Organism		Ecosystem	Metabolism
Chloroflexus		dense microbal mats in hot springs often in assocation with cyano, thermophilic; floculent surface layer in alkaline springs	e donors: sulfide, H2
Heliobacter		soil, dry paddy fields, occassionally from lakeshoremuds and hot springs
Green sulfur - Chlorobium		non-thermal aquatic ecosystems, hot springs	e donors: Reduced sulfur compounds, H2, Fe
Purple sulfur	alpha - aerobic	eutrophic marine environments
	alpha	non-thermal aquatic ecosystems, hot springs, do not generally form dense accumulations under natural conditions	e donors: wide variety of organic compounds and inorganic (H2, elemental sulfur, thiosulfate, sulfite, and iron), Fe
	beta
	Chromat.	fresh and marine waters,	e donors: sulfide reuction, intracellular S8, Fe
	Ectothior.	hypersaline	sulfide reduction, external S8 deposit
Cyano		everywhere

Multilayered microbial mat as it is regularly found in the sandflats of Great Sippewissett Salt Marsh (Cape Cod, Massachusetts, USA). A. In most instances, the mats consist of a top green layer, an intermediate purple layer, and a grayish to blackish bottom layer. B. Fully developed microbial mats consist (from top) of an olive-green layer of diatoms and cyanobacteria, a green layer consisting mostly of cyanobacteria, a purple layer of purple sulfur bacteria, a peach-colored layer formed by BChl b-containing purple sulfur bacteria (morphologically similar to Thiocapsa pfennigii), and a greyish to blackish bottom layer.

Sulfide acts as a strong poison of PSII activity in many algae and cyanobacteria. The ability of some Cyanobacteria to conduct anoxygenic photosynthesis with sulfide as an electron donor to PSI (Cohen et al., 1975; Padan, 1979; Padan and Cohen, 1982), or to continue oxygenic photosynthesis in the presence of sulfide (Cohen et al., 1986), may be one of the key traits that extend the habitat of sulfide-utilizing cyanobacteria into the temporarily anoxic, sulfide-containing, layers of hot springs (Castenholz and Utkilen, 1984), marine microbial mats (De Wit and van Gemerden, 1987a; De Wit et al., 1988), and the chemoclines of meromictic lakes (Jørgensen et al., 1979, Camacho et al., 1996). Sulfide is an inhibitor of PSII and induces the synthesis of a sulfide-oxidizing enzyme system. In contrast to phototrophic sulfur bacteria, cyanobacteria oxidize sulfide to elemental sulfur or thiosulfate but do not form sulfate (De Wit and van Gemerden, 1987b). However, the use of sulfide by cyanobacteria in anoxygenic photosynthesis must be regarded as a detoxification mechanism, since their low affinity for sulfide (De Wit and van Gemerden, 1987b, Garcia-Pichel and Castenholz, 1990) renders them unable to compete with purple or green sulfur bacteria for sulfide as an electron donor.