Taxonomy and Physiology of Filamentous Anoxygenic Phototrophs

Abstract

The filamentous anoxygenic phototrophs are a diverse group of photosynthetic bacteria that are of particular evolutionary significance. The best known species is the thermophilic Chloroflexus aurantiacus. This organism is a prominent member of hot spring microbial mat communities. Because it forms a deep division in the eubacterial line of descent and because it has an interesting combination of characteristics found in very different and diverse groups of phototrophic prokaryotes, it is of particular significance in addressing questions of evolutionary importance. There are several other strains of filamentous photosynthetic bacteria from a wide range of environments that are substantially different from Cf. aurantiacus yet have enough similarity in fundamental photosynthetic features to be likely relatives. Sequence data (16S rRNA) are needed to define the phylogenetic range of the family Chloroflexaceae. Some of the interesting biology of the diverse filamentous phototrophs is discussed in this chapter along with the taxonomic and phylogenetic problems they present. The physiology of Cf. aurantiacus is intriguing in several respects. The recently described autotrophic fixation pathway involving 3-hydroxypropionate is unlike any other known autotrophic mechanism. C. aurantiacus is also unique among all groups of phototrophs in lacking the capacity for nitrogen fixation. The regulation of pigment synthesis in response to changing growth conditions is particularly interesting due to the presence of two different photosynthetic pigments located in different sub-cellular environments. The fact that Cf. aurantiacus is a thermophile provides another dimension of complexity to its physiology. It is also quite resistant to UV radiation. Some of its characteristics may be relicts from Precambrian ancestors.