9.2 What are Algae?
Algae are eukaryotic organisms, which are organisms whose cells contain a nucleus and other structures (organelles) enclosed within membranes. They live in moist environments, mostly aquatic, and contain chlorophyll.
Algae are not terrestrial plants, which have 1) true roots, stems, and leaves, 2) vascular (conducting) tissues, such as xylem, and phloem, and 3) lack of non-reproductive cells in the reproductive structures. Algae are not cyanobacteria. Cyanobacteria are prokaryotes, which lack membrane-bound organelles and have a single circular chromosome. The figure below shows the cellular composition of blue-algae and the subsequent one shows a micrograph of the cells. The cell has a wall with a gelatinous coat. Just beneath the cell wall is a plasma membrane. Within the cell, there are layers of phycobilisomes, photosynthetic lamellae, ribosomes, protein granules, and circular DNA known as nucleoids. These are typical components of growing plants - however, the components we are interested in are lipid droplets, which are oils that can be extracted from the algae.
Algae is composed of ~ 50% carbon, 10% nitrogen, and 2% phosphorus. Table 10.3 shows the composition of various algae looking at the percentages of protein, carbohydrates, lipids, and nucleic acid.
|Scenedesmus obliquus (green alga)||50-56||10-17||12-14||3-6|
|Chlamydomonas rheinhardii (green alga)||48||17||21||-|
|Chlorella vulgaris (green alga)||51-58||12-17||14-22||4-5|
|Porphyridium cruentum (red alga)||28-39||40-57||9-14<||-|
So what are the characteristics of algae?
1. Eukaryotic organisms:
As mentioned above, algae are eukaryotic organisms. The structure of a eukaryote (a typical plant cell) is shown in the first figure below. The second figure below shows the cell structure of a prokaryote, a bacterium, one of two groups of prokaryotic life. Some do not consider the prokaryotes as true algae because they have a different structure, but most include these in the family of algae. There are labels for the different parts of the organisms, but I will not require you to know this information in detail - it is there so if you have a desire to look up more information, you can. The table shows a comparison of both these types of cells.
|--||Eukaryotic cells||Prokaryotic cells|
|Size||Fairly large in size||Very minute in size|
|Nuclear region||Nuclear materials surrounded by a membrane||The nuclear region (nucleoid) is not surrounded by a nuclear membrane|
|Chromosome||More than one chromosome present||Single chromosome present|
|Membrane||Membrane-bound cell organelles are present||Membrane-bound cell organelles are absent|
2. Live in moist environments
These organisms lack a waxy cuticle (the wax in terrestrial plants prevents water loss). There is a wide variety of growth environments for algae. The typical conditions for algae are moist, tropical regions and they can grow in marine and freshwater. Freshwater algae grow in animals, aquatic plants, farm dams, sewage, lakes, rivers, lagoons, snow, mud/sand, and soil.
3. Contain chlorophyll
Algae are mostly photosynthetic, like plants. They have five kinds of photosynthetic pigments (chlorophyll a, b, c, d, and f) and have many accessory pigments that are blue, red, brown, and gold. Chlorophyll is a green pigment found in almost all plant algae and cyanobacteria. It absorbs light and transfers light energy to ATP (adenosine triphosphate).
So how are algae classified?
Algae belong to the Protista kingdom. The figure below shows a schematic of where Protista fits with other classifications of Plantae, Animalia, Fungi, Eubacteria, and Archaebacteria.
Algae can also be classified based on chlorophyll content. The first type is Chromista. These types of algae contain chlorophylls a and c, and examples of the algae include brown algae (golden-brown algae), kelp, and diatoms. These materials are a division of Phaeophyta. These types have a habitat on rocky coasts in temperate zones or open seas (cold waters). The structure is multicellular and they can grow up to 50 m long.
Red algae are another type and contain chlorophyll a, such as marine algae (seaweed). These organisms are in the division of Rhodophyta, which has over 4000 species. These are some of the oldest eukaryotic organisms on Earth (there are 2 billion-year-old fossils). They are abundant in tropical, warm waters. They act as food and habitat for many marine species. The structure ranges from thin films to complex filamentous membranes. These algae have accessory pigments and the phycobilins (red) mask chlorophyll a. The second figure below shows various red algae. Dinoflagellates are unicellular protists, and these are associated with red tide and bioluminescence.
Green algae contain chlorophylls a and b. They are in the division Chlorophyta. This is the largest and most diverse group of algae. It is found mostly in freshwaters and also on land (rocks, trees, and soil). The structures are single cells (Micrasterias), filamentous algae, colonies (Volvox), and leaf-like shapes (Thalli). Terrestrial plants arose from a green algal ancestor. Both have the same photosynthetic pigments (chlorophyll a and b). Some green algae have a cell wall made of cellulose, similar to terrestrial plants.