4.2 Occurrence in Nature
69
cultivation of alginophytes in aquaculture systems have been introduced in recent
years, particularly in Asia (Charrier et al. 2017). Cultivation of some species of wild
seaweed has been restricted as a result of overexploitation. Other than being a source
of alginate and other phycocolloids, seaweed is a rich source of nutrients such as
proteins, minerals and vitamins, it is also the only non-fish source of omega-3 fatty
acid. It therefore has a high value as a food product for direct consumption by humans
and other animals.
Alginate provides seaweeds with the flexibility and strength needed to survive
at sea. Within the seaweed cell walls, alginates provide flexibility and mechanical
strength, needed by the algae to survive at sea, particularly where the water is particularly turbulent. They exist in the cell wall attached to metals such as magnesium,
calcium and sodium. The seaweed also contains cellulose which provides rigidity.
Alginate contents tend to be higher in seaweeds cultivated in more turbulent water
while seaweeds grown in calmer waters have lower alginate content. The properties
of the alginates extracted from seaweed such as viscosity and gel strength depend on
the structure of the alginate, and this in turn is affected by factors such as species of
seaweed, growth environment and the method of extraction. Alginate concentration
has also been shown to be affected by the period of harvest (Schiener et al. 2014;
Taelman et al. 2015).
While seaweeds naturally grow in aquatic environment, much of the world’s
seaweeds are grown on slow stagnant water with the right nutrients and environmental
conditions, to meet the growing demands for alginates and other products of brown
seaweed. Depending on the region and its demand, majority of the brown seaweed
cultivated is used for alginate while the rest is used for food. However, other statistics
show that in China, for example, most of the seaweed cultivated in this region is used
as food.
There are a variety of brown seaweed species which act as a source of alginates. These include Laminaria spp, Macrocystis pyrifera, Durvillaea antarctica,
Lessonia flavicans, Ecklonia maxima and Ascophyllum nodosum. The most commercially exploited ones are Laminaria, Macrocystis and Ascophyllum. Alginate is also
present as a protective extracellular structure of some bacteria such as Azotobacter
and pseudomonas (Aleksandra and Sanja 2017).
4.3 Chemistry of Alginates
4.3.1 Polymer Chain Structure
Alginates are linear ionic heteropolysaccharides copolymer of (1-4) linked beta-dmannuronic acid and alpha-l-guluronic acid arranged in block or alternating manner
along the polymer chain. Such that along the alginate polymer chain, some segments
of mannuronic acid, segments of guluronic acid and segments of mannuronic acids
and guluronic acid alternating exist. This is illustrated in Fig. 4.1.
69
cultivation of alginophytes in aquaculture systems have been introduced in recent
years, particularly in Asia (Charrier et al. 2017). Cultivation of some species of wild
seaweed has been restricted as a result of overexploitation. Other than being a source
of alginate and other phycocolloids, seaweed is a rich source of nutrients such as
proteins, minerals and vitamins, it is also the only non-fish source of omega-3 fatty
acid. It therefore has a high value as a food product for direct consumption by humans
and other animals.
Alginate provides seaweeds with the flexibility and strength needed to survive
at sea. Within the seaweed cell walls, alginates provide flexibility and mechanical
strength, needed by the algae to survive at sea, particularly where the water is particularly turbulent. They exist in the cell wall attached to metals such as magnesium,
calcium and sodium. The seaweed also contains cellulose which provides rigidity.
Alginate contents tend to be higher in seaweeds cultivated in more turbulent water
while seaweeds grown in calmer waters have lower alginate content. The properties
of the alginates extracted from seaweed such as viscosity and gel strength depend on
the structure of the alginate, and this in turn is affected by factors such as species of
seaweed, growth environment and the method of extraction. Alginate concentration
has also been shown to be affected by the period of harvest (Schiener et al. 2014;
Taelman et al. 2015).
While seaweeds naturally grow in aquatic environment, much of the world’s
seaweeds are grown on slow stagnant water with the right nutrients and environmental
conditions, to meet the growing demands for alginates and other products of brown
seaweed. Depending on the region and its demand, majority of the brown seaweed
cultivated is used for alginate while the rest is used for food. However, other statistics
show that in China, for example, most of the seaweed cultivated in this region is used
as food.
There are a variety of brown seaweed species which act as a source of alginates. These include Laminaria spp, Macrocystis pyrifera, Durvillaea antarctica,
Lessonia flavicans, Ecklonia maxima and Ascophyllum nodosum. The most commercially exploited ones are Laminaria, Macrocystis and Ascophyllum. Alginate is also
present as a protective extracellular structure of some bacteria such as Azotobacter
and pseudomonas (Aleksandra and Sanja 2017).
4.3 Chemistry of Alginates
4.3.1 Polymer Chain Structure
Alginates are linear ionic heteropolysaccharides copolymer of (1-4) linked beta-dmannuronic acid and alpha-l-guluronic acid arranged in block or alternating manner
along the polymer chain. Such that along the alginate polymer chain, some segments
of mannuronic acid, segments of guluronic acid and segments of mannuronic acids
and guluronic acid alternating exist. This is illustrated in Fig. 4.1.