Page 179 - Algae

162 Algae: Anatomy, Biochemistry, and Biotechnology

by nutrient availability but also by their proper relation, because changes in nutrient ratio cause
shifts in phytoplankton communities and subsequent trophic linkages. Nitrogen generally limits
overall productivity in the marine system. Nitrogen limitation occurs most often at higher salinities
and during low flow periods. However, because marine system is in stoichiometric balance, any
nutrient can become limiting. Phosphorus limitation occurs most often in freshwater system, in
environments of intermediate salinities, and along the coasts during periods of high fresh water
input. The occurrence of silicon limitation appears to be more spatially and temporally variable
than phosphorus or nitrogen limitation, and is more prevalent in spring than summer.
In the case of phosphorus, the limitation of algal growth can be at least twofold. First, there is
a limitation of nucleic acid synthesis. This limitation can be at the level of genome replication or
at the level of RNA synthesis (a form of transcriptional control). The limitation can affect photo-
synthetic energy conversion by reducing the rate of synthesis of proteins in the photosynthetic
apparatus, which is effectively a negative feedback on photosynthesis. This inhibition of protein
synthesis may thus have effects on cell metabolism and oxidative stress similar to those for inhi-
bition of protein synthesis under N limitation, except that the effect is indirect and less immediate.
Secondly, a more immediate response to phosphorus limitation is on the rate of synthesis and regen-
eration of substrates in the Calvin-Benson cycle, thereby reducing the rate of light utilization for
carbon fixation. Cells can undergo also a decrease in membrane phospholipids; moreover, the
inability to produce nucleic acids under P limitation limits cell division, leading to an increased
cell volume.
On a biochemical level, nitrogen limitation directly influences the supply of amino acids, which
in turn limits the translation of mRNA and hence reduces the rate of protein synthesis. Under
nitrogen-limited conditions also the efficiency of PSII decreases, primarily as a consequence of
thermal dissipation of absorbed excitation energy in the pigment bed. This appears to be due
mainly to a decrease in the number of PSII reaction centers relative to the antennae. The functional
absorption cross-section of PSII increases under nitrogen-limiting conditions, while the probability
of energy transfer between PSII reaction centers decreases. From a structural point of view, the
reaction centers behave as if they were energetically isolated with a significant portion of the
light-harvesting antenna disconnected from the photochemical processes. As nitrogen limitation
leads to a reduction of growth and photosynthetic rates, it also leads to a reduction in respiratory
rates. The relationship between the specific growth rate and specific respiration rate is linear
with a positive intercept at zero growth, which is termed maintenance respiration. The molecular
basis of the alterations is unclear; however, the demands for carbon skeletons and ATP, two of the
major products of the respiratory pathways, are markedly reduced if protein synthesis is depressed.
The requirement for silicon for the construction of diatom frustule makes this group uniquely
subject to silicate limitation. As silicic acid uptake, silica frustule formation, and the cell division
cycle are all tightly linked, under silica limitation, the diatom cell cycle predominantly stops at the
G 2 phase, before the completion of cell division. Thus, an inhibition of cell division linked to an
inability to synthesize new cell wall material under silicon limitation can lead to an increase in
the volume per cell. This increase could also be partly explained by the formation of auxospores
with a larger cell diameter.

ALGAE AND THE PHOSPHORUS CYCLE
The phosphorus cycle is the simplest of the biogeochemical cycles. Phosphorus is the eleventh-
most abundant mineral in the Earth’s crust and does not exist in a gaseous state. Natural inorganic
phosphorus deposits occur primarily as phosphates, that is, a phosphorous atom linked to four
oxygen atoms, in the mineral apatite. The heavy molecule of phosphate never makes its way
into the atmosphere; it is always a part of an organism, dissolved in water, or in the form of
rock. Cycling processes of phosphorus are the same in both terrestrial and aquatic systems.
When a rock with phosphate is exposed to water (especially water with a little acid in it), the