A PSYCHROPHILIC CRENARCHAEON INHABITS A MARINE SPONGE - CENARCHAEUM SYMBIOSUM GEN-NOV, SP, NOV

Archaea, one of the three major domains of extant life, was thought to comprise predominantly microorganisms that inhabit extreme environmen ts, inhospitable to most Eucarya and Bacteria, However, molecular phyl ogenetic surveys of native microbial assemblages are beginning to indi cate that the evolutionary and physiological diversity of Archaea is f ar greater than previously supposed, We report here the discovery and preliminary characterization of a marine archaeon that inhabits the ti ssues of a temperate water sponge, The association was specific, with a single crenarchaeal phylotype inhabiting a single sponge host specie s, To our knowledge, this partnership represents the first described s ymbiosis involving Crenarchaeota. The symbiotic archaeon grows well at temperatures of 10 degrees C, over 60 degrees C below the growth temp erature optimum of any cultivated species of Crenarchaeota. Archaea ha ve been generally characterized as microorganisms that inhabit relativ ely circumscribed niches, largely high-temperature anaerobic environme nts, In contrast, data from molecular phylogenetic surveys, including this report, suggest that some crenarchaeotes have diversified conside rably and are found in a wide variety of lifestyles and habitats, We p resent here the identification and initial description of Cenarchaeum symbiosum gen. nov., sp. nov., a symbiotic archaeon closely related to other nonthermophilic crenarchaeotes that inhabit diverse marine and terrestrial environments.