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Title: Assessing the potential of Methanotroph-dependent denitrification
Author: Hobkirk, Robert Ian
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2012
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Abstract:
Methane (CH4) and Nitrous Oxide (N2O) are both potent greenhouse gases which contribute to global warming by an estimated value of 20% and 6% respectively in addition to which their relative concentrations within the atmosphere are also on the increase at a rate of 0.8% and 0.3% yr-1 respectively over the past few decades (IPCC, 2007; Verma et al., 2006; Moiser et al., 1998). With the combined contribution of both CH4 and N2O constitute in relation to global warming, highlighting the importance of research into their reduction. Through the investigation of the process of methane oxidation and denitrification, both of which are bacterial processes which can lead to a reduction in the relative concentrations of both CH4 and N2O respectively within the atmosphere. Highlighting the importance of the research carried out within this thesis, in relation to the investigation of the potential coupling between methanotrophic and denitrifying bacteria, known as methanotrophic dependent denitrification (MDD) to occur within the environment. The initial experimental setup was designed to test whether soil derived and model denitrifying bacteria; Pseudomonas nitroreducens, Pseudomonas citronellolis and Paracoccus denitrificans, were able denitrify to N2 on the presumptive methanotrophic carbon exudates sodium acetate and sodium formate as their sole carbon source (Costa et a; 2000; Hanson & Hanson 1996; Rhee & Fuhs 1978). All of which was carried out in pure cultures, under ≤ 0.4 % O2 v/v headspace. 15N-labelling was carried out in order to obtain a more complete picture in relation to the production of N2, and the creation of a N2O:N2 product ratio for and whether there was any difference in the utilisation between the two carbon sources. The presented data demonstrated that Pseudomonas nitroreducens and Pseudomonas citronellolis were able to produce N2 on sodium acetate but not on sodium formate. This was followed by assessing how different oxygen headspace conditions 10, 3, 2, 1 and 0.4 % O2 v/v would affect the exudation of acetate by Type II soil derived methanotrophic bacteria Methylocystis parvus, Methylocystis rosea and Methylocystis trichosporium. The results demonstrated that the methanotrophic bacteria (i) exudate acetate (ii) the presence of the acetate exudate within the media was only detectable under microaerobic conditions of ≤ 2 % available O2 v/v within the headspace (iii) the concentration of acetate exudate present within the media increased as the available O2 v/v decreased from 2 % to ≤ 0.4 % O2 v/v available within the headspace. This was then followed by a series of experiments designed to assess the ability of Pseudomonas nitroreducens and Pseudomonas citronellolis denitrify to N2 on 0.22 μm filter sterilized acetate exudate bacteria from M. parvus, M. iv rosea and M. trichosporium grown under a ≤ 0.4 % O2 v/v headspace as their sole carbon source when also grown under the same ≤ 0.4 % O2 v/v headspace conditions. The results demonstrated that Pseudomonas nitroreducens and Pseudomonas citronellolis were able to denitrify on filter sterilized acetate exudate from the type II soil derived methanotrophic bacteria, under which the greatest concentration of acetate production was under a ≤ 0.4 % O2 headspace, demonstrating that the amount of acetate which is exuded by the Type II methanotrophic bacteria is great enough to support denitrification to N2. This was followed by a co-culture experimental setup in which both the methanotrophic and denitrifying bacteria were grown simultaneously in the same closed system, in which no bacterial mixing occurred.
Supervisor: Not available Sponsor: BBSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.577579  DOI: Not available
Keywords: Denitrification ; Methanotrophs
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