A phylogenetic relative of
Thermoplasma and
Ferroplasma is
Picrophilus.
Although
Thermoplasma and
Ferroplasma are extreme acidophiles,
Picrophilus is even more so, growing optimally at pH 0.7 and capable of growth to as low as pH -0.06!
Picrophilus differs from
Thermoplasma and
Ferroplasma in other ways as well, including having a cell wall (an S-layer) and has much lower DNA GC base ratio.
The physiology of
Picrophilus is clearly of interest as a model for acid tolerance. Studies of its cytoplasmic membrane suggest an unusual arrangement of lipids that form an extremely acid impermeable membrane at optimal pH values.
By contrast, at only moderately acidic pH such as pH 4, the membranes of cells of
Picrophilus quickly become leaky and literally disintegrate, clearly indicating that this organism has evolved to survive only in highly acidic habitats.
Taken from the text Brock Biology of Microorganisms (10th ed.). Madigan, M.T., Martinko, J.M., and Parker, J. 2003. Prentice Hall. 458p.