Part D | 24.3
602 Part D Marine Algal Biotechnology
cost, and its abundance in many parts of the world. Indeed, a comprehensive collection of algae for food and
algal phycocolloids (agar, carrageenans, alginates) is in
excess of 3 million tons per year, with a potential estimated at around 2:6 million collected for red algae and
16 million for brown algae [24.47].
The biosorbent ability of the macrophytes resides
in the structure of the cell walls of seaweeds and the
cuticle of seagrasses, which are responsible for this phenomenon. Indeed, in biosorption, the ability of active
sites on the surface of macrophytes to bind and concentrate heavy metals from even the most dilute aqueous
solutions is exploited. The cell wall intervenes in the
mechanisms of biosorption through physicochemical
processes like ion exchange, complexation, microprecipitation, and electrostatic interactions. Specifically, in
solution heavy metals can form ions (cations or anions), which create a passive binding with nonliving
biomass through a mechanism that is not metabolically controlled, in contrast to bioaccumulation by
living cells [24.48–50]. The use of dried nonliving
macrophytes as adsorbent material has considerable advantages, such as:
a) High efficiency in detoxification of diluted effluents
b) Minimizing the volume of waste, biological and/or
chemical
c) No demand for nutrients, and
d) Low cost.
An additional advantage is represented by the possible recovery of heavy metals: once the binding of
metals reaches saturation, the biomass can be regenerated with acid solutions and/or hydroxyl, which release
small volumes of heavy metal concentrates [24.51–53].
These may be subsequently treated with techniques of
co-precipitation, flocculation, and electro-deposition.
24.3.1 Cell Wall of the Algae
The cell wall is the portion of the algal cell directly involved in biosorption; its chemical structure has a major
effect on the mechanisms by which this phenomenon
takes place. The electrostatic attraction and the complexation of the groups present on the surface of the
cell wall with the metal in solution play a key role
in this regard [24.25]. The polysaccharides present in
the cell walls of algae contain chemical groups such
as carboxyl, hydroxyl, and sulfate, which may act as
binding sites [24.54]. They are effective ion exchangers and, therefore, important sites for complexation of
metal cations [24.55, 56]. Proteins, lipids and, occasionally, nucleic acids may also be present on the surface of
the cell walls. These molecules, however, occur mainly
in the plasma membrane and in the cytoplasm; they
are, therefore, bound to metal ions through their functional groups (aminic, carboxylic, imidazolic, thiolic,
thioesteric, nitrogen, and oxygen in peptidic bonds)
mostly within the cell [24.57]. In all algae and plants
the cell wall consists of a fibrillar fraction, which is responsible of the mechanical strength of the wall, and
an amorphous fraction, which provides flexibility. The
composition and relative amounts of these two fractions
vary in different taxonomic groups [24.6].
Chlorophyta (Green Algae)
The composition of the cell wall of the Ulvophyceae
shows some variation between different orders (in some
cases even between different life history stages in the
same species) [24.6]. In many groups, the fibrillar component forms an internal layer that lies directly against
the plasmalemma, whereas the amorphous portion is
predominantly on the outer side of the wall, forming
a slimy outer layer and containing relatively few and
randomly orientated microfibrils [24.6]. The fibrillar
layer consists of microfibrils with diameters varying between 3 and 35 nm. In many ulvophytes, the microfibrils consist of crystalline cellulose and are arranged in
numerous parallel layers; in orders with siphonous construction (Bryopsidales and Dasycladales), however,
the most abundant constituents are mannans, xylans,
and glucans. The structural component is embedded
in an amorphous, mucilaginous matrix, which also
varies in different orders. This may be formed by
acidic, partially sulfated heteropolysaccharides (as in
the case of the Ulvales) or highly sulfated arabinogalactans (e.g., in the Cladophorales) [24.6]. Polysaccharides
extracted from Ulva (order Ulvales) contain 16% sulfates and 1519% uronic acid [24.58]. Several genera
of siphonous ulvophytes (Acetabularia, Avrainvillea,
Penicillus, Halimeda, Udotea) are encrusted with calcium carbonate, which is deposited on the cell wall in
the form of aragonite crystals. Members of these genera
are among the most important contributors to calcium
carbonate deposition on tropical reefs.
Rhodophyta (Red Algae)
In the red algae the fibrillar fraction is constituted by
cellulose, which is deposited in the form of microfibrils
arranged to produce an irregular felt-like network [24.6,
59] immersed in the amorphous matrix. In some genera,
such as Bangia and Porphyra, cellulose is replaced by
xylans and mannans. The amorphous matrix forms up
602 Part D Marine Algal Biotechnology
cost, and its abundance in many parts of the world. Indeed, a comprehensive collection of algae for food and
algal phycocolloids (agar, carrageenans, alginates) is in
excess of 3 million tons per year, with a potential estimated at around 2:6 million collected for red algae and
16 million for brown algae [24.47].
