Physiology and biochemistry of symbiotic and free-living chemoautotrophic sulfur bacteria

Abstract

SYNOPSIS. In this chapter, the known mechanisms that enable diverse sulfur-oxidizing chemoautotrophic bacteria to conserve energy are summarized. The mechanisms known to be utilized by symbionts constitutea relatively small subset of those used by free-living chemoautotrophic bacteria; therefore, a search for additional pathways in symbionts should be fruitful. The emerging evidence for the use of nitrate as an alternative electron acceptor in sulfur oxidation by two types of symbionts is also discussed. Thus far, the data are not completely consistent with the operation of either a classical dissimilatory or classical assimilatory nitrate reductase. Lastly, previous literature calculations regarding the efficiency of coupling between sulfur compound oxidation and carbon dioxide fixation are reexamined. For both free-living and symbiotic sulfur bacteria, the published efficiencies are shown to have been overestimates by up to four-fold due to an inappropriate assumption about the source of electrons used for carbon reduction. © 1995 by the American Society of Zoologists.