Page 185 - Algae Anatomy, Biochemistry, and Biotechnology

168 Algae: Anatomy, Biochemistry, and Biotechnology

form. Following translocation across the plasmalemma (which is an energy-dependent process), the
þ
2
assimilation of NO 3 requires chemical reduction to NH 4 . This process is mediated by two
enzymes, namely, nitrate reductase and nitrite reductase. Nitrate reductase is located in the
cytosol and uses NADPH to catalyze the two-electron transfer:

NO 3 þ 2e þ 2H þ ! NO 2 þ H 2 O (4:3)
In cyanobacteria nitrate reductase is coupled to the oxidation of ferredoxin rather than a pyridine
nucleotide as in eukaryotic algae. The nitrite formed by nitrate reductase is reduced in a six-electron
transfer reaction:

þ

NO 2 þ 6e þ 8H þ ! NH 4 þ 2H 2 O (4:4)
Nitrite reductase utilizes ferredoxin in both cyanobacteria and eukaryotic algae; in the latter, the
enzyme is localized in the chloroplasts. In both cyanobacteria and eukaryotic algae photosynthetic
electron flow is an important source of reduced ferredoxin for nitrite reduction. The overall
stoichiometry for the reduction of nitrate to ammonium can be written as:

þ

NO 3 þ 8e þ 10H þ ! NH 4 þ 3H 2 O (4:5)
The incorporation of ammonium into ammino acids is primarily brought about by the sequential
action of glutamine synthetase (GS) and glutamine 2-oxoglutarate aminotransferase (GOGAT).
Ammonium assimilation by GS requires glutamate as substrate and ATP, and catalyzes the irrever-
sible reaction:

þ
Glutamate þ NH þ ATP ! Glutamine þ ADP þ P i (4:6)
4
The amino nitrogen of glutamine is subsequently transferred to 2-oxoglutarate, and reduced,
forming two moles of glutamate:

2-Oxoglutarate þ Glutamine þ NADPH ! 2½GlutamateŠþ NADP þ

Both GS and GOGAT are found in chloroplasts, although isoenzymes (multiple forms of an enzyme
with the same substrate specificity, but genetic differences in their primary structures) of both
enzymes may also be localized in the cytosol. Whatever the location of the enzymes, however, glu-
tamate must be exported from the chloroplast to the cytosol where transamination reaction (the
reversible transfer of an amino group of a specific amino acid to a specific keto acid, forming a
new keto acid and a new amino acid) can proceed, thereby facilitating the syntheses of other
amino acids.

ALGAE AND THE SILICON CYCLE
The biogeochemical cycle of silicon (Si) might be interpreted as those processes that link sources
and sinks of silicic acid [Si(OH) 4 ]. Silicic acid is the only precursor in the processing and deposition
of silicon in biota. The biogeochemical cycle of silicon does not facilitate a high biospheric abun-
dance of the element, in fact silicon cycle differs from the cycles of carbon, nitrogen, and sulfur and
it is similar to phosphorus in that there is no atmospheric reservoir. The silicon cycle, like those for
phosphorus and the divalent metals calcium and magnesium, has a significant abiotic drain. It actu-
ally consists of two parts: the terrestrial or freshwater cycle and the marine cycle, the former feeding
the latter. However, its replenishment can only occur via the marine sedimentary cycle. This is
dependent on geotectonic processes, such as mountain building and subduction, and, as such,