The Genus Leucothrix

Abstract

Leucothrix, a large-diameter, morphologically distinct, marine gliding bacterium, has been known in natural material since the time of Oersted in 1844, but its modern history began with the remarkable study Harold and Stanier published in 1955. In the introduction to their paper, Harold and Stanier state: “Leucothrix may be characterized succinctly as a chemoheterotrophic counterpart of the colorless sulfur-oxidizing organism Thiothrix. It has been observed on a few occasions ... in the century since its original description, but the existing accounts of its morphology and development, based entirely on the examination of crude cultures, are either incomplete or inaccurate. Thanks to the ease with which it can be grown in pure culture, we have been able to determine its complete cycle of development, which includes a unique and hitherto undescribed process of gonidial aggregation to form many-celled rosettes.”

Leucothrix is fascinating not only because of its morphological distinctiveness, but also because of its large size and the ease with which it can be recognized in natural material. The organism seems to be entirely marine and is widespread as an epiphyte of marine algae (Fig. 1) (Brock, 1966). It also causes an extensive infestation of benthic crustacea and fish eggs (Johnson et al., 1971), and has become a problem in the field of aquaculture, especially in the artificial cultivation of lobsters. Leucothrix appears to be related to the cyanobacteria (Pringsheim, 1957; Raj, 1977), as determined by morphological similarities. However, no detailed studies of molecular relationships between Leucothrix and any cyanobacteria have been carried out, although the DNA base composition of Leucothrix mucor isolates, 47–49 mol% GC (Brock and Mandel, 1966), is similar to that of a number of filamentous cyanobacteria (Edelman et al., 1967), which cluster in the range 42–51 mol% GC. This similarity is made even more striking when it is compared with the wide variation in DNA base compositions of the unicellular cyanobacteria (Edelman et al., 1967; Stanier et al., 1971) and with the wide variation found in the narrow-diameter gliding bacteria (Edelman et al., 1967).