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 246

1.3 Effects of Hydrogen Sulfide on Gene Expression
and Physiology of Desulfovibrio vulgaris Hildenborough

H 2 S is toxic to most life forms including the SRB themselves for which its presence
presents a stress, which these organisms must overcome. The activity of SRB is
influenced by the presence of metals such as iron, manganese, copper, cadmium,
nickel, lead, and zinc. Inhibitory or even lethal effects are observed at high
concentrations of heavy metals, while a low concentration could promote the
SRB activity [ 59 ]. Most SRB tolerate more than 10 mM sulfide and sulfate reducers
growing on aromatic hydrocarbons formed as much as 25 mM sulfide before growth
ceased. Some Desulfotomaculum species are more sensitive to sulfide, which
affects their growth at concentrations of 4-7 mM [ 59 ]. Sulfidic sulfur can be present
in three different forms (H 2 S, HS , and S 2 ) of which the relative fractions depend
on the pH of the environment, see reactions ( 4 ) and ( 5 )[ 60 ].

H 2 S ! H þ þ HS p K a 1 ¼ 6 : 97 25 C

ð Þ ð 4 Þ

HS ! H þ þ S 2 925 C

p K a 2

: ð Þ ð Þ

¼ 12 5

The response of Desulfovibrio vulgaris Hildenborough (hereafter referred to as
D. vulgaris H) cells to high sulfide stress has been determined [ 60 ]. The growth of
D. vulgaris H cells was compared in an open system, where sulfide was removed as
H 2 S by continuous gassing (low sulfide, 1 mM), with a closed system, where sulfide
was accumulated (high sulfide, 10 mM). High sulfide decreased the final cell
density by 33 % and the specific growth rate constant by 52 %, indicating a decrease
in bioenergetics fitness. Under high sulfide conditions the transcription of ribosomal
protein-encoding genes was decreased, in agreement with the lower D. vulgaris H
growth rate. The expression of the DsrD gene, located downstream of the Dsr genes
was also strongly down-regulated. In contrast, the expression of many genes
involved in proteolysis, stress response, and iron accumulation were increased.
High sulfide represents a significant stress condition, in which the bioavailability of
metals like iron may be lowered [ 60 ].

2 Enzymology of Hydrogen Sulfide Production
from Sulfate

2.1 Enzymology of Dissimilatory Sulfate Reduction

The hallmark characteristic of microorganisms utilizing dissimilatory sulfate reduc-
tion is the production of copious amounts of H 2 S. Prior to reduction, sulfate must be
activated by ATP sulfurylase to produce adenosine 5'-phosphosulfate (APS) plus
pyrophosphate (PP i ), reaction ( 6 ).