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