Definition, Groups and Classification
Definition: What is Phycology?
For this reason, it has become increasingly important to learn about these organisms and the role they play in their ecology. In doing so, it has also become possible to learn the impact of algae on life in general. This is particularly due to the fact that apart from aquatic environments, algae can be found in almost all environments across the globe.
* The word "Phycology" comes from the Greek word "Phykos" which means seaweed.
* Cryptogamic plants and consequently, flowering plants originated from algae (prokaryotic algae).
The development of phycology (as a field of study) may be divided into four major phases that include:
- Phase 1 - Greek writings of Theophrastus and Pedanius Dioscorides - This phase came to an end in the 18th Century
- Phase 2 - 1800 to 1880 - Western sciences
- Phase 3 - Early 1950
- Phase 4- This is the modern phase of phycology that started in the 1950s
The classification of algae has proved to be a complex process that has experienced several revisions over the years. Several systems have also been proposed based on different aspects of the organisms.
This section will represent the classification of algae-based on four major categories.
- Prokaryotic algae
- Eukaryotic algae - Divided into three groups that include:
- Chloroplasts surrounded by double membranes (belonging to the chloroplast)
- Chloroplasts surrounded by a single membrane (of chloroplast E.R)
- Chloroplasts surrounded by a double membrane (of chloroplast E.R)
Read more on Chloroplasts here.
Group 1: Prokaryotic Algae
This is the first group of algae and consists of cyanobacteria (Cyanophyta). Although cyanobacteria are closely related to bacteria as opposed to algae (eukaryotic algae), they are also classified under the blue-green algae.
Cyanobacteria is the only organism classified in this group and are characterized by their ability to carry out photosynthesis through the use of chlorophyll a (while some have b and d).
* Although cyanobacteria are included as a group of algae in some books, this classification remains controversial.
Group 2: Archaepastida
Eukaryotic algae whose chloroplasts are surrounded by two membranes of the chloroplast envelope.
This group is made up of three phyla that include:
· Glaucophyta - A small group of unicellular algae found in freshwater. Photosynthesis occurs in a modified endosymbiotic cyanobacteria.
· Rhodophyta - This is a group of red algae and consists of over 7,000 identified species. Photosynthetic pigments of Rhodophyta are collectively arranged in the phycobilisomes. Compared to some of the other groups, Rhodophyta is one of the oldest groups among eukaryotic algae.
· Chlorophyta - Are green algae that contain chlorophylls a and b. Members of this group are in some books referred to as seaweed.
The chlorophyll of algae in this group is surrounded by a single chloroplast E.R membrane.
Also known as Rhizaria, this group is made up of two phyla that include:
· Euglenophyta - Euglenids are freshwater flagellates that contain chlorophylls a and b. They are also characterized by strips of proteinaceous pellicle located beneath the plasma membrane.
· Dinophyta - Is a green algae normally found in seawater (however, they can also be found in freshwater). They also contain chlorophylls a and b and in their environment, feed on smaller protists and bacteria. Like Euglenids, Chlorarachniophytes also possess flagella. However, some are amoeboid in nature.
· Apicomplexa - This group is largely composed of parasitic alveolates that are characterized by an apical complex structure referred to as apicoplast. This is an important structure for the organisms that allow them to penetrate and infect the host.
Also referred to as Chromista, group 4 is composed of algae whose chloroplasts are surrounded by a double membrane of chloroplast E.R.
As compared to the other groups, this group also carries the majority of subgroups (phyla) that include:
· Cryptophyta - Members are commonly found in freshwater environments (but can also be found in marine habitats). Like many other algae, they also contain chlorophylls a and b. However, they are, for the most part, characterized by a flattened shape and their anterior groove.
· Heterokontophyta - Members of this group are known as heterokonts and significantly vary in size. For instance, multicellular kelp and unicellular diatoms are all found in this category. They also possess a flagellum for movement and chlorophyll a and b for movement.
· Prymnesiophyta - Members have two flagella for movement and chlorophylls a and c for photosynthesis. They are characterized by scale-like fucoxanthin located outside the cell.
· Phaeophyceae - Multicellular brown algae commonly found in the marine cold waters in the Northern Hemisphere.
· Chrysophyceae - Are golden-brown algae typically found in freshwater habitats. They also possess two flagella used for movement.
· Phaeothamniophyceae - Members of this group are golden-brown in color and may appear filamentous, coccoid or pseudofilamentous in shape.
· Synurophyceae - Some members have four surrounding membranes. They are also characterized by fucoxanthin and chlorophyll a and c and are commonly found in fresh water.
· Xanthophyceae - Yellow-green algae commonly found in freshwater. However, some species can also be found in other environments (marine and soil). Movement is made possible by the presence of a single flagellum or in some cases, filamentous structures.
· Eustigmatophyceae - This is a small group whose members can be found in various habitats (freshwater, marine, soil). This group is also characterized by unicellular organisms that are surrounded by a polysaccharide wall.
· Raphidophyceae - Commonly known as chloromonads, members of this group found in both fresh and marine environments. Also composed of unicellular and multicellular organisms that lack cell walls.
· Pinguiophyceae - Includes a range of unicellular organisms found in marine environments. They are characterized by a high concentration of fatty acids whose hydrocarbon chain contains several carbon-carbon double bonds.
· Bacillariophyceae - Composed of diatoms commonly found in such habitats as soil and oceans.
· Dictyochophyceae - Also known as silicoflagellates, members of this group, which are small in number, are commonly found in marine habitats and move from one place to another using flagellum..
· Bolidophyceae - With about 20 identified species, this group is composed of aquatic organisms that are characterized by a flagellum that is positioned at an angle
· Pelagophyceae - Composed of heterokont algae commonly found in marine environments. They also range from filamentous to single-celled organisms that move through the use of flagella
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G. R. South, A. Whittick. (2008). An Introduction to Phycology.
Robert Edwards Lee. (2008). Phycology. Colorado State University, USA.
S. M. Satpute, et al. (2016). Microbiology And Phycology.