1 Algae

Algae have been used in animal and human diets since very early times. Filamentous algae are usually considered as ‘macrophytes’ since they often form floating masses that can be easily harvested, although many consist of microscopic, individual filaments of algal cells. Algae also form a component of periphyton, which not only provides natural food for fish and other aquatic animals but is actively promoted by fishers and aquaculturists as a means of increasing productivity.

This topic is not dealt with in this section, since periphyton is not solely comprised of algae and certainly cannot be regarded as macroalgae. However, some ancillary information on this topic is provided in Annex 2 to assist with further reading. Marine ‘seaweeds’ are macro-algae that have defined and characteristic structures.
Microalgal biotechnology only really began to develop in the middle of the last century but it has numerous commercial applications. Algal products can be used to enhance the nutritional value of food and animal feed owing to their chemical composition; they play a crucial role in aquaculture. Macroscopic marine algae (seaweeds) for human consumption, especially nori (Porphyra spp.), wakame (Undaria pinnatifida), and kombu (Laminaria japonica), are widely cultivated algal crops. The most widespread application of microalgal culture has been in artificial food chains supporting the husbandry of marine animals, including finfish, crustaceans, and molluscs.

The culture of seaweed is a growing worldwide industry, producing 14.5 million tonnes (wet weight) worth US$7.54 billion in 2007 (FAO, 2009). The use of aquatic macrophytes in treating sewage effluents has also shown potential. In recent years, macroalgae have been increasingly used as animal fodder supplements and for the production of alginate, which is used as a binder in feeds for farm animals. Laboratory investigations have also been carried out to evaluate both algae and macroalgae as possible alternative protein sources for farmed fish because of their high protein content and productivity.
Microalgae and macroalgae are also used as components in polyculture systems and in remediation; although these topics are not covered in this paper, information on bioremediation is contained in many publications, including Msuya and Neori (2002), Zhou et al. (2006) and Marinho-Soriano (2007). Red seaweed (Gracilaria spp.) and green seaweed (Ulva spp.) have been found to suitable species for bioremediation. The use of algae in integrated aquaculture has also been recently reviewed by Turan (2009).

 

1.1 Classification

The classification of algae is complex and somewhat controversial, especially concerning the blue-green algae (Cyanobacteria), which are sometimes known as blue-green bacteria or Cyanophyta and sometimes included in the Chlorophyta. These topics are not covered in detail this document. However, the following provides a taxonomical outline of algae.
Archaeplastida
• Chlorophyta (green algae)
• Rhodophyta (red algae)
• Glaucophyta
Rhizaria, Excavata
• Chlorarachniophytes
4 Use of algae and aquatic macrophytes as feed in small-scale aquaculture – A review
• Euglenids
Chromista, Alveolata
• Heterokonts
• Bacillariophyceae (diatoms)
• Axodine
• Bolidomonas
• Eustigmatophyceae
• Phaeophyceae (brown algae)
• Chrysophyceae (golden algae)
• Raphidophyceae
• Synurophyceae
• Xanthophyceae (yellow-green algae)
• Cryptophyta
• Dinoflagellates
• Haptophyta
The following sections discuss the characteristics and use of both ‘true’ algae and the Cyanophyta, hereinafter referred to as ‘blue-green algae’).