The paper:
S. Leininger et al., "Archaea predominate among ammonia-oxidizing prokaryotes in soils," Nature, 442:806-9, 2006 (Cited in 84 papers)
The surprise:
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.
The extent:
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.
The genetics:
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 |
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