Characterisation of a stable laboratory co-culture of acidophilic nanoorganisms

This study describes the laboratory cultivation of ARMAN (Archaeal Richmond Mine Acidophilic
Nanoorganisms). After 2.5 years of successive transfers in an anoxic medium containing ferric sulfate
as an electron acceptor, a consortium was attained that is comprised of two members of the order
Thermoplasmatales, a member of a proposed ARMAN group, as well as a fungus. The 16S rRNA identity
of one archaeon is only 91.6% compared to the most closely related isolate Thermogymnomonas
acidicola. Hence, this organism is the first member of a new genus. The enrichment culture is dominated
by this microorganism and the ARMAN. The third archaeon in the community seems to be present in
minor quantities and has a 100% 16S rRNA identity to the recently isolated Cuniculiplasma divulgatum.
The enriched ARMAN species is most probably incapable of sugar metabolism because the key genes
for sugar catabolism and anabolism could not be identified in the metagenome. Metatranscriptomic
analysis suggests that the TCA cycle funneled with amino acids is the main metabolic pathway used by
the archaea of the community. Microscopic analysis revealed that growth of the ARMAN is supported by
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the formation of cell aggregates. These might enable feeding of the ARMAN by or on other community
members.