In the interview with Ken Nealson by managing editor Eva Emerson ("From fringe to electromicrobiological mainstream," SN: 12/5/09, p. 32), I fear that some misinformation may well have been conveyed.
Professor Nealson said of his finding, "it simply wasn't in the textbooks 20 years ago and still is not there in most textbooks." Namely, he was referring to the fact that bacteria could reduce solid substrates, "... the bugs would just settle down and respire the rock," and he intimated that this was unheard of at that time. I call attention to two books published more than 50 years ago: Biochemistry of Autotrophic Bacteria by H. Lees (Butterworths Scientific Publications, 1955) and An Introduction to Bacterial Physiology by E.L. Oginsky and W.W. Umbreit (W.H. Freeman & Co., 1954). These provide summaries of research since the early 1920s on the autotrophic sulfur-oxidizing bacterium Thiobacillus thiooxidans and other bacteria that oxidize and reduce sulfur and iron. Bacteria have been known to derive their energy from oxidizing these substances and using the energy derived to fix COs into carbon compounds used by the cells.
Thus, electron transfer by bacterial metabolic activity to insoluble compounds, leading to their degradation, has been known for more than the 25 years since Nealson's findings.
T. thiooxidans was isolated and characterized in 1921. When I was a graduate student with Umbreit, we, along with Pauline Holbert, demonstrated, using electron photomicrography, the firm attachment to, and degradation of, sulfur crystals by a pure culture of Z thiooxidans (W.I. Schaeffer, P.E. Holbert and W.W. Umbreit. The Journal of Bacteriology, January 1963). This letter in no way is meant to denigrate the findings and subsequent research by Nealson, only to point out that the paradigm is far older than this interview would lead one to believe.
Warren I. Schaeffer, Edison, N.J. Schaeffer is an emeritus professor of microbiology and molecular genetics at the University of Vermont in Burlington.
Ken Nealson responds: I am afraid Dr. Schaeffer caught me in my own myopic world of solid metal oxide reduction. I was specifically referring to the extracellular electron transfer of cellular reducing power (electrons) to solid metal oxides, such as iron or manganese oxides, and the conceptual difficulty that was imagined with truly insoluble substrates as electron acceptors. Dr. Schaeffer is quite correct to point out that the other side of the electron flow issue--namely extracting electrons from a solid substrate--has been seen since it was first realized that elemental sulfur could serve as a source of electrons for both photosynthetic and chemolithotrophie microbes. But with regard to reductive interaction with solids, what I said was correct, though incomplete in the broader context. The issue raised, however, is a very important one, as it is conceivable that microbes may exist that could do both, and could operate in the subsurface using minerals of different redox potentials both as a source of electrons and as the ultimate electron acceptors. That would be really something.