Microalgae

From Future Food to Cellular Factory

Jöel Fleurence

• Publisher: Wiley-ISTE
• Year: 2021
• ISBN:
  9781119854920
• Edition: 1
• Topic: Scienze biologiche
• Subtopic: Ecologia

Microalgae and cyanobacteria are the first organisms in the oceanic food chain and are essential producers of oxygen and effective carbon dioxide traps. They are traditional sources of food proteins for Aztec, African and Asian populations, and some of them have even acquired the status of superfoods. Microalgae reviews the biological, ecological and biochemical characteristics of microalgae and cyanobacteria. They are true cellular factories, producing substances of interest such as original pigments, proteins and polysaccharides with biological activities. Their use covers many sectors of human activity including aquaculture, livestock breeding, agri-food, and human and veterinary medicine. This book presents their mode of production and the transformation processes that are applied to them, as well as the traditional and future valorization of algae. As they are a source of lipids and fatty acids, microalgae have become the focus of attention for the development of green fuels, such as biofuel.

1
Biology and Ecology of Microalgae

1.1. Biological characteristics

1.1.1. General characteristics

Microalgae are mainly photo-synthetic single-celled organisms living in aquatic environments (marine, brackish, fresh water) or humid or aerial terrestrial environments (atmosphere, soils, trees, building facades, etc.) (Sharma et al. 2006). They can also associate together to form colonies or undifferentiated multicellular organisms. The morphology and size of microalgae vary greatly according to species and taxonomic groups.

Microalgae are eukaryotic organisms that possess the main characteristics of the vegetable eukaryotic cell. They can be flagellated as in the case of algae belonging to the genus Chlamydomonas (see Figure 1.1) or not (see Figure 1.2). As eukaryotes, microalgae can be distinguished from cyanobacteria, which are prokaryotic organisms with a long life span and are called “blue-green algae” or Cyanophyceae. Spirulina (Arthrospira sp.), a well-known representative of this group, is, therefore, a photosynthetic bacterium and not an alga, as is often stated in commercial communication (Fleurence 2018).

Microalgae are also characterized by a great morphological diversity. This is the case of diatoms, which are distinguished from other algae by the presence of a siliceous shell called a frustule and whose architecture differs according to the species considered (see Figure 1.3) (Loir 2004). This unique morphological feature is the biological signature of diatoms, which are also known as “siliceous algae”. This morphological heterogeneity is also found in size since microalgae generally vary in size from less than 1 μm to 1 mm. The smallest size, 0.8 μm, is observed for the marine species Ostreococcus tauri (Borowitzka 2018a).

Figure 1.1. Cellular organization of the microalga Chlamydomonas sp. (source: Pouchus Y.-F.). For a color version of this figure, see www.iste.co.uk/fleurence/microalgae

Figure 1.2. Cellular organization of the microalga Chlorella sp. (source: Pouchus Y.-F.). For a color version of this figure, see www.iste.co.uk/fleurence/microalgae

1.1.2. The different groups in traditional and phylogenetic classification

Like all organisms, microalgae are classified into several taxonomic groups according to traditional or phylogenetic systematics. The traditional systematics establishes botanical phyla based on the pigment position of the microalgae. The pigment criterion, according to its nature, is an agglomerating or discriminating character. Thus, all algae, micro- or macroalgae, have a common pigment: chlorophyll a. This pigment is also present in cyanobacteria and terrestrial plants. The presence of additional pigments, called supernumerary pigments, is used as a distinguishing criterion to define the main algal phyla (see Table 1.1). These phyla, of which there are three, are, respectively, the phylum of Chlorophyta (green algae), Chromophyta (golden-brown algae) and Rhodophyta (red algae). Each phylum consists of a single phylum branch, as is the case for...