The fact that Ferroplasma acidiphilum, a single-celled organism lacking a protective cell wall, is capable of living in sulfuric acid is already extraordinary. But what really makes the microbe unique is its unusual relationship to iron. Researchers in Braunschweig and Madrid have discovered that Ferroplasma acidiphilum not only extracts energy from iron – it "eats" the metal and leaves rust behind – but also uses it as an essential structure organising element for most of its cellular proteins. This biochemical apparatus differentiates Ferroplasma acidiphilum from all other known organisms. Quite possibly the microorganism has retained a primordial characteristic from the earliest days of evolution. The research findings have now been published in the most recent edition of the scientific journal Nature.
"Ferroplasma acidiphilum is a so-called archaebacterium, a microbe that exists in unusual, mostly highly extreme environments," explains Dr. Olga Golyshina from the Helmholtz Centre for Infection Research in Braunschweig, who discovered the germ in pyrite ore a few years ago. Ferroplasma, as the name suggests, lives in acidic iron-rich environments, like drainage streams from abandoned mines. "Because the energy yield from iron oxidation is so minimal, Ferroplasma metabolizes iron-containing rock by the ton, to extract energy for its growth," notes Professor Ken Timmis, the lead scientist for this project at the Helmholtz Centre for Infection Research. "It thereby performs an astounding biogeochemical work," adds Timmis.
Together with colleagues in Braunschweig and the CSIC Institute of Catalysis in Madrid, Prof. Timmis studied key components of the cell – the proteins – of Ferroplasma, and made an amazing discovery: "Almost all of the proteins of Ferroplasma acidiphilum contain atoms of iron," says Dr. Peter Golyshin, who works at both the Braunschweig Technical University and Helmholtz Centre. "In all other organisms surveyed, including other archaebacteria, only a minor fraction of the cell’s proteins contain iron." In most instances, the iron atoms in Ferroplasma proteins serve as anchors that hold together the flexible protein chains. The term "iron rivet" was coined for this property.
The discovery of this iron rivet-dominated protein machinery of Ferroplasma acidiphilum suggests new ideas about the early stages of evolution. "One current theory on how life began," says Prof. Timmis, "proposes that the first biological molecules would have formed on iron-sulfur, energy-rich surfaces, such as pyrite. Indeed, iron-sulfur mediated catalysis is a feature of some current biochemical reactions. Our results suggest that the first primordial cells may not only have exploited iron in iron-sulfur catalysis, but also as a primitive protein structure organiser. Later, as life form radiated to other habitats containing little iron, evolution will have selected other types of structure organiser". "An exception to the normal iron-limited world is the environment in which Ferroplasma acidiphilum can be found, even today," says Timmis, "where soluble iron is freely available. Perhaps Ferroplasma belongs to a branch of evolution that never left this environment and therefore never needed to replace its iron rivets."