Distribution and Activity of Ammonia-Oxidizing Archaea in Natural Environments

One of the major challenges in studying microorganisms from natural habitats is the inability to cultivate many of them in the laboratory. Recently, the complete genome sequence of C. symbiosum was determined from a metagenomic library providing further insights into the potential physiological properties of uncultured ammonia-oxidizing archaea (AOA). The amo genes of ammonia-oxidizing bacteria (AOB) and AOA are distantly related, and molecular approaches to study the distribution and diversity of ammonia oxidizers rely on specific PCR primers that amplify the archaeal amoA or the bacterial amoA variant. Nitrification is an important process in sedimentary biogeochemistry and particularly in estuarine sediments, which can be exposed to high loads of nutrients from agricultural runoff. There is now unambiguous evidence for the occurrence of AOA in environments of elevated temperature. The mechanisms by which soil pH influences the growth and activity of many microbial functional groups have been determined through a combination of physiological and soil microcosm studies. With their involvement in ammonia oxidation, the majority of mesophilic crenarchaeota (or thaumarchaeota) are most likely major players in biogeochemical cycling. Most studies that attempted to evaluate the actual activity of AOA (and bacteria) seem to reveal that there could, in fact, be an ecological niche differentiation between AOA and AOB.