Page 186 - Algae Anatomy, Biochemistry, and Biotechnology

Biogeochemical Role of Algae 169

will incur delays of tens to hundreds of millions years before marine silicon is returned to the
terrestrial environment.
The substantial losses of biospheric silicic acid to abiotic sinks may be compensated for in
nature by its overall abundance in the Earth’s crust. It is the second most abundant element in
the lithosphere (28%), the iron being the first one (35%). It is found in the Earth’s crust in silicate
minerals; the most prevalent of which are quartz, the alkali feldspars, and plagioclase. The latter
two minerals are aluminosilicates and contribute significantly to the aluminium content of the
crust. All of these minerals are broken down by the process of weathering. Important feedbacks
exist between autotrophs (algae and plants), weathering, and CO 2 .
The dominant form of weathering is the carbonation reaction involving carbonic acid (H 2 CO 3 ),
which results in enhanced removal of CO 2 from the atmosphere, because the net effect of silicate
mineral weathering is to convert soil carbon, derived ultimately from photosynthesis, into dissolved
2
HCO 3 . On a geological timescale, this transfer is an important control on the CO 2 content of the
atmosphere and hence the global climate. Weathering is a complex function of rainfall, runoff,
lithology, temperature, topography, vegetation, and magnitudes. Algae, plants, and their associate
microbiota directly affect silicate mineral weathering in several ways: by the generation of organic
substances, known as chelates, that have the ability to decompose minerals and rocks by the
removal of metallic cations; by modifying pH through the production of CO 2 or organic acids
such as acetic, citric, phenolic, etc., and by altering the physical properties of the soil, particularly
the exposed surface areas of minerals and the residence time of water. The significance of this
natural process for biota can be found in the detailed geochemistry of the weathering reactions
and, in particular, in the rates at which these reactions occur. The rate of mineral weathering is
dependent on a number of factors including the temperature, pH, ionic composition of the
solvent (or leachate), and hydrogeological parameters such as water flow.
Silicification occurs in three clades of photosynthetic heterokonts: Chrysophyceae (Parmales),
Bacillariophyceae, and Dictyochophyceae, with diatoms being the world’s largest contributors to
biosilicification. Because amorphous silica is an essential component of the diatom cell wall,
silicon availability is a key factor in the regulation of diatom growth in nature; in turn, the use
of silicon by diatoms dominates the biogeochemical cycling of silicon in the sea, with each
atom of silicon weathered from land passing through a diatom on an average of 39 times before
burial in the sea bed.
Several thousand million years ago little if any of the life on Earth was involved in the proces-
sing of silicic acid to amorphous silica (SiO 2 nH 2 O). The concentration of silicic acid in the

aqueous environment was high, of the order of millimolar, and reflected equilibration according
to the dominant mineral weathering reactions at that time. The prevalence of these environments
rich in silicic acid is indicated in the fossil record by evidence of blue-green algae found
encased in silica cherts. It is important to recognize that implicit in this observation is the accep-
tance that early biochemical evolution proceeded within environments that, relative to the con-
ditions which prevail today, were extremely rich in silicic acid. Concomitant with the advent of
dioxygen, and its subsequent gradual increase in atmospheric concentration from approximately
1% towards the level of 21% which is characteristic of today, an increasing number of organisms
occurred within which silicic acid was processed to silica. The most important of these, in the
terms of their diversity, ubiquity (both freshwater and marine species) and biomass, were the
diatoms. The diatoms are characterized by a silica frustule that surrounds their cell wall. Silicic
acid is freely diffusible across the cell walls and membranes and, in most cell types of most
organisms, the intracellular concentration of silicic acid equilibrates with the extracellular
environment according to a Donnan equilibrium (the equilibrium characterized by an unequal
distribution of diffusible ions between two ionic solutions separated by a membrane, which is
impermeable to at least one of the ionic species present). However, while the intracellular concen-
tration of the silicic acid in the diatom has not been measured it is likely that it is under kinetic as
opposed to thermodynamic control and that it is maintained at an extremely low level, probably less