Uncultured archaea in spruce rhizospheres and mycorrhizas

Archaea are microorganisms belonging to the third domain of cellular life on earth. Despite being unicellular microorganisms, they differ from the bacteria in many ways, and actually share an evolutionary history with the Eukaryotic branch of life. Since their discovery in extreme habitats, they have during the last two decades been found to inhabit almost all environments on Earth. Specific linages of archaea have been found to live forest soil ecosystems, and they have been found to be especially associated with boreal forest tree roots and mycorrhizospheres. These archaea belong to the so called Group I.1c of uncultured Thaumarchaeota (formerly included in the Phylum Crenarchaeota), and euryarchaeotal linages phylogenetically falling with the generally extremely halophilic Halobacteriales, as well as with methylotrophic and acetoclastic methanogens belonging to the Methanolobus and Methanosaeta, respectively. Norway spruce is one of the most common forest trees in the Fennoscandian boreal forest. Only few studies have thus far been conducted on archaea inhabiting the rhizosphere and mycorrhizosphere of boreal forest trees, and even fewer have concentrated on Norway spruce. However, it has been shown that the tree species has profound effect on the community composition of the archaea in the rhizosphere. When the tree roots are colonized by ectomycorrhizal fungi, the effect of the fungus dominates and the effect of the tree decreases. A spruce seedling grown in natural humus from a Scots pine stand gathered more detectable archaea in its roots than a Scots pine seedling, but when grown in natural humus from a Norway spruce stand the detectable number of archaea in the Norway spruce roots was much reduced. It has been shown that different tree species affect the soil they live in, and that Norway spruce has a tendency to acidify the soil, while for example silver birch increase the soil pH. Acidic spruce needle litter also bring recalcitrant organic matter and phenolics to the soil, while birch litter increase soil organic matter and nitrogen. An even greater difference between tree species can be detected in the composition of bacterial groups in the rhizospheres and mycorrhizospheres of different boreal forest tree species. The fact that archaea are generally found only in the roots and rhizosphere, and not in the soil uncolonized by mycorrhizal fungi or tree roots indicate a relationship between the archaea and the ectomycorrhizal fungi and the tree. This chapter will concentrate on the archaea detected in the roots and mycorrhizosphere of boreal forest Norway spruce in comparison to other boreal tree species, but will also shortly touch on the subject of bacteria.