Page 192 - Algae Anatomy, Biochemistry, and Biotechnology

Biogeochemical Role of Algae 175

rocks (e.g., limestone and dolomite). Carbon is also present in the mineral soil, in the bottom sedi-
ments of water bodies, in peat, bogs and mires, in the litter and humus, which contain 3000 gigatons
(0.005%) of the world’s carbon.
Carbon dioxide enters the ocean from the atmosphere because it is highly soluble in water; in
the sea, free dissolved CO 2 combines with water and ionizes to form bicarbonate and carbonate
ions, according to the following equilibrium:

þ
2

þ
CO 2 þ H 2 O !H 2 CO 3 !HCO 3 þ H !CO 3 þ H !HCO 3 !CO 2 þ OH (4:7)
These ions are bound forms of carbon dioxide, and they (especially bicarbonate) represent
by far the greatest proportion of dissolved carbon dioxide in seawater. On average, there
are about 45 ml of total CO 2 in 1 l of seawater, but because of the equilibrium of chemical
reactions, nearly all of this occurs as bound bicarbonate and carbonate ions which thus act
as a reservoir of free CO 2 . The amount of dissolved CO 2 occurring as gas in 1 l of seawater is
about 0.23 ml. When free CO 2 is removed by photosynthesis, the reaction shifts to the left and
the bound ionic forms release more free CO 2 ; so even when there is a lot of photosynthesis,
carbon dioxide is never a limiting factor to plant production. Conversely, when CO 2 is released
by the respiration of algae, plants, bacteria, and animals, more bicarbonate and carbonate ions
are produced.
According to the general chemical reactions presented earlier, the pH of seawater is largely
regulated by the concentrations of bicarbonate and carbonate, and the pH is usually 8+0.5. The
seawater acts as a buffered solution, because when CO 2 is added to seawater due to mineralization
processes and respiration, the number of hydrogen ions increases and the pH goes down (the
solution becomes more acidic). If CO 2 is removed from water by photosynthesis, the reverse
happens and the pH is elevated.
Some marine organisms combine calcium with carbonate ions in the process of calcification to
manufacture calcareous skeletal material. The calcium carbonate (CaCO 3 ) may either be in the
form of calcite or aragonite, the latter being a more soluble form. After death, this skeletal material
sinks and is either dissolved, in which case CO 2 is again released into the water, or it becomes
buried in sediments, in which case the bound CO 2 is removed from the carbon cycle. The
amount of CO 2 taken up in the carbonate skeletons of marine organisms has been, over geological
15
time, the largest mechanism for absorbing CO 2 . At present, it is estimated that about 50 10 tons
15 12
of CO 2 occurs as limestone, 12 10 tons in organic sediments, and 38 10 tons as dissolved
inorganic carbonate.
Calcification is not confined to a specific phylogenetically distinct group of organisms, but
evolved (apparently independently) several times in marine organisms. Carbonate sediments
blanket much of the Atlantic Basin, and are formed from the shells of both coccolithophorids
and foraminifera. As the crystal structures of the carbonates in both groups is calcite (as
opposed to the more diagenically susceptible aragonite), the preservation of these minerals and
their co-precipitating trace elements provides an invaluable record of ocean history. Although on
geological time scales, huge amounts of carbon are stored in the lithosphere as carbonates, on eco-
2þ
logical time scales, carbonate formation depletes the oceans of Ca , and in so doing, potentiates
the efflux of CO 2 from the oceans to the atmosphere. This calcification process can be summarized
by the following reaction:

Ca 2þ þ 2HCO !CaCO 3 þ CO 2 þ H 2 O (4:8)
3
Among the marine organisms responsible for calcification, coccolithophores play a major role,
especially Emiliania huxleyi. When the blooms of this Haptophyta appear over large expanses of
the ocean (white water phenomenon), myriad effects on the water and on the atmosphere above