type. (Science of the Total Environment)
Raji, S.G., Dörsch, P.
Effect of legume intercropping on N2O emission and CH4 uptake during maize production in the Ethiopian Rift valley. (Biogeosciences)
Tzanakakis, V.A., Taylor, A.E., Bakken, L.R., Bottomley, P.J., Myrold, D.D., Dörsch, P.
Relative activity of ammonia oxidizing archaea and bacteria determine nitrification-dependent N2O emissions in Oregon forest soils. (Soil Biology and Biogeochemistry)
Roco, C.A., Dörsch, P., Booth, J.G., Pepe-Ranney, C., Groffman, P.M., Fahey, T.J., Yavitt, J.B., Shapleigh, J.P.
Using metagenomics to reveal landscape scale patterns of denitrifiers in a montane forest ecosystem. (Soil Biology and Biochemistry)
Yu, L., Zhu, J., Zhang, X., Wang, Z., Dörsch, P., Mulder, J.
Humid subtropical forests constitute a net methane source: a catchment-scale study. (Journal of Geophysical Research: Biogeosciences)
Schlüter, S., Zawallich, J., Vogel, H.J., Dörsch, P.
Physical constraints for respiration in microbial hotspots in soil and their impact on denitrification. (Biogeosciences)
Russenes, A., Korsaeth, A., Bakken, L.R., Dörsch, P.
Effects of nitrogen split application on seasonal N2O emissions in southeast Norway. (Nutrient Cycling in Agroecosystems)
Cavicchioli, R., Ripple, W.J., Timmis, K.N., Azam, F., Bakken, L.R., Baylis, M., Behrenfeld, M.J., Boetius, A., Boyd, P.W., Classen, A.T.
Scientists’ warning to humanity: microorganisms and climate change. (Nature Reviews Microbiology)
Conthe, M., Lycus, P., Arntzen, M.Ø., Ramos da Silva, A., Frostegård, Å., Bakken, L.R., Kleerebezem, R., van Loosdrecht, M.C.M.
Denitrification as an N2O sink. (Water Research)
Graves, D.B., Bakken, L.R., Jensen, M.B., Ingels, R.
Plasma Activated Organic Fertilizer. (Plasma Chemistry and Plasma Processing)
Torregrosa-Crespo, J., Pire, C., Martínez-Espinosa, R.M., Bergaust, L.
Denitrifying haloarchaea within the genus Haloferax display divergent respiratory phenotypes, with implications for their release of nitrogenous gases. (Environmental Microbiology)
Woliy, K., Degefu, T., Frostegård, Å.
Host range and symbiotic effectiveness of N2O reducing Bradyrhizobium strains. (Frontiers in Microbiology)
Raji, S.G., Tzanakakis, V., Dörsch, P.
Bradyrhizobial inoculation and P application effects on haricot and mung beans in the Ethiopian Rift Valley. (Plant and Soil)
Weldon, S., Rasse, D.P., Budai, A., Tomic, O., Dörsch, P.
The effect of a biochar temperature series on denitrification: which biochar properties matter? (Soil Biology and Biochemistry)
Yu, L., Mulder, J., Zhu, J., Zhang, X., Wang, Z., Dörsch, P.
Denitrification as a major regional nitrogen sink in subtropical forest catchments: Evidence from multi-site dual nitrate isotopes. (Global Change Biology)
Zhu, J., Yu, L.F., Bakken, L.R., Mørkved, P.T., Mulder, J., Dörsch, P.
Controlled induction of denitrification in Pseudomonas aureofaciens: A simplified denitrifier method for dual isotope analysis in NO3-.
Schlüter, S., Henjes, S., Zawallich, J., Bergaust, L., Horn, M., Ippisch, O., Vogel, H.J., Dörsch, P.
Denitrification in Soil Aggregate Analogues - Effect of Aggregate Size and Oxygen Diffusion. (Frontiers in Environmental Science)
Jacobsen, C.S., Nielsen, T.K., Vester, J.K., Stougaard, P., Nielsen, J.L., Voriskova, J., Winding, A., Baldrian, P., Liu, B., Frostegård, Å., Pedersen, D., Tveit, A.T., Svenning, M.M., Tebbe, C.C., Øvreås, L., Jakobsen, P.B., Blazewicz, S.J., Hubablek, V., Bertilsson, S., Hansen, L.H., Cary, S.C., Holben, W.E., Ekelund, F., Bælum, J.
Inter-laboratory testing of the effect of DNA blocking reagent G2 on DNA extraction from low-biomass clay samples. (Scientific Reports)
Lim, N.Y., Frostegård, Å., Bakken, L.R.
Nitrite kinetics during anoxia: the role of abiotic reactions versus microbial reduction. (Soil Biology and Biochemistry)
Lycus, P., Soriano-Laguna, M.J., Kjos, M., Richardson, D.J., Gates, A.J., Milligan, D.A., Frostegård, Å., Bergaust, L., Bakken, L.R.
