Page 178 - Algae Anatomy, Biochemistry, and Biotechnology

Biogeochemical Role of Algae 161

In general, growth rate of a population of organisms would be proportional to the uptake rate of
that one limiting factor. Nutrient-limited growth is usually modeled with a Monod (or Michaelis-
Menten) equation:

m max ½LNŠ
m ¼ (4:1)
½LNŠþ K m

where m is the specific growth rate of the population as a function of [LN]; [LN] is the concentration
of limiting nutrient; m max is the maximum population growth rate (at “optimal” conditions) and K m
is the Monod coefficient, also called the half-saturation coefficient because it corresponds to the
concentration at which m is one-half of its maximum. When the concentration of limiting nutrient
[LN] equals K m , the population growth rate is m max /2.
As [LN] increases, m increases and so the algal population (number of cells) increases. Beyond
a certain [LN], m tends asymptotically to its maximum (m max ), and the population tends to its
maximum yield. If this concentration is not maintained, rapidly primary productivity returns to a
level comparable to that prior to the nutrient enrichment. This productivity variation is the seasonal
blooming. Normal becomes abnormal when there is a continuous over-stimulation of the system by
excess supply of one or more limiting nutrients, which leads to intense and prolonged algal blooms
throughout the year. The continuous nutrient supply sustains a constant maximum algal growth rate
(m max ). Therefore, instead of peaks of normal blooms, followed by periods when phytoplankton is
less noticeable, we have a continuous primary production. When this occurs, we refer to it as eutro-
phication. In this process, the enhanced primary productivity triggers various physical, chemical,
and biological changes in autotroph and heterotroph communities, as well as changes in processes
in and on the bottom sediments and changes in the level of oxygen supply to surface water and
oxygen consumption in deep waters. Eutrophication is considered to be a natural aging process
for lakes and some estuaries, and it is one of the ways in which a water body (lake, rivers, and
seas) transforms from a state where nutrients are scarce (oligotrophic), through a slightly richer
phase (mesotrophic) to an enriched state (eutrophic).
Eutrophication can result in a series of undesirable effects. Excessive growth of planktonic
algae increases the amount of organic matter settling to the bottom. This may be enhanced by
changes in the species composition and functioning of the pelagic food web by stimulating the
growth of small flagellates rather than larger diatoms, which leads to lower grazing by copepods
and increased sedimentation. The increase in oxygen consumption in areas with stratified water
masses can lead to oxygen depletion and changes in community structure or death of the benthic
fauna. Bottom dwelling fish may either die or escape. Eutrophication can also promote the risk
of harmful algal blooms that may cause discoloration of the water, foam formation, death of
benthic fauna and wild or caged fish, or shellfish poisoning of humans. Increased growth and
dominance of fast growing filamentous macroalgae in shallow sheltered areas are yet another
effect of nutrient overload, which will change the coastal ecosystem, increase the risk of local
oxygen depletion, and reduce biodiversity and nurseries for fish.
Human activities can greatly accelerate eutrophication by increasing the rate at which nutrients
and organic substances enter aquatic ecosystems from their surrounding watersheds, for example
introducing in the water bodies detergents and fertilizers very rich in phosphorus. The resultant
aging, which occurs through anthropogenic activity, is termed cultural eutrophication.
Globally, nitrogen and phosphorus are the two elements that immediately limit, in a Liebig
sense, the growth of photosynthetic organisms. Silicon could also become a more generally limiting
nutrient, particularly for diatom growth. These nutrients are present in algal cells in a species-
specific structural ratio, the so-called Redfield ratio, which determines the nutrient requirement
of the species, and whose value depends on the conditions under which species grow and
compete. Consequently, the species composition of an environment will be determined not only