S. Leininger et al., "Archaea predominate among ammonia-oxidizing prokaryotes in soils," Nature, 442:806-9, 2006 (Cited in 84 papers)
To quantify the presence of ammonia-oxidizing archaea (AOA) in soil, Christa Schleper at the University of Bergen and colleagues sifted through 12 types of soil from three climate zones for amoA - a gene for a subunit of a key ammonia-oxidizing enzyme. PCR studies revealed that archaeal amoA is up to 3,000 times more abundant in soil than bacterial amoA, overturning a decades-old belief that bacteria are the largest contributors to soil nitrification.
Follow-up studies have detected AOA in ammonium-rich estuaries in Mexico, fertilized red soil in China, and sand from a Tennessee watershed. In July, researchers found that AOA are abundant even in sea floor sediments (Nature,...
The missing piece:
However, "the real direct proof is still missing" - that soil archaea are actually fixing nitrogen, says Michael Wagner, a microbiologist at the University of Vienna. "The presence of a gene doesn't prove function." To study the physiology of the archaea, first the microorganisms must be isolated. "It's not trivial that there's no pure culture," says Wagner.
In early 2008, Stahl and Wagner each cultivated a separate AOA from hot springs, both closely related to soil AOA. The researchers are now working to sequence the two genomes for a comparative genomic analysis, which could help define the relationships among the diverse lineages of AOA, says Wagner, as well as provide insight to the physiology of the organisms.
|Ratios of AOA to AOB amoA gene copies in sandy soil:|
|0-10 cm deep:||55:1|
|20-30 cm deep:||170:1|
|40-50 cm deep:||1,125:1|