Nutrient Use and Remineralization - Freshwater Ecology: Concepts and Environmental Applications - page 313

and hydrogen, rarely are these elements considered nutrients because they
seldom limit primary production. When oxygen limits ecosystem activity,
this is due to lack of availability for use as an oxidant of organic C, not a
lack of availability as material to build cells. Likewise, primary producers
can utilize CO 2 directly, and many researchers think carbon supply ulti-
mately does not limit ecosystems driven by photoautotrophs and should
not be considered a “nutrient.” Nitrogen, phosphorus, silicon, and iron are
the nutrients most often studied by aquatic ecologists.
The major classes of biological molecules require different amounts of
nutrients. Lipids are phosphorus and carbon rich. Amino acids and pro-
teins require relatively more nitrogen than most molecules. Nucleotides
and nucleic acids require significant amounts of nitrogen and phosphorus
relative to carbohydrates, such as starch and sugars, that are composed en-
tirely of carbon, oxygen, and hydrogen. Other elements such as iron are
required in very small amounts as cofactors in enzymes. Specialized re-
quirements include carbonate for mollusks and silicon for diatoms. Each
organism must acquire nutrients to survive and grow.
Nutrients are acquired in a variety of forms by organisms. Most primary
producers and many bacteria have the ability to utilize inorganic nutrients,
such as nitrate or phosphate. In a case that stretches the definition “au-
totroph,” algae such as the dinoflagellates can ingest particulate material for
its nitrogen and phosphorus content and to meet part of their carbon re-
quirements. Most animals and heterotrophic eukaryotic unicellular organisms
acquire their nutrients in organic form (e.g., nitrogen must be assimilated as
proteins or amino acids, and carbon must be assimilated as carbohydrates).
Nutrients in organic form can be dissolved in the surrounding water
or contained in particulate material (including organisms). Algae can uti-
lize some of these dissolved organic forms directly (Berman and Chava,
1999). Individual bacteria have different degrees of ability to utilize inor-
ganic or organic compounds, but the bacterial assemblage found in natural
waters is generally able to utilize a wide variety of organic and inorganic
nutrient sources. Here, I consider the general principles of assimilation and
uptake of dissolved nutrients. Uptake of materials in the form of particles
(e.g., living and dead organisms) will be considered in later chapters.
Nutrients need to be taken into cells from the water surrounding them
(uptake) and then incorporated into organic molecules used for growth (as-
similation). Generally, each of these steps requires energy. Three equations
are commonly used to describe the functional relationships among nutri-
ent concentrations, uptake, and assimilation.
The Michaelis-Menten relationship is used to describe the influence of
nutrient concentration on uptake rate:

[ S ]
K S

V V max

[ S ]

where V is the uptake rate of substrate, [ S ] is the concentration of substrate
(nutrient), 1 and K s is the concentration of S where V

1 ⁄ 2 V max . The shape

1 A reminder for those who have forgotten chemical conventions: The square brackets around
[ S ] indicate the concentration of the substrate S , generally in moles per liter. Moles per liter
can be converted to grams per liter by multiplying by molecular weight. Units of molecular
weight are in grams per mole.