Geochemical and Microbiological Studies of Nitrous Oxide Variations within the New NEEM Greenland Ice Core during the Last Glacial Period

ABSTRACT

Deep polar ice cores provide atmospheric records of nitrous oxide (N₂O) and other trace gases reflecting climate history along with a parallel archive of microbial cells transported with mineral dust, marine and volcanic aerosols from around the globe. Our interdisciplinary study of 32 samples from different depths of the recently drilled NEEM Greenland ice core addressed the question whether the identified microorganisms were capable of post-depositional biological production of N₂O in situ. We used high-resolution geochemical and microbiological approaches to examine the N₂O concentrations, the quantitative distributions of dust, Ca⁺², NH₄⁺ and NO₃⁻ ions related to N cycle pathways, the microbial abundance and diversity at specific NEEM core depths from 1758 m to 1867.8 m. Results showed varying concentrations of N₂O (220–271.5 ppb). Microbial abundance fluctuated between 3.3 × 10⁴ and 3.3 × 10⁶ cells mL⁻¹ in direct correlation with dust and Ca²⁺ concentrations with higher cell numbers deposited during colder periods. The average values of NH₄⁺ and NO₃⁻ indicated that substrates were available for the microorganisms capable of utilizing them. PCR amplification of selected functional genes involved in bacterial and archaeal nitrification and denitrification was not successful. Sanger and Illumina MiSeq sequence analyses of SSU rRNA genes showed variable representation of Alpha-, Beta- and Gammaproteobacteria, Firmicutes, Actinobacteria, chloroplasts and fungi. The metabolic potential of the dominant genera of Proteobacteria and Firmicutes as possible N₂O producers suggested that denitrification activity may have led to in-situ production and accumulation of N₂O.

KEYWORDS:

• Geochemistry
• Greenland
• metabolic activity
• microbial diversity
• N₂O
• NEEM ice core

Acknowledgments

The ice core samples used on this study were retrieved during the NEEM project in Greenland. NEEM is directed and organized by the Centre for Ice and Climate at the Niels Bohr Institute and the Office of Polar Programs of the U.S. National Science Foundation (NSF). It is supported by funding agencies and institutions in Belgium (FNRS-CFB and FWO), Canada (NRCan/GSC), China (CAS), Denmark (FIST), France (IPEV, CNRS/INSU, CEA and ANR), Germany (AWI), Iceland (RannIs), Japan (NIPR), Korea (KOPRI), The Netherlands (NWO/ALW), Sweden (VR), Switzerland (SNF), United Kingdom (NERC) and the United States (US NSF, Office of Polar Programs). We thank Greg Grove, Deb Grove, and Craig Praul from the Pennsylvania State University Genomics Facility for the libraries preparation and next-generation sequencing. We acknowledge the valuable help of Aswathy Sebastian and Istvan Albert with the Illumina sequence analyses. We also thank Kaitlyn Rinehold and Caroline Burlingame for their input in this research.

Funding

This study was supported by NSF grants ARC0909323 and 0806407 and the NASA-NAI agreement NNA09DA76A. The Division for Climate and Environmental Physics, Physics Institute, University of Bern acknowledges the long-term financial support of ice core research by the Swiss National Science Foundation (SNSF) and the Oeschger Centre for Climate Change Research.