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Novel sinks for the atmospherically potent gas nitrous oxide

About the Project

Nitrous oxide is an atmospherically potent gas, having 350 times the warming potential of CO2 and destroying stratospheric ozone. As a consequence, research has focused on important atmospheric sources of this potent gas. For example, agricultural soils, estuaries and coastal ecosystems, with high loadings of human derived nitrogen (e.g. ammonia and nitrate) are often associated with high emissions of N2O. Recently, however, as part of our work in pristine, nitrogen limited parts of the Arctic, we have unearthed evidence for a novel sink for N2O. Here, across geothermally heated streams (2°C to 17°C), we found that colder streams contained less N2O than expected, relative to atmospheric equilibration, while the warmer streams contained more. Bio-available, fixed-N, is scarce in these pristine streams and their ecosystems are fuelled by an abundance of N-fixing, diazotrophs (cyanobacteria, diatoms) which readily fix N2 gas. The ecological problem here though is that N2 fixation has a very high activation energy – for example, 2-3 eV, compared to that typical for photosynthesis of 0.45eV - which makes fixing N2 in the cold energetically costly. In contrast, nitrous oxide is partially activated relative to N2 and fixing it should be energetically more favourable in the cold. We propose, therefore, that the under-saturation in N2O in cold Arctic streams is due to it being fixed as an alternative to the energetically more costly N2. At warmer temperatures this advantage is lost, N fixation shifts back to N2, total production increases and the oversaturation in N2O may result from the oxidation of the now more abundant ammonia – though we don’t know for sure. In addition, our experimental ponds (East Stoke, Dorset) are also impoverished with fixed-N and they too have a very high potential to consume N2O. We can discount dissimilatory reduction of N2O to N2 (no 15N2 from 15N2O), so the most likely explanation here is also N2O dependent, N-fixation. The only other account of N2O-fixation is in the N limited surface waters of the tropical south Pacific.

Objective and hypotheses:

Here the primary objective is to characterise the fundamental microbial ecology and biogeochemical significance of this novel sink for N2O. The PhD student will use 15N incubations to trace the fixation of both 15N2 and 15N2O into microbial community biomass – bulk and their DNA – as a function of temperature. They we will then use 15N and 13CO2 stable-isotope-probing (SIP) combined with taxonomic marker gene sequencing and functional metagenomics to identify whether this N2O fixation represents a physiological response at cooler temperatures - in otherwise N2 fixing organisms - or whether there is an entirely novel, N2O sustained, microbial community. The student will work in both the controlled setting of our long-term experimentally warmed ponds, where microbial communities have been exposed to an offset of 4°C for 11 years, and along the temperature gradient of geothermally heated streams in Iceland to test:

H1 – The apparent activation energy of N2O-fixation is lower than that for N2-fixation and, as a consequence, at colder temperatures a higher proportion of total N-fixation is dependent on N2O.

H2 – The diazotrophs responsible for N2O fixation (15N2O SIP) will be taxonomically distinct from those fixing N2 (15N2 SIP) – or, alternatively, N2O-fixation represents a physiological response at cooler temperatures in diazotrophs that typically fix N2.

The successful candidate will be trained in tracer biogeochemistry (Trimmer) and molecular microbial ecology (Hanson). The student will join a vibrant, productive and well-funded research group working on nitrogen and carbon cycling in a variety of aquatic ecosystems.

Funding

This position is funded by a QMUL Principal's Postgraduate Research Studentship and is available to EU, UK and International citizens. It will cover tuition fees as well as provide an annual tax-free maintenance allowance for 3 years at Research Councils UK rates (£16,777 in 2018/19). 

Eligibility and Applying

Applications are invited from outstanding candidates with or expecting to receive a first or upper-second class honours degree in an area relevant to the project. An masters degree is desirable, but not essential.

Informal enquiries can be sent to Professor Mark Trimmer (m.trimmer@qmul.ac.uk). For formal applications, please submit an online application before the stated deadline.

Apply Online

References

  • Yvon-Durocher G, Hulatt C, Woodward G, and Trimmer M (2017) Long-term warming amplifies shifts in the carbon cycle of experimental ponds. Nature Climate Change. doi:10.1038/nclimate3229. 
  • Trimmer M, Chronopoulou PM, Maanoja ST, Upstill-Goddard RC, Kitidis V, Purdy KJ (2016) Nitrous oxide as a function of oxygen and Archaeal gene abundance in the North Pacific. Nature Communications. DOI: 10.1038/ncomms13451. 
  • Lansdown K, McKew BA, Whitby C, Heppell CM, Dumbrell AJ, Binley A, Olde L, Trimmer M (2016) Importance and controls of anaerobic ammonium oxidation influenced by riverbed geology. Nature Geoscience. 9: 357-360. doi:10.1038/ngeo2684. 
  • Cavan EL, Trimmer M, Shelley F, Sanders R (2017) Remineralization of particulate organic carbon in an ocean oxygen minimum zone. Nature Communications. 10.1038/ncomms14847. 
  • Trimmer M, Shelley FC, Purdy KJ, Maanoja ST, Chronopoulou P-M, Jonathan G. Riverbed methanotrophy sustained by high carbon conversion efficiency. The ISME Journal. 9: 2304–2314. doi:10.1038/ismej.2015.98. 
  • Hanson CA, Marston M, Martiny JBH (2016) Biogeography of host range phenotypes in marine cyanophages. Frontiers in Microbiology. DOI: 10.3389/fmicb.2016.00983. 
  • Clasen JL, Hanson CA*, Ibrahim Y, Weihe C, Marston M, Martiny JBH (2013) Diversity and temporal dynamics of Southern California coastal marine cyanophage isolates. Aquatic Microbial Ecology. 69:17-31. *Corresponding author 
  • Hanson CA, Fuhrman JA, Horner-Devine MC, Martiny JBH (2012) Beyond biogeographic patterns: processes shaping the microbial landscape. Nature Reviews Microbiology. 10: 497-506.
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