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 262

(e.g., Pyrodictium species and A. ambivalens ) is similar to that of some eubacteria,
such as W. succinogenes . This dissimilatory process involves two multi-subunits,
membrane-bound enzymes: a nickel-iron-containing hydrogenase and a SR or PSR.
These two enzymes together reduce sulfur to H 2 S with H 2 as electron donor.
The composition of the described electron transfer chain shows participations of
similar [NiFe] hydrogenases and similar PSR in the case of A. ambivalens and
W. succinogenes , whereas the Py. abyssi SR seems to be different. Nine major
subunits constitute the hydrogenase-sulfur multienzyme complex isolated from Py.
abyssi DSM 6158 [ 153 ] and the subunits from Py. abyssi range from 24 to 82 kDa
with 550 kDa for the entire complex. Analysis of this complex reveals the presence
of a [NiFe] hydrogenase, a c -type heme, one or two b -type hemes, and an
undetermined number of [Fe-S] centers.
3.2.3 Sulfur Reductase from Pyrococcus furiosus
P. furiosus has been studied as the model organism for the mechanism of the
fermentation-based sulfur reduction. Two enzymes play key roles in the sulfur
metabolism: a membrane-bound oxidoreductase complex (MBX) and a cytoplas-
mic coenzyme A-dependent NADPH sulfur oxidoreductase (NSR). MBX is
encoded by an operon with 13 open reading frames and plays an essential role in
mediating electron flow to sulfur [ 154 ]. NSR is a homodimeric flavoprotein (M r
100,000) and reduces elemental sulfur to H 2 S with NADPH as the electron donor
[ 154 ]. This MBX-NSR complex sulfur-reduction system has only been reported so
far in the Thermococcales.
4 Microbial Oxidation of Hydrogen Sulfide to Sulfate
The oxidative inorganic sulfur metabolism has been recently and extensively
described both in eubacterial and archaebacterial microorganisms [ 28 , 29 , 49 ,
155 ]. Here, we briefly describe enzymes or multienzyme systems involved in sulfur
compound oxidation in Bacteria and Archaea.
4.1 Archaebacterial Inorganic Sulfur Compound Oxidation
Aerobic dissimilatory sulfur oxidation is common in the order Sulfolobales of the
Crenarchaeota which frequently thrive in continental solfataric fields [ 29 ,
155 ]. Mechanisms of archaeal inorganic sulfur compound oxidation were almost
exclusively studied in Acidianus species with the thermoacidophilic A. ambivalens
as the model organism [ 28 , 29 ].