Filamentous Anoxygenic Phototrophs in Hot Springs

Abstract

Microbial mats often seen in neutral and alkaline hot springs are generally formed by phototrophic bacteria. These mat-forming thermophiles comprise oxygenic phototrophs, namely cyanobacteria, and the other group of phototrophic bacteria, so-called filamentous anoxygenic phototrophs (FAPs). FAPs contain bacteriochlorophyll(s) as photosynthetic pigments and conduct photosynthesis without producing oxygen. They are all affiliated to the order Chloroflexales in the phylum Chloroflexi, and are classified into three genera, Chloroflexus, Heliothrix and Roseiflexus. Chloroflexus species have greenish brown to brown filaments and a typical light-harvesting apparatus called the chlorosome, while the others are red to reddish orange and completely lack chlorosomes. FAPs form colorful and variously shaped microbial mats with cyanobacteria. Distinctive microbial mats composed of these bacteria, e.g. complexly layered or ruffled fur-like mats, can be seen in some Japanese hot springs. While FAPs phylogenetically differ from all other phototrophs, the group is known to have a "chimeric" photosynthetic system: The photochemical core complex in the FAPs, essential to photosynthesis, resembles that in the purple bacteria; their light-harvesting apparatus is similar to that in green sulfur bacteria. The photosynthetic peculiarity found in the FAPs is important to solving the evolution of photosynthesis.