Euglena is a large genus of unicellular protists: they have both plant and animal characteristics.
All live in water, and move by means of a flagellum. This is an animal characteristic. Most have chloroplasts, which are characteristic of algae and plants.
Some characteristic features of Euglenoids are as follows.
• Euglenoids (such as Euglena) are unicellular protists commonly found in fresh water.
• Instead of cell wall, a protein-rich cell membrane known as pellicle is present.
• They bear two flagella on the anterior end of the body.
• A small light sensitive eye spot is present.
• They contain photosynthetic pigments such as chlorophyll and can thus prepare their own food. However, in absence of light, they behave similar to heterotrophs by capturing other small aquatic organisms.
• They have both plant and animal-like features, which makes them difficult to classify and hence they are called as connecting link between plants and animals.
Euglenids are believed to descend from an ancestor that took up green algae by secondary endosymbiosis.
Over 1,000 species of Euglena have been described, and there are more to be discovered. Their relationships are being analyzed.
When acting as a heterotroph (animal), the Euglena surrounds a particle of food and consumes it by phagocytosis. When acting as an autotroph, the Euglena has chloroplasts which produce sugars by photosynthesis. The chloroplasts use the pigments chlorophyll a and chlorophyll b.
As a whole, this phylum includes a decidedly heterogeneous assortment of organisms. Most possess flagella or whiplike structures that serve for locomotion (up to 300 microns per second) although some loose the flagella structure in early development and move in amoeboid fashion.
The euglenoids are unicellular organisms that show a combination of plant-like and animal-like characteristics. They are plant like in that many species have chlorophyll and are photosynthetic. They are animal-like in lacking a cell wall and being highly motile, and the species that lack chlorophyll are heterotrophic like animals.
Zoologists have traditionally regarded euglenoids as animals and placed them among the flagellated protozoa. Botanists on the other hand have regarded them as plants and placed them among the algae. They seem to have no close relatives among other algae, however, and for this reason botanical classifications usually put them in a division by themselves.
The euglenoid species that lack chlorophyll are obligate heterotrophs. The species that have chlorophyll are facultive heterotrophs and can survive in the dark if they have a source of organic nutrients. Reproduction in euglenoids is by longitudinal miotic cell division.
Almost every known type of nutrition is to be found in the euglenoids. Some are holophytic, carrying on photosynthesis, others are holozoic ingesting particulate food. Many are saprophytic, absorbing decay products through cell surfaces and quite a number are parasitic. Certain flagellates are capable of alternating modes of nutrition. Euglena, for example, can be photosynthetic or saprophytic. Within this division, three classes are commonly mentioned.
The number of flagellates that are photosynthetic, some of which are nonphotosynthetic, and others which may be either are placed in the class phytomastigina. The lumping of these into one class is based largely on biochemical and morphological similarities.
Ecologically, the contributions of the members of this class are varied. Photosynthetic forms such as Euglena, Cryptomonas and others are important in the cycles of respiratory gasses. Some in the role of saprophytes aid in the breakdown of complex organic compounds. Many serve as food for larger organisms and thereby enter into the food chains in the community structure.
Photosynthetic members of this class and the dynoflagellates, which will be mentioned next, share with certain of the true algae some of the fundamental roles as producers in the food cycles.
The class dynoflagellates includes forms which are holozoic, holophytic, and saprophytic. The dynoflagellates typically have one or more flagella lying in a groove about the body. The photosynthetic dynaflagellates possess chlorophyll A, chlorophyll C, carotene, four xanthophylls and other yellow-brown pigments.
Most of the dynoflagellates are unicells, some possessing covering plates of cellulose while others are naked. Peridinium and Ceratium are examples of the armored type.
In fresh waters, blooms of Peridinium or a host of others impart fishy odors to reservoirs or lakes. Ceratium in abundance produces a particularly obnoxious odor.
This class is characterized by flagellated forms that are free living or have entered into various symbiotic relations. Because of their nutrition, these are often placed with Protozoa. They typically represent a successional stage in a series of population changes, and in natural waters often show seasonal cycles of abundance.