Review of Literature
2.4 Microalgae: classification and biology
plant systems. Thus, the net generation of CO2 will equal the net accumulation of CO2 in algae leading to a carbon neutral process and at higher efficiencies of CO2 sequestration the process is expected to be carbon negative in which the net generation from burning of fuel will be lesser than the accumulation (Taylor et al., 2013). These properties of algae are providing chances for the algal biomass to be a viable third generation feedstock for liquid transportation fuels (Subhadra, 2010). Therefore, microalgae are believed to be the only potential feedstock that can fulfil the current energy requirements (Chisti, 2007; Mata et al., 2010). But several bottlenecks remains ahead to develop a sustainable process for feasible biodiesel production in large quantities and there is still a lot to learn about these primitive and diverse group of species.
Fig. 2.5 Various value added products and biofuels (shown in red) generated from microalgal biomass
macroscopic multicellular loose or filmy conglomerations, matted or branched colonies and more complex leafy or blade forms categorized as macroalgae. Microalgae measures about 0.2 to 50 µm length in diameter while macroalgae can grow in to giant kelps which may measure around 60 m in length (McHugh, 2003). Around 1 to 10 million species of algae exists in earth among which 40,000 species were identified (Hu et al., 2008) and many remaining unexploited. Unlike plants, these organisms do not show remarkable variations in their vegetative structures such as roots, stems, leaves, vascular structures and complex sex organs.
Classification of algae is a difficult task with more than 20 classes of algae have been described since the introduction of classification systems by Linnaeus. Many approaches were used to classify these complex systems at higher levels, which may be broadly considered under the morphological concept (organization in vegetative states), ultra-structural concept (basal body orientations in flagellated cells) and molecular concept (smaller and larger subunit Ribosomal DNA, 5.8S, including internal transcribed spacers ITS-1 and ITS-2, chloroplast and mitochondrial genes) (Bordie and Lewis, 2007).
Prokaryotic members of this assemblage are grouped into two divisions: Cyanophyta and Glaucocystophyta (Croft et al., 2006), whereas eukaryotic members are grouped into nine divisions: Glaucophyta, Rhodophyta, Heterokontophyta, Haptophyta, Cryptophyta, Dinophyta, Euglenophyta, Chlorarachinophyta and Chlorophyta (Fig. 2.6). The most primitive, prokaryotic photosynthetic cyanobacteria are believed to fill the earth with oxygen and removing all the toxic gases in earth’s atmosphere. These cyanobacteria colonise in wide habitats which include salty oceans, hot springs, mountains, and glaciers while they multiple through fission process and no sexual reproduction was reported (Barsanti and Gualtieri, 2006). In general, most of the cyanobacteria are photoautotrophs with few growing under heterotrophic and/or mixotrophic conditions (Alagesan et al.,
2013). One of the important inherent properties of these organisms is to form the heterocyst which are the non-photosynthetic nitrogen fixing cells that supplies nitrogen to all growing photosynthetic cells in the system. The eukaryotic algal strains were believed to evolve through endosymbiosis that exist between the prokaryotic cyanobacteria in a non- photosynthetic eukaryote which generated three major group of algae glaucocystophyta, chlorophyta (green algae) and rhodophyta (red algae) as shown in Fig. 2.6.
Fig. 2.6 Classification of algal systems based on the evolution. Three major groups of algae glaucocystophyta, chlorophyta and rhodophyta generated from primary endosymbiosis of a prokaryotic cyanobacteria with a non-photosynthetic eukaryote. These groups are further divided in to several via secondary endosymbiosis (details obtained and modified from Croft et al., 2006). The group haptohyta, cryptophyta and heterokontophyta undergone tertiary symbiosis to form other species which are not shown
Chlorophyta is the largest group which are well characterized and includes many common species such as Chlorella, Dunaliella, Hematococcus, Chlamydomonas, Tetraselmis and Scenedesmus. The other group members are generated by repeated endosymbiotic relationship that existed among these variants (Croft et al., 2006).
Haptophyta are yellow, green or brown in color due to xanthophylls and the heterokontophyta includes brown algae (Phaeophyta), yellow algae (Xanthophyta), golden algae (Chrysophyta) and diatoms (Bacillariophyta). The algal communities which are actively involved in the accumulation of lipids are:
(i) Diatoms (Bacillariophyceae) which stores carbon in the form of natural oil or as a polymer of carbohydrate (Matsumoto et al., 2010). These diatoms rich in lipid eventually degraded to form the conventional fossil fuel resources as per the literatures (Ramachandra et al., 2009).
(ii) Green algae (Trebuxiophyceae, Chlorophyceae) are commonly found in fresh water and brackish water with very few in marine habitat (Matsumoto et al., 2010). They store their energy in the form of starch and they store neutral lipids under certain stress/growth conditions (Illman et al., 2000). The most widely studied oleaginous microalgal species belongs to the group Chlorophyta (green algae).
(iii) Golden algae (Chrysophyceae) appears yellow, orange or brown in color and produce natural oil and carbohydrates as storage compounds.
On basis of the polyphyletic nature, microalgae show a distinct, complex biology and physiology. Fig. 2.7 shows the generalized structure of a unicellular prokaryotic cyanobacteria and eukaryotic green algal cell. The prokaryotic blue-green algae is characterized by the absence of intracellular organelles and prominent nuclear membrane while, the thylakoid membranes are arranged as a network in the peripheral region of the cell with phycobilisomes on their surface for light harvesting (Fig. 2.7A). This arrangement of thylakoids in cyanobacteria is termed as chromatoplast. The starch granules and lipid bodies are found all over the cytoplasm. The cell wall of cyanobacteria is also covered by an extracellular mucilage layer which is absent in many other higher algae. On the other hand, in eukaryotic algae the thylakoid membranes are stacked in a network resembling the plant thylakoids along with a short chloroplast DNA in it (Fig. 2.7B). The eukaryotic cells comprise complex intracellular organelles such as endoplasmic reticulum, golgi bodies and mitochondria for their effective functioning. The starch granules are observed in the chloroplasts while the lipid bodies are observed all over the cytoplasm. The cell wall of
eukaryotic cells may or may not have mucilage layers in their extracellular matrix and it varies with species to species.
Fig. 2.7 Generalized structural morphology of an unicellular (A) prokaryotic blue-green algae and (B) eukaryotic green algae (Adopted and modified from Barsanti and Gualtieri, 2006)
Reproduction in microalgae differs with different species and it may be vegetative by the division of a single cell or fragmentation of a colony or asexual by the production of motile spore or sexual by the union of gametes (Barsanti and Gualtieri, 2006). The common oleaginous green algae Chlorella sp. (Chlorophyta) and Nannochloropsis
(Heterokontophyta) reproduces asexually by autosporulation. The mother cells forms four daughter cells with separate cell walls and allows them to mature inside. After maturation, the daughter cells are liberated by rupture of mother’s cell wall and the residual debris of the mother’s cell is consumed by the daughter cells (Safi et al., 2014). The other modes of asexual reproduction involve binary fission in which the mother cell divides in to two equal parts with same nucleic acid contents to form two daughter cells. In Chlamydomonas sp. the multiplication takes place through the formation of flagellate motile spores termed as zoospores inside the vegetative mother cell. Distinct gametes with haploid genome are also observed in many algal species which is the characteristic feature in sexual reproduction.
Detailed biology of all the group members is available in Barsanti and Gualtieri (2006).