Nitrogen, Sulfur, Phosphorus, and Other Nutrients - Freshwater Ecology: Concepts and Environmental Applications - page 257

Oxic processes

Dimethylsulfide

Biological and abiological sulfur oxidation

A A A

A

Organic S Sulfide

S 2-

Sulfur
S 0

Thiosulfate
S 2 O 3 2-

Sulfate
SO 4 2-

Fermentation

Biological sulfur reductions

Disproportionation

Metal sulfide
(e.g. FeS)

Anoxygenic photosynthesis

Anoxic processes

FIGURE 13.8

A conceptual diagram of the sulfur cycle. A assimilation.

of iron). The precipitation occurs only in the presence of O 2 , and the com-
plex dissociates under anoxic conditions. Precipitation of ferric phosphate
leads to the deposition of phosphorus in sediments when surface water is
oxygenated. When the precipitate settles into an anoxic zone (such as an
anoxic hypolimnion of a lake) the phosphate dissociates. Processes that
move dissolved materials then move the phosphate. Thus, phosphate is
brought back up to the surface when fall mixing breaks down an anoxic
hypolimnion, and phosphate deposited into wetland sediments can diffuse
back toward the water column.
Phosphate is assimilated at very low concentrations by cells under
phosphorus limited conditions. Evidently, billions of years of natural se-
lection under phosphorus limiting conditions have created a selective pres-
sure for very efficient uptake mechanisms. Uptake of nutrients will be dis-
cussed in Chapter 16.
Organisms have phosphatase enzymes that cleave dissolved organic
phosphorus compounds to liberate phosphate. The phosphatases are com-
mon inside of cells but can also be excreted outside the cell (extracellular)
or be associated with the exterior cell surface (Chróst, 1991; Olsson,
1991). These enzymes increase the availability of phosphate to cells, so the
excretion of extracellular phosphatases increases when phosphorus be-
comes scarce. The ubiquitous nature of these compounds in natural waters
leads to rapid turnover of many organic phosphorus compounds.
Heterotrophy results in rearrangement of organic phosphorus com-
pounds as in all other cycles (Fig. 13.9). In contrast to other nutrient cycles,
phosphate cannot serve to oxidize organic carbon (e.g., denitrification and
dissimilatory sulfate reduction). Organisms excrete excess phosphorus as
phosphate or organic phosphorus in both oxic and anoxic environments.