A bet-hedging strategy for denitrifying bacteria curtails their release of N2O. (Proceedings of the National Academy of Sciences)
Torregrosa-Crespo, J., Bergaust, L., Pire, C., Martínez-Espinosa, R.M.
Denitrifying haloarchaea: sources and sinks of nitrogenous gases. (FEMS Microbiology Letters)
Robotized solutions for field- and laboratory experiments.
In collaboration with ADIGO, we have developed an autonomous vehicle that measures emissions of N2O and CO2 from the soil surface in field plot experiments.
The robot measures N2O with TDL with a frequency of 1 Hz, allowing emissions to be quantified with a closure time of 3 min.
Tha alpha-version is now in operation in our field experiments, where we test the effects of pH adjustments by lime and mafic minerals.
Documentation of the performance of the robot is found here.
A beta version of the robot will be made commercially available by ADIGO, in collaboration with NMBU.
We are inviting research institutions to join in on this project, named TEMBO.
This video shows the robot in action and how it is controlled from the office:
The video also shows some interesting details regarding the "fast box" technique developed.
Robotics for gas kinetics in laboratory experiments
We have constructed a robotized incubation system (Molstad et al 2007) which allows tracking of electron flow to various electron acceptors (O2, NO2, NO and N2O) in 15 parallel cultures. The system can also be used to measure other relevant trace gases (CO2, CH4, NO and N2O) at or below ambient concentrations, and thus serves in situ flux measurements. Our lab is also equipped with tools for stable isotope analyses (13C, 15N, 18O, D/H) of organic materials and gases (N2O, CH4, CO2).
We have designed several more versions of this system (described here), replacing the microGC with traditional GC and an improved incubator/sampling system that allows parallel incubations of 30 stirred batches (+14 without stirring). One of these also includes H2 and H2S detection. Another, similar system will be installed in 2020.
Gas kinetics analysis
Links calibration tutorials
We strive to unravel how microbes regulate oxic and anoxic respiration. Why is this environmental research?
The increasing concentrations of N2O in the atmosphere is due to mankind's interference with the nitrogen cycle.
The ultimate cause is nitrogen enrichment, the proximal N2O sources are nitrogen red/ox reactions by prokaryotes.
We believe that there exist options for reducing N2O emissions, but only if we improve our understanding of the regulatory network at the cellular-, microhabitat-, whole soil-, soil-plant- and ecosystem level. This a goal for NMBU Nitrogen Group.
We study the regulatory biology and ecology of denitrifcation, dissimilatory reduction of nitrite to ammonium, and ammonium oxidation.
We use model organisms, such as Paracoccus denitrificans;Nitrosomnas europea, Agrobacterium tumefaciens.
.. not because we believe that they are representative for organisms in nature, but because they are "fast tracks" to new insight, novel phenomena, new concepts.
We are aware that phenomena discovered in model organisms must be validated by experiments with natural populations; which we do.
This is best illustrated by an example:
We discovered that the model organism, Paracoccus denitrificans was unable to make functional N2O reductase at pH 6, although it clearly "tried" by transcribing the gene coding for the enzyme.
Through a series of expeiments, we found that the most plausible explanation is that low pH interferes with the assembly of the enzyme in the periplasm, and we hypothesize that Cu-insertion is the critical step. The enzyme, if successfully assembled at pH 7, was perfactly functional at pH<6! This could of course be a peculiarity for Paracoccus denitrificans, but we have provided ample evidence for the opposite by studying natural populations: in all soils tested, we find that the expression of N2O reductase declines with pH, and experiments with bacteria extracted from soil confirmed the peculiar post translatoric phenomenon described for Paracoccus.
We would not need the model strain to discover the effect of pH an the N2O/N2 product ratio of denitrification. The phenomenon was observed more than 60 years ago, but never understood, hence forgotten and rediscovered several times since then. But the experiments with the model strain helped us to understand the cause.
Why is this so intresting? Because the pervasive effect of soil pH on the N2O/N2 product ratio means that we can reduce N2O emissions from soils by liming! Liming increase soil pH, which enhance the expression of N2O reductase, thus lowers the N2O/N2 product ratio of denitrification, hence reduces the N2O emission to the atmosphere!
Environmental research at its best unravels causations.
Our research on denitrification in heterotrophic prokaryotes was initiated in 2005, and our aim was to characterize and understand how they regulate their transition from aerobic to anaerobic respiration in response to oxygen depletion.
At that time, many "nuts and bolts" of the regulatory networks controlling this transition had been unravelled (albeit limited to a few model organisms), but few had attempted to characterize the regulatory phenotypes under stringent experimental conditions.
We developed such routines, and investigated both model organisms and newly isolated organisms. We discovered a range of novel regulatory phenomena and putative regulatory fitness traits such as bet hedging and the formation of persister cells. We also discovered the reason for the apparent inhibition of N2O reductase at low pH. This research is about to change our understanding of denitrifying organisms as sources or sinks for N2O.
