Bomberg and Timonen 1 Title ; Effect of tree species and mycorrhizal colonisation on the archaeal population of boreal forest

8 Group 1.1c Crenarchaeota are the predominating archaeal group in acidic boreal forest soils. In this 9 study we show that the detection frequency of 1.1c crenarchaeotal 16S rRNA genes increased in the 10 rhizospheres of the boreal forest trees following colonisation by the ectomycorrhizal fungus Paxillus 11 involutus. This effect was very clear in the fine roots of Pinus sylvestris, Picea abies and Betula 12 pendula, the most common forest trees in Finland. The non-mycorrhizal fine roots had a clearly 13 different composition of archaeal 16S rRNA genes compared to the mycorrhizal fine roots. In the 14 phylogenetic analysis, the 1.1c crenarchaeotal 16S rRNA gene sequences obtained from the fine roots 15 formed a well-defined cluster separate from the mycorrhizal ones. Alnus glutinosa differed from the 16 other trees by having a high diversity and detection level of Crenarchaeota both on fine roots and on 17 mycorrhizas as well as by harbouring a distinct archaeal flora. Similarity of the archaeal populations in 18 rhizospheres of the different tree species was increased upon colonisation by the ectomycorrhizal 19 fungus. A minor part of the sequences obtained from the mycorrhizas belonged to Euryarchaeota, order 20