Page 177 - Algae

160 Algae: Anatomy, Biochemistry, and Biotechnology

to kerogens, a source of petroleum under appropriate geochemical condition. Moreover, we are still
using the remains of calcareous microorganisms, deposited over millions of years in ancient ocean
basins, for building material. Diatomaceous oozes are mined as additives for reflective paints,
polishing materials, abrasives, and for insulation. The fossil organic carbon, skeletal remains,
and oxygen are the cumulative remains of algae export production that has occurred uninterrupted
for over 3 billions years in the upper ocean.
In total, 99.9% of the biomass of algae is accounted for by six major elements such as carbon
(C), oxygen (O), hydrogen (H), nitrogen (N), sulfur (S) and phosphorus (P), plus calcium (Ca),
potassium (K), sodium (Na), chlorine (Cl), magnesium (Mg), iron (Fe), and silicon (Si). The
remaining elements occur chiefly as trace elements, because they are needed only in catalytic
quantities.
All elements that become incorporated in organic material are eventually recycled, but on
different time scales. The process of transforming organic materials back to inorganic forms of
elements is generally referred to as mineralization. It takes place throughout the water column
as well as on the bottom of water bodies (lakes, streams, and seas), where much of the detrital
material from overlying waters eventually accumulates. Recycling of minerals may take place rela-
tively rapidly (within a season) in the euphotic zone (i.e., the portion of water column supporting
net primary production) or much more slowly (over geological time) in the case of refractory
materials which sink and accumulate on the seabed. In the water column, where there is usually
plenty of oxygen, decomposition of organic material takes place via oxidative degradation
through the action of heterotrophic bacteria. Carbon dioxide and nutrients are returned for reutili-
zation by the phytoplankton. Ecologically, the most important aspect of recycling in the water
bodies is the rate at which growth-limiting nutrients are recycled. Among the nutrients that are
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in short supply, nitrate (NO 3 ), iron (bioavailable Fe), phosphate (PO 4 ), and dissolved silicon
[Si(OH) 4 ] are most often found in a concentration well below the half-saturation levels required
for maximum phytoplankton growth. In particular, algae are important for the biogeochemical
cycling of the chemical elements they uptake, assimilate, and produce, such as carbon, oxygen,
nitrogen, phosphorus, silicon, and sulfur. We will now briefly consider some aspects of these
elements in relation to biosynthesis and photosynthesis.

LIMITING NUTRIENTS
Among the elements required for algal growth, there are some that can become limiting. The orig-
inal notion of limitation was introduced by von Liebig more than a century ago to establish a cor-
relation between the yield of a crop and the elemental composition of the substrate required for the
synthesis of that crop. Von Liebig stated that if one crop nutrient is missing or deficient, plant
growth will be poor, even if the other elements are abundant. That nutrient will be defined “limiting
nutrient.” This concept is known as Liebig’s “Law of the Minimum.” Simply stated, Liebig’s law
means that growth is not controlled by the total of nutrients available but by the nutrient available in
the smallest quantity with respect to the requirements of the plant. Liebig likens the potential of a
crop to a barrel with staves of unequal length. The capacity of this barrel is limited by the length of
the shortest stave and can only be increased by lengthening that stave. When that stave is length-
ened, another one becomes the limiting factor.
The concentration of a nutrient will give some indication whether the nutrient is limiting, but
the nutrient’s supply rate or turnover time is more important in determining the magnitude or degree
of limitation. For example, if the concentration of a nutrient is limiting, but the supply rate is
slightly less than the uptake rate by the algae, then the algae will only be slightly nutrient-
limited. Not all the algae are limited by the same nutrient, but it occurs at the species level, for
example, all the diatoms are limited by silicate. Moreover, there is a considerable variation in
the degree, kind, and seasonality of nutrient limitation, which is related to variations in riverine
input, but also to conditions and weather in the outflow area.