IRON BACTERIA
Authors: PRINGSHEIM E. G. 
Published: January 2008

The iron bacteria, characterized by depositing iron compounds in a morphologically defined way, are a biological group, the knowledge of which has grown rather unmethodically as shown in the historical survey. Morphological, ecological, practical and biochemical investigations were never co‐ordinated.

Taxonomically, the Chlamydobacteriaceae, the Gallionellaceae, and the unicellular iron bacteria are unrelated. The extraordinary variability of the first group has not been realized by most investigators, who have expected every appearance to be connected with a definite taxonomic form. Reversible modifications of a few species have been described under a multitude of names as different genera, species and varieties. It is proposed to go back to Kützing's Sphaerotilus as the oldest named genus, to which all the forms called Leptothrix, Cladothrix Chlamydothrix, etc. also belong. Crenothrix is an exception, being another genus of the Chlamydobacteriaceae. With respect to the Gallionellacae, the definition of the species needs further investigation. The position in the unicellular forms is still worse.

The ecology of iron bacteria is of special interest because it is connected with the iron cycle in nature. Iron is a physiologically essential chemical element, which may, oftener than any other, be deficient in natural habitats of algae and higher plants owing to its insolubility in the higher oxidation state. Iron‐depositing organisms are found only where oxygen has access, but near positions where chemical reduction is prevalent.

Such conditions are difficult to maintain in experimental circumstances. Attempts made to replace ferrous by manganous compounds have failed because even in the presence of manganous compounds ferrous compounds are also required. Under natural conditions humus and other substances may help to keep iron in a suitable state by their reducing power and by their capacity to form more or less stable and at the same time soluble ferrous compounds. If such conditions were imitated in experiments, autotrophy would of course not be warranted. These are some of the reasons why the question has not yet been answered as to whether iron bacteria can grow without organic substances by utilizing the oxidation energy of iron compounds.

New culture investigations have been undertaken to obtain pure strains from growths of Sphaerotilus, Leptothrix and Cladothrix. All of them could be shown to belong to one or the other of two species: Sphaerotilus natans Kützing and S. disco‐phorus (Schwers) nov. comb. From the former all the appearances described as different genera could be obtained by providing suitable conditions. S. discophorus behaves in a similar way. Both can grow without producing inorganic sheaths. The sheaths when present are different in the two species, only those of the latter containing manganese. This admixture causes a characteristic brown hue, while the sheaths of S. natans are almost colourless.

Similar investigations are necessary in the other groups of iron bacteria, and on such investigations methodical and elaborate biochemical and morphological studies could be based.

An index of names and synonyms with critical remarks is given, to show how many incomplete diagnoses have been made, and which of the species are likely to be maintained and which discarded. In order to facilitate the finding of information, the list of references to literature contains some titles not mentioned in the text.

I am much indebted to Dr F. M. Haines, Queen Mary College, University of London, for correcting the manuscript.