Environmental Engineering Reference
In-Depth Information
sulfide is a significant gaseous product of phytoplankton metabolism that
forms an important component of the global sulfur budget.
Sulfur Transformations
Numerous transformations are possible because of the many redox
states that sulfur can take. A few of these transformations are illustrated
in Table 13.1. The general fluxes of heterotrophy and remineralization
(aerobic or anaerobic) are present, as for all other nutrient cycles.
Reduced forms of sulfur can combine with O
2
with a net release of
potential energy (Table 13.1).
Abiotic oxidation
occurs spontaneously, but
more slowly than
biotic sulfur oxidation,
so the biotic oxidation of re-
duced sulfur compounds by chemoautotrophic organisms is generally con-
fined to areas at the interface between oxic and anoxic habitats where the
supply of reduced sulfur compounds is high. In one unique case, this
led to an entire cave ecosystem supported by sulfur-oxidizing bacteria
(Sidebar 13.2).
Oxidized sulfur can be used as an electron acceptor to respire organic
carbon in a process similar to denitrification.
This
dissimilatory sulfur reduction
is a pri-
mary source of sulfide found in anoxic sedi-
ments and waters. Sulfate forces the redox
to remain moderately high and can indicate
conditions in which methanogenesis will not
occur.
Several unique transformations exist in
the sulfur cycle. One is called
disproportion-
ation,
an anoxic transformation in which
thiosulfate is converted to sulfate and sulfide,
yielding energy (Table 13.1). Another is the
use of sulfide as an electron donor for
anoxy-
genic photosynthesis
that yields sulfide in a
process analogous to using water as an elec-
tron donor for oxygenic photosynthesis. The
production of sulfate in anoxic habitats leads
to the possibility of complete autotrophic and
heterotrophic components and a complete
sulfur cycle with no O
2
present. Such
processes may have been central to early life
in the anoxic habitats of primordial Earth.
Like methane and nitrous oxide, di-
methylsulfide is a gaseous by-product of or-
ganisms and can lead to a net loss of sulfur
from some ecosystems. The significance of
this production has not been well studied in
freshwaters; it is of interest to those who
link global sulfur budgets to marine primary
producers (Malin and Kirst, 1997). Di-
methylsulfide is ultimately oxidized to sul-
fate in the atmosphere.
were sampled, and 98% of those exceeded the
recommended levels of 10 mg N liter
1
. World-
wide, 10% of rivers exceed 9 mg N liter
1
(Meybeck
et al.,
1989). Some of the large con-
tributors to nitrate in groundwater and streams
include agricultural fertilization and feedlots.
Some sewage treatment plants release large
amounts of nitrate as well.
Once an aquifer is contaminated with ni-
trate, treating the problem is very difficult. One
possibility is to fertilize with organic carbon to
cause respiratory consumption of O
2
by mi-
crobes and promote denitrification. The nitro-
gen then is lost as N
2
. The problem with this
treatment is that it leads to anoxic groundwa-
ter. Anoxia can result in other problems, such
as high iron content and bad tastes and odors
related to sulfide and other chemicals.
Perhaps the best way to control the problem
of nitrate contamination is to do so at the
source. Options include lining livestock feed-
lots and treating all runoff before it reaches
the groundwater or streams, only adding as
much fertilizer to crops as is necessary, leav-
ing intact riparian buffer zones (Hedin
et al.,
1998), better design of septic drain fields, and
treating sewage to remove nitrogen (discussed
in later chapters). All the treatment and control
options require a basic understanding of nitro-
gen cycling and cycling of other nutrients.
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