“Life has evolved to thrive in environments that are extreme only by our limited human standards: in the boiling battery acid of Yellowstone hot springs, in the cracks of permanent ice sheets, in the cooling waters of nuclear reactors, miles beneath the Earth’s crust, in pure salt crystals, and inside the rocks of the dry valleys of Antarctica.” -Jill Tarter, astronomer
Yellowstone National Park (YNP) is well known for its geothermal features and diverse ecosystems. With features such as hot spring, geysers, canyons and forests, it’s no wonder that the biodiversity (microbial and not) at YNP is enormous.
Isolation of Nanopusillus acidilobi
Earlier this month, a group from the Oak Rige National Laboratory published a paper in Nature Communications on the identification of a Nanoarchaeota species from a hotspring in YNP. The Nanoarchaeota phylum previously had only one representative species (Nanoarchaeum equitans). The new Nanoarchaeota species, Nanopusillus acidilobi, is amongst the smallest known organisms. The size of an N. acidilobi cell is 100-300 nm in diameter. Optimal growth of N. acidilobi is at 82C and pH 3.6 making the acidic hotspring a hotspot (no pun intended) for this organism.
Not only is the physical size of N. acidilobi cells minuscule, its genome is only 605,887 nucleotides in length and contains 656 genes. This compares ~4 million nucleotides and ~3,000 protein coding regions of the E. coli genome. N. acidilobi possesses genes for processing genetic information but does not possess genes for biosynthetic and energetic capabilities. Most surprisingly, N. acidilobi lacks components of the respiratory chain and ATP synthesis complexes required for creating ATP, the universal energy currency needed for life. So how does N. acidilobi even exist?
Symbiosis of course!
Symbiosis describes the close association between two or more species. Symbiotic relationships can take on several forms:
(1) Mutualism: both species involved benefit from the interaction (ex: flowers provide bees with food (nectar) and the bees pollinating flowers as a result)
(2) Commensalism: one organism benefits from the other without affecting it (ex: barnacles attach onto whales for transport but the whale is neither harmed nor helped)
(3) Parasitism: one species benefits from the interaction at the expense of the other (ex: fleas on a dog)
In the case of N. acidilobi, it forms a (probable) commensalistic relationship with another archaeon, Acidilobus sp. 7A. N. acidilobi acquires ATP. nucleotides, co-factors, amino acids and lipids from Acidilobus. It may seem that because N. acidilobi sequesters many of these metabolic precursors away from its host, the Acidilobus cells would be affected negatively. Surprisingly, the growth rate of Acibilobus is the same in monoculture and in co-culture with N. acidilobi.
Electron microscopy imaging revealed the attachment of N. acidilobi to its host via a stretched membrane. The exact mechanisms and structures that facilitate the transfer of metabolites to N. acidilobi remains unknown but it is impressive that such a diverse pool of metabolites can be acquired to sustain N. acidilobi cells through this mechanism.
