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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 Chlorophyta and Rhodophyta, or three separate branch, as is the case for Chromophyta (see Table 1.2). Each branch is itself divided into several classes. The number of classes may vary according to the criteria used by the different authors. This is particularly true for Rhodophyta, where complementary criteria to the pigmentary characteristics (nature of the starch, life cycle and reproduction, etc.) lead to the distinction of seven classes instead of one.

Table 1.1. Pigment distribution according to the botanical phyla of microalgae (from Morançais et al. (2018))

• - Chlorophylls c, e
• - Excess carotenoids
• (ß-carotene, fucoxanthin, zeaxanthin, etc.)

• - Chlorophyll d
• - R or B Phycoerythrin
• - Phycocyanin
• - Allophycyanin

1.1.2.1. Chlorophyta

The phylum of Chlorophyta is characterized by the simultaneous presence of chlorophylls a and b. This group comprises 6,429 species and is divided into 11 distinct classes (Sexton and Lomas 2018) (see Table 1.2 where only the main ones are mentioned):

• - the Chlorophyceae class with 3,653 species represents the main phylum taxonomic group. Microalgae belonging to the genera Chlamydomonas, Volvox and Dunaliella are the most representative members. The alga Dunaliella sp. is also valued in animal and human food (see sections 3.1 and 3.2);
• - the Ulvophyceae class mainly includes macroalgae and some microscopic unicellular forms (Sexton and Lomas 2018);
• - the Trebouxiophyceae class includes the genus Chlorella, a microalga valued in particular as a food supplement in human nutrition.

Table 1.2. Taxonomic distribution of algae according to the traditional classification enriched with the addition of secondary traits such as endosymbiotic origin plastid (based on Loir (2004) and Sexton and Lomas (2018))

• - Chlorophyceae (3,653)
• - Ulvophyceae (1,725)
• - Trebouxiophyceae (794)
• - Prasinophyceae (105)
• - Mamiellophyceae (18)

• - Bacillariophyceae (11,000-100,000) (diatoms)
• - Chrysophyceae (670)
• - Xanthophyceae (450-600)

1.1.2.2. Chromophyta

The Chromophyta phylum is divided into three phylum branches, each of which is subdivided into several distinct classes (see Table 1.2). In the branch - of Pheophycophyta - there is one class, that of Phaeophyceae. Phaeophyceae include 2,040 species and are known as brown algae. They do not include unicellular forms, except for the presence of biflagellated spores during the reproductive cycle of these algae, the best-known members of which belong to the genera Macrocystis, Laminaria and Fucus.

In the Chrysophycophyta branch, there are three well-classified classes. Among them, the class of Bacillariophyceae is one of the most studied for its biological diversity and ecological importance. This class, known as diatoms, includes unicellular brown-yellow algae with sizes ranging from 2 µm to 1 mm (Loir 2004). The number of species of diatoms is estimated to be at least 11,000, but some authors estimate the number to be around 100,000 (Mann and Vanormelingen 2013). Diatoms are morphologically characterized by the presence of an external envelope of siliceous nature called a frustule. This name is derived from the Latin frustulum, which means "piece" or "small end" (Round et al. 1990). The frustule is a shell composed of two valves called the epivalve and hypovalve (see Figure 1.3). This structural element has different shapes and symmetries in different species.

The frustule is responsible for the dichotomous separation of diatoms into two distinct groups. When the latter is in the form of a disc or tube with radial symmetry, the diatoms are referred to as centric or central diatoms (see Figure 1.4). On the other hand, diatoms with a more or less elongated frustule and mainly showing bilateral symmetry are classified as pennate or pennal diatoms (see Figure 1.5).

The species Haslea ostrearia involved in the greening mechanism of oysters (see section 3.1) belongs to the latter group (see Figure 1.6). Diatoms are photosynthetic organisms, but some species living in light-poor environments are heterotrophic for carbon. These diatoms incapable of synthesizing chlorophyll represent less than 10 species belonging to the genera Nitzschia and Hantzschia (Hantzschia achroma) (Li and Volcani 1987).

Figure 1.3. Frustule (epivalve and hypovalve) of the diatom Nitzschia sp. visualized by a scanning electron microscope (SEM) (photo credit © Gaudin P., 2010)

Figure 1.4. Example of a central diatom Skeletonema sp. visualized by a scanning electron microscope (SEM) (photo credit © Petit A., 2010)

Figure 1.5. Example of a pennate Haslea...