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Schematic representation of a Euglena cell with red eyespot (9) Schematic representation of a Chlamydomonas cell with chloroplast eyespot (4). The eyespot apparatus (or stigma) is a photoreceptive organelle found in the flagellate or (motile) cells of green algae and other unicellular photosynthetic organisms such as euglenids.
Euglena is a genus of single cell flagellate eukaryotes. It is the best known and most widely studied member of the class Euglenoidea, a diverse group containing some 54 genera and at least 200 species. [1] [2] Species of Euglena are found in fresh water and salt water.
Euglena gracilis is a freshwater species of single-celled alga in the genus Euglena. It has secondary chloroplasts, and is a mixotroph able to feed by photosynthesis or phagocytosis. It has a highly flexible cell surface, allowing it to change shape from a thin cell up to 100 μm long to a sphere of approximately 20 μm.
The stigma (2) of the euglena hides a light-sensitive spot. The earliest predecessors of the eye were photoreceptor proteins that sense light, found even in unicellular organisms, called " eyespots ". [ 23 ]
Euglenales consists mostly of freshwater organisms, in contrast to its sister Eutreptiales which is generally marine. Cells have two flagella, but only one is emergent; the other is very short and does not emerge from the cell, so cells appear to have only one flagellum. [3]
Phototaxis is mediated by simple eyespots that consists of a pigment cell and a photoreceptor cell. The photoreceptor cell synapses directly onto ciliated cells, which are used for swimming. The eyespots do not give spatial resolution, therefore the larvae are rotating to scan their environment for the direction where the light is coming from. [58]
As with other euglenids, cells in the Euglenaceae are surrounded by a series of proteinaceous strips called the pellicle; the pellicle can stretch in most genera, allowing the cell to contract, creating a type of movement called metaboly.
Euglena viridis is a freshwater, single cell, mixotroph microalgae bearing a secondary chloroplast. [1] Their chloroplast is bounded by three layers of membrane without a nucleomorph . [ 2 ] Normally, it is 40–65 μm long, slightly bigger than other well-known Euglena species: Euglena gracilis .