Page 187 - Algae Anatomy, Biochemistry, and Biotechnology

170 Algae: Anatomy, Biochemistry, and Biotechnology

than 1 mmol dm 23 . This kinetic control of the intracellular silicic acid concentration may be
achieved through the condensation and polymerization of silicic acid in a number of chemical
(e.g., pH controlled) and physical (e.g., membrane-bound) compartments eventually resulting in
amorphous silica. This biogenic silica is then deposited in a controlled manner to form the intricate
and elaborate silica frustules. How all of these remarkable feats of chemistry are achieved within
the diatom remains largely unknown. However, what is known, and is becoming more apparent, is
the formative role played by diatoms and other silica-forming organisms, such as silicoflagellates,
radiolarian, and sponges, in the biogeochemical cycle of silicon.
The reactions of condensation of silicic acid and subsequent polymerization to form biogenic
silica eventually (i.e., upon the death of the organism) result in a net loss of silicic acid to the bio-
sphere. The rate of the forward reaction (condensation and polymerization) is several orders of
magnitude higher than that of the reverse (regeneration of silicic acid pool) with the result that con-
comitant with the rise of the diatoms and other silica-forming organisms was a significant reduction
in the environmental silicic acid concentration. Silica frustules are formed in a matter of hours to
days whereas the rate at which silicic acid is returned to the biosphere through the dissolution of the
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frustules of dead diatoms as they sink in the water column is of the order of 10 mmol m sec .
In addition, the dissolution of these sinking frustules can be greatly influenced by the chemistry of
the water column. For example, the frustule is a highly adsorptive surface and is implicated in the
removal of metal ions, for example, aluminium, from the water column. These adsorptive processes
tend to stabilize the frustule surface towards dissolution and thereby reduce the amount of silicic
acid returned to the biosphere during sedimentation. Once the silica frustules have settled to the
bottom their silica enters the sedimentary cycle whereupon it is unlikely to reappear in the bio-
sphere for tens of millions of years.
The biologically induced dramatic decline in the environmental silicic acid concentration had
the effect of accelerating the rate of mineral weathering. This, in turn, consumed more carbon
dioxide and precipitated a gradual reduction in the atmospheric concentration of this “greenhouse”
gas. The impact of the emergence of diatoms and other silica-forming organisms, and latterly the
spread of rooted vascular plants, on the biogeochemical cycle of silicon contributed significantly to
the global cooling, which has resulted in the climate of today. The diatoms, in particular, are extre-
mely successful organisms and will continue to deposit silica frustules of varying silica content at
micromolar concentrations of environmental silicic acid. In this way they are a continuous accel-
erant of mineral weathering almost regardless of how low the environmental silicic acid concen-
tration may fall. From the advent of the silica-forming organisms the process of biochemical
evolution has continued in silicon-replete, though no longer of millimolar concentration, environ-
ments. Diatoms in sedimentary deposits of marine and continental, especially lacustrine, origin
belong to different geologic ranges and physiographic environments. Marine diatoms range in
age from Early Cretaceous to Holocene, and continental diatoms range in age from Eocene to
Holocene; however, most commercial diatomites, both marine and lacustrine, were deposited
during the Miocene. Marine deposits of commercial value generally accumulated along continental
margins with submerged coastal basins and shelves where wind-driven boundary currents provided
the nutrient-rich upwelling conditions capable of supporting a productive diatom habitat. Commer-
cial freshwater diatomite deposits occur in volcanic terrains associated with events that formed
sediment-starved drainage basins. Marine habitats generally are characterized by stable conditions
of temperature, salinity, pH, nutrients, and water currents, in contrast to lacustrine habitats, which
are characterized by wide variations in these conditions. Marine deposits generally are of higher
quality and contain larger resources, owing to their greater areal extent and thickness, whereas
most of the world’s known diatomites are of lacustrine origin.
Unlike many other algae, whose division cycles are strongly coupled to the diel light cycle,
diatoms are capable of dividing at any point of the diel cycle. This light independence extends
to their nutrient requirements, with nitrate and silicic acid uptake and storage continuing during
the night through the use of excess organic carbon synthesized during the day. Moreover, the