The Ammonia-Oxidizing Gene
|"… some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity etc…", Charles Darwin, on the origins of life in tidal pools |
A genetic analysis of soil samples indicates that a group of microorganisms called crenarchaeota are the Earth’s most abundant land-based creatures that oxidize ammonia, according to an international team of researchers from Norway, Germany, United Kingdom and the United States.
Soil microbes, in a process known as nitrification, combine ammonia with oxygen to form nitrates, which are used as nutrients by plants.
"Ammonia oxidation is an important step in the nitrogen cycle that was believed for the last 100 years to be solely performed by bacteria," said Christa Schleper, full professor of Molecular biology of Archaea at University Bergen, Norway.
The discovery was made possible by a combination of different techniques ranging from molecular biology, biochemistry to metagenomics. Using a novel sequencing technique and bioinformatics tools, Stephan C. Schuster, associate professor of biochemistry and molecular biology at Penn State, and his co-workers accurately measured the quantities of active bacteria and archaea in the complex mixtures of soil organisms. The international research team reported their findings in the Aug. 17 issue of Nature.
Archaea are single-celled microbes that, along with bacteria, comprise a category of small organisms whose genetic material, or DNA, is not stored in a nucleus (as it is in animals and plants). Crenarchaeota, which belong to the archaea, are found in various habitats, including soil.
"We think crenarchaeota in soil gain their energy from oxidizing ammonia," said Schuster. "But we don’t know yet if they can also gain energy by other means. The bacterial counterparts can only do ammonia (and urea) oxidation, nothing else."
During a recent study of a collection of genes in microorganisms, researchers had stumbled on a particular gene, which is responsible for the production of a key enzyme used for the oxidation of ammonia.
The gene was subsequently found in a marine strain of archaea that uses ammonia as its sole source of energy. Researchers examined soil samples from 12 pristine and agricultural lands across three climatic zones to see if such ammonia-oxidizing microorganisms were present in terrestrial ecosystems as well.
"We measured the abundance of the particular crenarchaeota gene alongside the same type of gene from bacteria," explained Schleper.
The tally suggested that copies of the archaeal gene in the soil samples were up to 3,000 times more abundant than copies of the bacterial gene. High amounts of lipids specific to crenarchaeota confirmed the organism’s presence.
|Tree of life, divided between three major cell types, those with and without a nucleus (Bacterial Prokaryotes and Animal Eukaryotes), preceded by the root of the tree, Archea.|
At Penn State, Schuster used a novel technique to directly sequence only the transcribed portion of the genomes from soil organisms, thus giving proof that crenarchaeota are in fact active and not just dormant residents in the soil.
Crenarchaeotal gene counts also do not change with soil depth, while bacterial gene counts drop significantly as one goes deeper.
"It might mean that they can oxidize ammonia at least with less oxygen and probably also with less ammonia, but we don’t know for sure. Our data clearly say that the archaea are more versatile in their lifestyle than bacteria," said Schuster, also a researcher at Penn State’s Centers for Infectious Disease Dynamics and Comparative Genomics and Bioinformatics.
Despite their abundance, it is not yet clear if crenarchaeota oxidize more ammonia than regular bacteria, and what that might mean for the ecological impact of ammonia oxidation, or the nitrogen cycle. "We will have to study the nitrification activity of archaea and their underlying biochemistry," said Schleper, who initiated the study.
"Perhaps the measured amounts of greenhouse gases such as nitric oxide and nitrous oxide are not produced by bacteria, but by a very different group of organisms, namely archaea," said Schleper. "But it is not clear, if and in what amounts the archaea form these gases as byproducts. This is only known from some of the respective bacteria," Schleper added.