Environmental Engineering Reference
In-Depth Information
Sulfur Cycle
The cycling of sulfur is studied mainly because of interactions with
other nutrients. The sulfur cycle in aquatic systems is indirectly important
to ecosystem productivity because sulfur is rarely a limiting nutrient for
primary producers; the requirement is low and the levels available are gen-
erally relatively high. The sulfur cycle can be diagrammed using the method
presented previously (Fig. 13.8).
PHOSPHORUS
Phosphorus is the key nutrient that lim-
its primary production in many aquatic habi-
tats. Nutrient pollution and use by organ-
isms are discussed extensively in Chapters
16 and 17. Unlike carbon, nitrogen, and sul-
fur, phosphorus is mainly found in only one
inorganic form (phosphate), so most re-
search has centered on organic transforma-
tions and interaction of other inorganic
chemicals with phosphate.
Sidebar 13.2.
A Romanian Cave Ecosystem Fueled by
Sulfide-Oxidizing Primary Producers
In 1986, access was obtained to a groundwater
ecosystem within Movile Cave, which has a con-
siderable influx of springwater rich in H
2
S. The
cave has a terrestrial fauna with 30 obligate
cave-dwelling invertebrates, of which 24 are en-
demic. The aquatic fauna includes 18 inverte-
brates, with 9 endemic species. Air pockets in
the cave have floating microbial mats (composed
of bacteria, fungi, and five nematode species;
Riess
et al.,
1999) and other mats are attached to
walls. Scientists hypothesized that sulfide-
oxidizing bacteria form the base of the food web
in these caves.
Subsequent studies demonstrated that the
microbial communities were able to assimilate
14
C-labeled bicarbonate into microbial lipids, ver-
ifying that the assemblages growing in the cave
were autotrophic. The ratios of stable isotopes of
carbon (
13
C/
12
C) and nitrogen (
15
N/
14
N) were used
to ascertain that the carbon fixed by microbial
autotrophs was entering the food webs. This
analysis demonstrated that the microbial biofilms,
the invertebrate consumers of the microbes, and
the invertebrate predators had isotopic ratios
more similar to each other than to those found in
nearby surface aquatic or terrestrial samples
(Sarbu
et al.,
1996). Furthermore, isotopic signa-
tures were consistent with the hypothesis that
invertebrates subsisted on microbes or other
invertebrates that feed on microbes in the cave.
This unusual cave ecosystem is one of the few
known systems in the world that is not depen-
dent on carbon fixed by photosynthesis.
Phosphorus Forms
Phosphate (PO
4
3
) is a dominant form
of inorganic phosphorus in natural waters,
but concentrations are often below detection
(about 1
g liter
1
). Determining the precise
level of phosphate is difficult because stan-
dard methods of analysis also detect a vari-
able and poorly defined group of phosphate
compounds (Rigler, 1966). Cells can also
store phosphate as a polymer (polyphos-
phate).
Organic phosphorus occurs in a variety
of organic compounds. The crudest classifi-
cation is dissolved organic phosphorus (DOP)
and particulate phosphorus (PP). The sum of
DOP, PP, and phosphate is total phosphorus
(TP). Phosphorus is used in cells for nucleic
acids, phospholipids, and other compounds.
Phosphorus Transformations
The interaction of phosphate with iron
is important in determining the availability
of phosphorus in many aquatic systems.
Phosphate will precipitate with some metals,
including ferric iron (Fe
3
, an oxidized form
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