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Dissertation: Dissertation: 28 Mar 2015 Microbial controls of greenhouse gas emissions from boreal lakes (Saarenheimo)


28.3.2015 12:00 — 15:00

Location: Ylistonrinne, YAA303
M.Sc. Jatta Saarenheimo defends her doctoral dissertation in Aquatic Sciences ”Microbial controls of greenhouse gas emissions from bo­real lakes”. Opponent Professor Stefan Bertilsson (Evolutionary Biology Centre, Uppsala) and custos Professor Marja Tiirola (University of Jyväskylä).

Jatta Saarenheimo. Photographer: Sampo SaarenheimoM.Sc. Jatta Saarenheimo defends her doctoral dissertation in Aquatic Sciences ”Microbial controls of greenhouse gas emissions from bo­real lakes”. Opponent Professor Stefan Bertilsson (Evolutionary Biology Centre, Uppsala) and custos Professor Marja Tiirola (University of Jyväskylä).

The event is in English.

Biogeochemical processes in stratified boreal lakes promote or reduce greenhouse gas (GHG; CO2, CH4 and N2O) emissions in distinct compartments. In this thesis environmental and biotic controls of microbially-mediated GHG processes were studied at a multi-lake scale as well as in a whole-lake experiment. Microbial communities, specific microbial groups and functional genes were studied using molecular methods, and data were combined with environmental and gas concentration measurements.

Aerobic, microaerobic and anaerobic zones of a small stratified lake were shown to support distinct microbial communities, which were mostly controlled by seasonal succession and oxygen concentration High biomasses of anaerobic photosynthetic green sulphur bacteria (GSB) of the genus Chlorobium were found where light intensities were sufficient to support their growth in the anoxic zone. During a three year whole-lake experiment, the effect of trophic cascades on the microbial communities and processes was studied by adding fish (European perch,Perca fluviatilis) to one basin of an experimentally divided fishless lake. This removed nearly all zooplankton and released methanotrophic bacteria from grazing by Daphniasp., allowing more methane oxidation. These cascading impacts reduced CH4 efflux from the basin to which fish were added to one quarter the efflux from the fishless basin.

The genetic potential for denitrification (studied with functional genes nirSnirKnosZI and nosZII), whereby N2O is produced as an intermediate product, was wide spread in all studied lakes and was driven by only a few core proteobacterial species. Inter-lake variation in N2O accumulation was connected to the relative abundance of nitrite versus N2O reductase genes, which co-varied with the nitrate concentration. Thus the abundance of GSB, methanotrophs, and denitrifiers were all found to impact on GHG emissions from boreal lakes, with a strong control by dissolved oxygen concentration.

The dissertation is published in the series of Jyväskylä Studies in Biological and Environmental Science, number 299, 55 p., Jyväskylä 2015, ISSN 1456-9701; 299, ISBN 978-951-39-6127-5. It is available at the University Library’s Publications Unit, tel. +358 40 805 3825,


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Jatta Saarenheimo