Microbial Ecology Distribution of Cren- and Euryarchaeota in Scots Pine Mycorrhizospheres and Boreal Forest Humus Malin Bomberg 1 and Sari Timonen 1,2 (1) Department of Applied Biology, University of Helsinki, P.O. Box 27, Latokartanonkaari 7, FIN-00014, Helsinki, Finland (2) Department of Applied Chemistry and Microbiology, University of Helsinki, P.O. Box 56, 00014, Helsinki, Finland Received: 12 February 2007 / Accepted: 14 February 2007 / Online publication: 6 March 2007 Abstract Archaeal 16S rRNA gene sequences have been found in a variety of moderate-temperature habitats including soil and rhizospheres. In this study, the differences of archaeal communities associated with Scots pine (Pinus sylvestris L.) short roots, different types of mycorrhizo- spheric compartments, and uncolonized boreal forest humus were tested by direct DNA extraction, polymerase chain reaction–denaturing gradient gel electrophoresis (PCR–DGGE), and sequencing. The results indicated that mycorrhizal colonization of Scots pine roots substantially influence the archaeal community of pine rhizospheres. Colonization of short roots by most mycorrhizal fungi tested increased both archaeal frequency and diversity. Most of the archaeal sequences encountered in mycor- rhizas belonged to the phylum Euryarchaeota, order of Halobacteriales. The difference in archaeal diversity between the mycorrhizospheric compartments and hu- mus was profound. Most compartments with fungal components contained euryarchaeotal 16S rRNA gene sequences, whereas a high diversity of crenarchaeotal sequences and no euryarchaeotal sequences were found in forest humus outside mycorrhizospheres. Introduction Archaea, bacteria, and eukaryotes construct the three domains of life. The domain of archaea is the least well- known of these three groups. It consists of two main Phyla, Crenarchaeota and Euryarchaeota, and two minor Phyla, Korarchaeota and Nanoarchaeota. Thus far, mainly Crenarchaeota and some Euryarchaeota have been found in moderate environments. Since the first findings in soil and fresh water habitats, an ever increasing number of 16S rRNA gene sequences of Crenarchaeota have been detected in various moderate-temperature environments [1–12]. Buckley et al.[13] estimated that the relative abundance of crenarchaeotal 16S rRNA gene sequences was up to 1 to 2% of the total 16S rRNA gene pool in field soils, whereas Ochsenreiter et al. [10] gave an estimation of 0.3–0.5% in sandy soil. Both studies suggest that Crenarchaeota represent a stable and specific component of the microbial communities in soils. Crenarchaeota have also been detected from rhizo- spheres of agricultural plants [5, 6] and mycorrhizo- spheres of Scots pine growing in forest humus [8]. In tomato rhizospheres, Crenarchaeota have been shown to be more abundant on senescent parts of roots than on the young extending root tips [5]. However, rhizospheres do not appear to be the most favored environments for Crenarchaeota. In a study conducted on sandy soil covered with mixed grass, it was estimated that the bulk soil contained between 3 and 18 times more crenarch- aeotal rDNA than the rhizospheric soil [10]. Recently, moderate environments such as fresh water springs, marshlands, and soils have been shown to contain Euryarchaeota belonging to the order Halobac- teriales and other usually halophilic taxa [10, 14, 15]. Several halophilic pure cultures have been obtained from moderate environments, such as sulfide- and sulfur-rich fresh water springs [14] and estuarine marsh mud pans [15]. Some of these pure culture isolates were able to grow on salt concentrations as low as seawater (2.5% NaCl) [15], although members of this group are usually found in salt-saturated environments and are extremely halophilic. Euryarchaeota belonging to halophilic taxa Correspondence to: Sari Timonen; E-mail: sari.timonen@helsinki.fi DOI: 10.1007/s00248-007-9232-3 & Volume 54, 406–416 (2007) & * Springer Science + Business Media, LLC 2007 406