Aquifex

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Aquifex is a genus of bacteria in the phylum Aquificota. There is one valid species, Aquifex pyrophilus. A. aeolicus is sometimes listed as a species, but its name isn’t validly published.

Aquifex bacteria are extreme thermophiles, growing best at 85–95°C. They are rod-shaped, about 2–6 micrometers long and 0.5 micrometers wide, and they are motile. They do not form spores and are Gram-negative autotrophs. The name Aquifex means “water-maker,” referring to how their respiration makes water.

They often form small aggregates of up to about 100 cells and are found near underwater volcanoes or hot springs. A. aeolicus requires oxygen but can thrive with very low oxygen levels (around 7.5 parts per million), while A. pyrophilus can respire anaerobically by reducing nitrogen instead of oxygen. Both species have nitrate reductase and nitrate transporter genes.

The genome of A. aeolicus has been sequenced. It is about 1.55 million base pairs long, with over 1,500 genes. More than 90% of the genome is coding for proteins, and there are few non-coding repeats. Many respiration-related enzyme subunits are organized in separate operons, and DNA repair in Aquifex uses proteins similar to those in eukaryotes.

Metabolically, Aquifex fixes carbon using the reductive TCA cycle to make acetyl-CoA and other building blocks. Some common gluconeogenic pathways are not clearly present, which suggests Aquifex may use a different route for producing sugars.

Both A. aeolicus and A. pyrophilus can oxidize hydrogen, thiosulfate, and sulfur as part of their energy production. Phylogenetic studies give mixed results: small-rRNA trees suggest Aquifex is among the earliest bacteria, possibly linked to Archaea, but protein-based trees give different pictures, so their exact position is debated. They are often considered among the earliest hyperthermophiles.

A. aeolicus is used as a model organism for studying hyperthermophilic bacteria. Research on Aquifex hydrogenases shows they can catalyze hydrogen reactions at very high temperatures, which has raised interest in their potential use as renewable biocatalysts for hydrogen-based fuel cells. Discovery locations include A. aeolicus near Sicily and A. pyrophilus near Iceland.