Nitrogen, Sulfur, Phosphorus, and Other Nutrients - Freshwater Ecology: Concepts and Environmental Applications - page 264

The complex interactions among organisms can be thought of as con-
strained by redox but also as tied together by organic carbon because it
provides energy and materials for organism growth. For example, chemoau-
totrophic organisms, such as those that oxidize ammonium, sulfide, or fer-
rous iron, use the energy obtained to assimilate CO 2 . Assimilatory processes
can provide new nutrients to a habitat, as in the case of nitrogen fixation.
The redox series of metabolic interactions discussed previously is in large
part a series of increasingly less efficient ways for organisms to oxidize or-
ganic carbon to CO 2 and release energy.
In addition, key links occur between the cycles that are mediated by
inorganic chemical reactions. These include interactions between ferric iron
and phosphate, and between ferrous iron and sulfide.
SUMMARY
1. The major forms of inorganic nitrogen are N 2 gas, nitrate, nitrite,
and ammonium. Organic nitrogen occurs in many forms, including
amino acids, proteins, nucleic acids, nucleotides, and urea.
2. The major fluxes in the nitrogen cycle include denitrification (using
nitrate to oxidize organic C yielding N 2 ), oxidation of ammonium to
nitrate by chemosynthetic bacteria, assimilation of ammonium,
fixation of N 2 by bacteria, and excretion of ammonium by
heterotrophs.
3. Forms of inorganic sulfur include sulfide, thiosulfate, sulfate,
elemental sulfur, and metal sulfides. Organic sulfur is found in
proteins and amino acids.
4. The most important fluxes in the sulfur cycle are biological and
abiological sulfur oxidation, biological sulfur reduction (a form of
anoxic respiration), production of sulfide by fermentation,
disproportionation, and metal pyrite precipitation and deposition.
5. Phosphorus is a key element that determines the productivity of
many aquatic ecosystems and can be found in the forms of
phosphate and organic phosphate. Organic phosphate can be cleaved
to phosphate by phosphatase enzymes produced by organisms.
Phosphate forms a low-solubility precipitate with ferric iron in the
presence of O 2 that can cause its removal from oxic environments.
6. Silicon is a vital component of the cell walls of diatoms and can be a
key factor in controlling composition of phytoplankton communities.
It is redissolved slowly; thus, when frustules sink out of the photic
zone in lakes, it takes months for the silicon to become available
again.
7. Iron occurs as ferric and ferrous ions in oxic and anoxic habitats,
respectively. It can also occur as a metal pyrite (FeS) in anoxic
habitats and a flocculent precipitate [Fe(OH) 3 ] in oxic habitats. Iron
is an important component of many proteins, including those for
electron transport, nitrate assimilation, and chlorophyll synthesis.
8. Bacteria in oxic environments can oxidize ferrous iron. Ferric iron
combines with OH or PO 4 3 to form low-solubility precipitates.
Chelators can keep ferric iron in solution (prevent it from