Page 191 - Algae Anatomy, Biochemistry, and Biotechnology

174 Algae: Anatomy, Biochemistry, and Biotechnology

sunlight that reaches the Earth’s surface leads to the consequent reduction of global temperature.
This drop in temperature is suggested to cause a decrease in the primary production of DMSP
(in other words DMS). Thus, DMS is considered to be counteractive to the behavior of greenhouse
gases like CO 2 and CH 4 . Removal of the algae-DMS-derived sulfur from the air by rain and depo-
sition of particles is a significant source of this biologically important element for some terrestrial
ecosystems.

ALGAE AND THE OXYGEN/CARBON CYCLES
The oxygen and carbon cycles are closely related, because they are directly associated with photo-
synthesis and respiration processes.
The natural oxygen cycle is determined by the aerobic respiration of glucose (taking place in all
living organisms), which consumes oxygen in free form (O 2 ) using it as electron sink and produces
carbon dioxide and water, and by photosynthesis, which consumes carbon dioxide and water to
produce carbohydrates and molecular oxygen as a by-product, as we have seen in Chapter 3.
Today oxygen constitutes about 21% of the atmosphere, 85.8% of the ocean, and 46.7% by
volume of the Earth’s crust. It has not always been like that. The primordial atmosphere of the
Earth is thought to have contained mainly CO 2 ,N 2 ,H 2 O, and CO with traces of H 2 , HCN, H 2 S,
and NH 3 , but to have been devoid of O 2 (only small amounts were derived from the photolysis
of water), thus being neutral to mildly reducing. Today, the atmosphere contains 78% N 2 , 21%
O 2 , and 0.036% CO 2 by volume, and is strongly oxidizing. All of the molecular oxygen present
in the Earth’s atmosphere has been produced as the result of oxygenic photosynthesis, the
source of the original O 2 being photosynthetic activity in the primordial oceans. The development
of aquatic photosynthesis coincided with a long and reasonably steady drawdown of atmospheric
CO 2 , from concentration approximately 100-fold higher than in the present-day atmosphere to
approximately half of the present levels. This drawdown was accompanied by a simultaneous evol-
ution of oxygen from nil to approximately 21%, comparable to that of the present day. The current
atmospheric oxygen concentration is maintained in equilibrium between the production by photo-
synthesis and the consumption by respiration, with annual fluctuations of +0.002%. Over geologi-
cal time scales, the drawdown of CO 2 was not stoichiometrically proportional to the accumulation
of O 2 because photosynthesis and respiration are but two of the many biological and chemical
processes that affect the atmospheric concentrations of these two gases. The removal rate of
CO 2 from the atmosphere by photosynthesis on land is about 60 gigatons C yr 21 , worldwide.
The concentration of oxygen in the oceans (85.8%) is influenced horizontally and vertically by
physical features such as the thermocline (i.e., a layer in a large body of water, such as a lake,
that sharply separates regions differing in temperature), which isolates deep water from exchange
with the atmosphere and can be a zone of significant decomposition causing an oxygen minimum.
Oxygen is only sparingly soluble in water (oxygen solubility is inversely proportional to the temp-
4
erature) and diffuses about 10 times more slowly in water than air. Deep water masses are pro-
duced at the sea surface in the polar zones where cooling gives rise to increased gas solubility
and convection currents. These waters remain largely intact and move through the ocean basins
with their oxygen concentration decreasing with time due to the decomposition of organic matter.
Carbon, the key element of all life on Earth, has a complex global cycle that involves both phys-
ical and biological processes, made up of carbon flows passing back and forth among four main
natural reservoirs of stored carbon: the atmosphere, storing 735 gigatons (0.001%) of the
world’s carbon as carbon dioxide (CO 2 ), carbon monoxide (CO), methane (CH 4 ), longer chain
volatile hydrocarbons, and halogen compounds (CFC and HCFC compounds); living organisms,
storing 8000 gigatons (0.001%) of the world’s carbon as compounds like fats, carbohydrates,
and proteins; the hydrosphere, storing 39,000 gigatons (0.06%) of the world’s carbon, as dissolved
carbon dioxide; the lithosphere, storing 1000 gigatons (0.002%) of the world’s carbon in the form of
fossils (e.g., oil, natural gas, lignite, and coal), and 62,000,000 gigatons (99.9%) in sedimentary