A panoply of phototrophs: An Overview of the thermophilic chlorophototrophs of the microbial mats of Alkaline siliceous hot springs in Yellowstone National Park, WY, USA

Chlorophototrophs are organisms that can synthesize chlorophylls or bacteriochlorophylls, and they use these molecules to harvest and convert light energy into stored chemical potential energy. Some of these organisms also perform photosynthesis, in which light provides the energy (ATP) and reducing power (NAD(P)H or reduced ferredoxin) required for inorganic carbon (C_i) fixation. Over the past decade, we have studied the chlorophototrophs found in two alkaline siliceous hot springs in Yellowstone National Park, WY, USA. The microbial mats that occur at temperatures of 40-73 °C in Mushroom and Octopus Springs have proven to contain a surprisingly diverse array of chlorophototrophs. These include members of six of the seven bacterial phyla known to have members capable of synthesizing (bacterio)-chlorophylls: Acidobacteria, Cyanobacteria, Chlorobi, Chloroflexi, Firmicutes, and Proteobacteria. More than 16 chlorophototrophs have now been associated with these microbial mats, and this does not include the many ecotypes of these organisms that occur within these communities. In this chapter we will briefly describe the panoply of phototrophic organisms that occur in these mat communities and will provide an introduction to their morphological appearance and other basic properties. Metagenomic analyses have revealed several novel organisms, e.g., Chloracidobacterium thermophilum, "Candidatus Thermochlorobacter aerophilum," "Candidatus Chloranaerofilum corporosum," "Candidatus Roseovibrio tepidum," and "Candidatus Roseilinea gracile," which were hitherto unknown to microbiologists because they escaped isolation by classical, culture-based methods. However, by combining molecular methods, in situ physiological observations, metabolic reconstruction, and enrichment techniques, we are now making remarkable progress toward the isolation of these chlorophototrophic organisms.