Abstract
Algae encompass a phylogenetically diverse group; its members are placed in the domains Bacteria and Eukarya, in the latter dispersed in five supergroups. Contrary to common assumption, algae are found almost everywhere from aquatic to terrestrial habitats, some of them in symbiotic relationship, others thriving in harsh environments such as deserts, hot springs, or on ice. Around 45% of atmospheric oxygen is released by algae, which underlines their pivotal role in global element cycles. This high diversity is also reflected in different morphologies from microscopic single cells to parenchymatous organisms measuring several meters in size. The total species number is estimated from 70,000 to more than 10,000,000. Algae produce various valuable compounds, but this high natural potential is only marginally being used. To date, only a few taxa are commercially exploited.
TopDelimitation Of Algae
The term algae comprises organisms that release oxygen as a by-product of photosynthesis. Moreover, algae do not show the characteristic features of vascular plants – they are not differentiated into leaves, roots and shoots and do not develop vascular tissues.
More closely examining this definition reveals that algae do not have a common, unique feature. Algae are polyphyletic, they show diverse evolutionary lineages. Based on this definition, cyanobacteria are included in this group, which is understandable in that chloroplasts in all eukaryotic phyla originated from cyanobacteria. Moreover, cyanobacteria resemble eukaryotic algae; they share ecological niches and show similarities in morphology and physiology. Nonetheless, opinions differ in this respect because sometimes solely the eukaryotic forms are considered as algae. Another point deserves mention here: many heterotrophic taxa are included in this group, as they are closely related to their photoautotrophic counterparts. Prime examples for phagotrophy and osmotrophy are found in the phyla Euglenophyta and Dinophyta. Mixotrophic organisms are also represented, among them the chrysophyte genus Dinobryon, which switches between phagotrophy and autotrophy depending on the irradiance level (Caron et al., 1993). Mixotrophic cultivation is a promising method for generating high value products (Patel et al., 2021).
Key Terms in this Chapter
Uniseriate: Arranged in a single row.
Oogamy: Fusion of two gametes of different shape and size. The small gamete is mobile and treated as male. The large gamete is immobile and female.
Diplont: The diploid generation of the life cycle is multicellular, the only haploid stage is the gametes after meiosis.
Phagotrophy: Engulfment of particulate food.
Homotrichal: Filaments – sometimes branched – have the same morphology.
Heterokont Flagellation: The ultrastructure of flagella inserting in a flagellate differs from each other. In the Ochrophyta, an acronematic (naked) flagellum inserts next to a pleuronematic flagellum (covered by fine hairs = mastigonema).
Heteromorphic Life Cycle: Alternate generations show different morphology.
Colony: Group of cells held together by cell wall structures or mucilage. Usually more than one generation, variable cell number and no defined shape.
Multinucleate: Also called coenocytic. More than one nucleus per cell.
Heterotrichal: Filaments – sometimes branched – have different morphology (e.g., erect and creeping filaments, main axis and side branches, the latter with terminal hairs).
Form Resistance: Empirical variable to describe the frictional resistance of a body. A sphere has a form resistance of 1, the bigger the deviation from this shape, the larger the form resistance.
Heterocyte: Special type of cyanobacterial cell for dinitrogen fixation with unique features.
Anisogamy: Fusion of two gametes of different shape and size. Both gametes are motile. The larger gamete is treated as female, the smaller as male.
Osmotrophy: Uptake of dissolved organic substances as energy source.
Isomorphic Life Cycle: Alternate generations show same morphology.
Multiseriate: Arranged in more than a single row. Usually developing from uniseriate filaments.
Coenobnium: Group of cells held together by cell wall structures or mucilage. One generation, 2 n cell number and genetically defined shape.
Haplont: The haploid generation is multicellular, the only diploid stage is the zygote.
Seagrasses: Embryophytes (flowering plants) growing in marine habitats.
Haplo-Diplont: A haploid and a diploid multicellular generation alternates.
Seaweeds: General term for macroalgae growing in marine habitats.
Isokont Flagellation: The ultrastructure of flagella inserting in a flagellate does not differ from each other (although they may differ in length).
Isogamy: Fusion of two gametes which do not differ in size.