A popular hypothesis among microbial ecologists is that denitrifying communities in soils are different regarding their ability to achieve a balanced transition from oxic to anoxic conditions, and that their propensity to emit N2O can be predicted by their genetic "fingerprint". The most naïve version of this hypothesis is that the propensity to emit N2O can be predicted by gene abundance, i.e. that soils which emit much N2O have high nir/nosZ gene abundance and vice versa. Such hypotheses are commonly tested by searching for correlations between gene abundance data and emissions (or the N2O/N2 product ratio of denitrification). No consistent patterns have emerged, however: the numerous correlations found appear to be utterly spurious.
We believe that regulatory biology is a clue to understand the role of denitrifying communities as sources and sinks for N2O in the environment, and that stringent investigations of model organisms can enhance our understanding of how N2O emissions are regulated, hence strengthen our ability to develop mitigations, i.e. actions that reduce the emissions of N2O.
Our investigatins of Paracoccus denitrificans have revealed bet hedging response to oxygen depletion: all cells express nosZ (N2O reductase), while only a minority express NirS. As a result, the population becomes a strong sink for N2O. The phenomen appears to be widespread among denitrifying organisms (papers are in the pipeline).
Our investigation of nosZ-expression in Pa. denitrificans showed that low pH hampers this expression by interering with the maturation of the enzyme in the periplasm. This could be an anecdote for Paracoccus, but this is clearly not the case: All microbial communities tested so far show exactly the same post transcriptional effect of low pH on the making of N2O reductase! We are currently exploring in more detail what goes wrong at low pH. But the results so far suffice to claim that we know why soil pH has a pervasive effect on the N2O/N2 product ratio of denitrification.
A common notion is that N2O reductase is destabilized/destroyed by oxygen. Our experiments with Pa denitrificans show that this is not the case. During aerobic growth, cells with N2O reductase (expressed during anoxic spell) will preserve the enzyme (but diluting it if growing) during aerobic growth. We have also demonstrated that they preserve nitrite reductase during aerobic growth. This explains the "legacy of anoxia" often observed in soil: once exposed to anoxia, the soils responds faster to subsequent anoxic spells.
A long standing notion is that NOR (the enzyme reducing NO to N2O) is a necessity for survival as a denitrifying organism. This is certainly true when grown in pure culture (no organisms can survive for long in the presence of micromolar concentrations of NO). Nevertheless, denitrifying organisms without NOR have been isolated (Falk et al 2010, see publication list), and more will be published on this in the near future. Thus, it appears perfectly possible to survive as a denitrifier without NOR, probably because NO toxicity is not a problem under natural conditions (NO scavenged by others). The "Black queen hypothesis" seems relevant here...
Several observations of communities and pure cultures have convinced us that post- transcriptional regulation of electron flow to either O2 or NOx is a clue to understand N2O emissions from soils. Transcription of the denitrification genes appears to be universally repressed by oxygen, but aerobic denitrification is a significant phenomenon, provided that the denitrification proteome has been expressed during foregoing anoxia.
The "post anoxic trauma" is a term used for a community exposed to oxygen after an anoxic spell. The figure above shows that Thauera aminoaromatica is unable to stop denitifying if given a sudden dose of oxygen. The same phenomenon was deomstrated for a denitrifying community by Morley et al 2008 (see publication list). In both cases, nitrous oxide reductase activity was apparently inhibited by oxygen. Our understanding of this phenomenon has deepened over the last years: it appears that the "inhibition" is primarily a "deprivation": that the terminal oxidases compete successfully with N2O reductase for electrons. In contrast, NIR and NOR appear to be more competitive vi a vis terminal oxidases.
Seb Rishaug Strand
Odd Erik Varberg
Biology and enzymology of oxic/anoxic respiration
Oliver Einsle (University of Freiburg)
Jim Shapleigh (Cornell, USA)
David Richardson and Andrew Gates (University of East Anglia, UK)
Rosa María Martínez-Espinosa (University of Alicante, Spain)
Stephen Spiro (University of Texas, Dallas)
Jeff Cole (University of Birmingham)
Rob van Spanning (Vrije Universiteit, Amsterdam)
Mark van Loosdrecht (Technical University of Delft)
Jörg Simon (Technical University of Darmstadt)
Jim Prosser (School of Biological Sciences, University of Aberdeen)
Olaf Ippisch (Institut für Mathematik, TU Clausthal)
Frank Bruggeman (VU University, Amsterdam)
Soils and microbial communities
Sergio Morales (Ottago University, New zealand)
Kristina Lindström and Leena A. Räsänen (University of Helsinki, Finland)
Ricarda Kellerman - PhD scholar (2019-2023)
Marte W. Lindholm-Dahle - Master Student (2022-2023)