The biosorbent ability of the macrophytes resides
in the structure of the cell walls of seaweeds and the
cuticle of seagrasses, which are responsible for this phenomenon. Indeed, in biosorption, the ability of active
sites on the surface of macrophytes to bind and concentrate heavy metals from even the most dilute aqueous
solutions is exploited. The cell wall intervenes in the
mechanisms of biosorption through physicochemical
processes like ion exchange, complexation, microprecipitation, and electrostatic interactions. Specifically, in
solution heavy metals can form ions (cations or anions), which create a passive binding with nonliving
biomass through a mechanism that is not metabolically controlled, in contrast to bioaccumulation by
living cells [24.48–50]. The use of dried nonliving
macrophytes as adsorbent material has considerable advantages, such as:
a) High efficiency in detoxification of diluted effluents
b) Minimizing the volume of waste, biological and/or
chemical
c) No demand for nutrients, and
d) Low cost.
An additional advantage is represented by the possible recovery of heavy metals: once the binding of
metals reaches saturation, the biomass can be regenerated with acid solutions and/or hydroxyl, which release
small volumes of heavy metal concentrates [24.51–53].
These may be subsequently treated with techniques of
co-precipitation, flocculation, and electro-deposition.
24.3.1 Cell Wall of the Algae
The cell wall is the portion of the algal cell directly involved in biosorption; its chemical structure has a major
effect on the mechanisms by which this phenomenon
takes place. The electrostatic attraction and the complexation of the groups present on the surface of the
cell wall with the metal in solution play a key role
in this regard [24.25]. The polysaccharides present in
the cell walls of algae contain chemical groups such
as carboxyl, hydroxyl, and sulfate, which may act as
binding sites [24.54]. They are effective ion exchangers and, therefore, important sites for complexation of
metal cations [24.55, 56]. Proteins, lipids and, occasionally, nucleic acids may also be present on the surface of
the cell walls. These molecules, however, occur mainly
in the plasma membrane and in the cytoplasm; they
are, therefore, bound to metal ions through their functional groups (aminic, carboxylic, imidazolic, thiolic,
thioesteric, nitrogen, and oxygen in peptidic bonds)
mostly within the cell [24.57]. In all algae and plants
the cell wall consists of a fibrillar fraction, which is responsible of the mechanical strength of the wall, and
an amorphous fraction, which provides flexibility. The
composition and relative amounts of these two fractions
vary in different taxonomic groups [24.6].
Chlorophyta (Green Algae)
The composition of the cell wall of the Ulvophyceae
shows some variation between different orders (in some
cases even between different life history stages in the
same species) [24.6]. In many groups, the fibrillar component forms an internal layer that lies directly against
the plasmalemma, whereas the amorphous portion is
predominantly on the outer side of the wall, forming
a slimy outer layer and containing relatively few and
randomly orientated microfibrils [24.6]. The fibrillar
layer consists of microfibrils with diameters varying between 3 and 35 nm. In many ulvophytes, the microfibrils consist of crystalline cellulose and are arranged in
numerous parallel layers; in orders with siphonous construction (Bryopsidales and Dasycladales), however,
the most abundant constituents are mannans, xylans,
and glucans. The structural component is embedded
in an amorphous, mucilaginous matrix, which also
varies in different orders. This may be formed by
acidic, partially sulfated heteropolysaccharides (as in
the case of the Ulvales) or highly sulfated arabinogalactans (e.g., in the Cladophorales) [24.6]. Polysaccharides
extracted from Ulva (order Ulvales) contain 16% sulfates and 1519% uronic acid [24.58]. Several genera
of siphonous ulvophytes (Acetabularia, Avrainvillea,
Penicillus, Halimeda, Udotea) are encrusted with calcium carbonate, which is deposited on the cell wall in
the form of aragonite crystals. Members of these genera
are among the most important contributors to calcium
carbonate deposition on tropical reefs.
Rhodophyta (Red Algae)
In the red algae the fibrillar fraction is constituted by
cellulose, which is deposited in the form of microfibrils
arranged to produce an irregular felt-like network [24.6,
59] immersed in the amorphous matrix. In some genera,
such as Bangia and Porphyra, cellulose is replaced by
xylans and mannans. The amorphous matrix forms up