Hydrogen Sulfide: A Toxic Gas Produced by Dissimilatory Sulfate and Sulfur Reduction and Consumed by Microbial Oxidation - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 242

1.1.4.1 Anoxygenic Phototrophic Bacteria

Reduced inorganic sulfur compounds play an important role as electron donors
for photosynthetic carbon dioxide fixation in anoxygenic phototrophic sulfur
bacteria. Four major phylogenetic groups of anoxygenic phototrophic bacteria
can be distinguished: (a) The green sulfur bacteria (GSB) (family Chlorobiaceae ),
(b) the purple sulfur (PSB) and purple non-sulfur bacteria (PNSB), (c) the Gram-
positive Heliobacteria,

(d) the filamentous and gliding green bacteria

( Chloroflexaceae )[ 50 , 51 ].
Dissimilatory sulfur metabolism (i.e., the use of inorganic sulfur compounds,
such as sulfide, elemental sulfur, polysulfides, sulfite, or thiosulfate, as sources or
sinks of electrons) has been mainly investigated in the PSB of the families
Ectothiorhodospiraceae and Chromatiaceae . Many of the PNSB can also utilize
inorganic sulfur compounds as a source of electrons. The GSB of the family
Chlorobiaceae , some cyanobacteria, and some members of the filamentous
anoxygenic phototrophic bacteria (family Chloroflexaceae ) are also able to grow
phototrophically using reduced sulfur compounds as electron donors [ 50 , 51 ].
Anoxygenic phototrophic PSB constitute a major group of bacteria widely
distributed in nature, primarily in aquatic environments. The two families of PSB,
the Ectothiorhodospiraceae and Chromatiaceae , respectively, produce external and
internal sulfur granules. Typical habitats of PSB of the family Chromatiaceae are
freshwater lakes and intertidal sandflats. The family Ectothiorhodospiraceae is
found mainly in hypersaline waters. Many species of PSB are “extremophilic”,
growing at high salt and/or pH. PNSB have been isolated from almost every
environment, including freshwater, marine systems, soils, plants, and activated
sludge. PSB (more than 30 genera) consist of a variety of morphological types
and belong to the Gammaproteobacteria (order Chromatiales ) (Table 2 ). GSB are
found in various types of aquatic habitats such as the pelagial of lagoons or lakes,
bacterial mats in hot springs, or bottom layers of bacterial mats in intertidal
sediments.
More than twenty genera of PNSB are now recognized (Table 2 ). PNSB consti-
tute a physiologically versatile group of purple bacteria that can grow well both in
darkness and phototrophically.

1.1.4.2 Colorless Sulfur Bacteria

The name “colorless sulfur bacteria” (CSB) has been utilized since the time of
Winogradsky [ 49 ] to designate microorganisms able to use reduced sulfur com-
pounds (e.g., sulfide, elemental sulfur, and organic sulfides) as sources of energy for
growth. The adjective “colorless” is utilized because of the lack of photopigments
in these organisms, although colonies and dense cultures of these bacteria could
actually be brown or pink due to their high cytochrome content [ 49 ]. CSB play an
essential role in the oxidative side of the biological sulfur cycle (Figure 1 ).