Browsing by Subject "NarG"

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Community Structure and Activity of Nitrate-Reducing Microorganisms in Soils under Global Climate Change
(2006) Deiglmayr, Kathrin; Kandeler, Ellen
Since the beginning of the Industrial Revolution, atmospheric carbon dioxide concentrations have been steadily increasing and, thus, contributed to a warming of the climate and altered biogeochemical cycles. To study the response of soil microorganisms to altered environmental conditions under global climate change, the nitrate-reducing community was regarded as a model community in the present thesis. This functional group, which performs the first step in the denitrification pathway, was selected because it is phylogenetically very diverse. In particular rising levels of atmospheric carbon dioxide as the most important catalyst of temperature rise and the retreat of glaciers in the Alps as one of the most evident consequences of climate change were investigated. The behaviour of nitrate reducers was investigated in a biphasic approach: (i) at the level of its enzyme activity of the nitrate reductase and (ii) at the level of community structure, which was characterised by RFLP (Restriction Fragment Length Polymorphism)-fingerprints using the functional gene narG. The effect of elevated atmospheric carbon dioxide concentrations on nitrate-reducing micro-organisms was studied in the Swiss FACE (Free Air Carbon dioxide Enrichment) experiment including the rhizosphere of two functional plant types (Lolium perenne and Trifolium repens), two N fertilisation levels and two sampling dates (June and October 2002). Whereas in June no significant treatment effect was observed, the nitrate reductase activity proved to be significantly reduced under elevated atmospheric carbon dioxide at the autumn sampling date. Simultaneously, elevated enzyme activities were recorded under Trifolium repens and high N fertilisation pointing to a control of nitrate reductase activity by nitrate availability at the time of sampling. The community structure of nitrate reducers, however, showed a different response pattern with sampling date and the strongly varying pH of the different experimental plots constituting the main driving factors. With respect to the three experimental factors atmospheric carbon dioxide, plant type and N fertilisation the composition of the nitrate reducers revealed a high stability. The microbial succession of nitrate-reducing microorganisms was studied in the rhizosphere of Poa alpina across the glacier foreland of the Rotmoosferner/Oetz valley. Sampling was performed in August and at the end of the short period of vegetation in September. The nitrate reductase activity increased significantly with progressing successional age, whereas organic carbon together with nitrate concentrations in the soils explained the major part of this effect. The microbial community of nitrate reducers revealed a significant shift across the glacier foreland, with pH and organic carbon representing the most important environmental factors inducing this shift. A detailed analysis of the clone libraries that were constructed for the youngest and the oldest site in the glacier foreland pointed to the tendency of lower diversity in the late succession compared to the young succession. Possibly an increasing selective pressure due to higher densities of microorganisms and, hence, a higher competition for limited resources contributed to the decline in diversity. In conclusion, the functional group of nitrate reducers responded to changing environmental conditions under global climate change particularly through altered enzyme activities. The amount and the direction of this response depended strongly on the nitrate availability and the organic carbon content in soils. The community structure of nitrate-reducing microorganisms, however, proved to be resilient towards short-term substrate fluctuations. This indicates that the genetic pool of this group of soil microorganisms possesses a high functional stability characterized by a relatively persistent composition and an independent modulation of enzyme activity.