Euglena is a genus of unicellular protists, of the class Euglenoidea of the phylum Euglenozoa (also known as Euglenophyta). They are single-celled organisms. Currently, over 1000 species of Euglena have been described. There are many to be discovered. Marin et al. (2003) revised the genus to include several species without chloroplasts, formerly classified as Astasia and Khawkinea. Some Euglena are considered to have both plant and animal features. Due to these dual characteristics, much debate has arisen to how they have evolved, and into which clade they should be placed. Euglena were originally placed in the kingdom Protista but now are classified above the kingdom Excavata, which contains both Kinetoplastids and Euglenids.
Form and Function
A Euglena is a protist that can both eat food as animals by heterotrophy; and can photosynthesize, like plants, by autotrophy. When acting as a heterotroph, the Euglena surrounds a particle of food and consumes it by phagocytosis. When acting as an autotroph, the Euglena utilizes chloroplasts, (hence green color) containing Chlorophyll a, Chlorophyll b, and some carotenoid pigments, to produce sugars by photosynthesis. Each chloroplast has three membranes, and exist in thylakoid stacks of three. The number and shape of chloroplasts within euglenozoa varies greatly due to environmental conditions and evolutionary history. Euglena are able to move through aquatic environments by using a large, rear mounted flagellum for locomotion. To observe its environment, the cell contains an eyespot, a primitive organelle that filters sunlight into the light-detecting, photo-sensitive structures at the base of the flagellum; allowing only certain wavelengths of light to hit it. This photo-sensitive area detects the light that is able to be transmitted through the eyespot. When such light is detected, the Euglena may accordingly adjust its position to enhance photosynthesis. The mobility of Euglena also allows for hunting capability, because of this adaptation, many Euglena are considered mixotrophs: autotrophs in sunlight and heterotrophs in the dark. Euglena also structurally lack cell walls, but have a pellicle instead. The pellicle is made of protein bands that spiral down the length of the Euglena and lie beneath the plasma membrane.
Euglena can survive in fresh and salt water. In low moisture conditions, a Euglena forms a protective wall around itself and lies dormant as a spore until environmental conditions improve. Euglena can also survive in the dark by storing paramylon granules in pyernoid bodies within the chloroplast.
Euglenas reproduce asexually by fission, and there has been no existence of sexual reproduction. Reproduction includes transverse division and longitudinal division, which both occur in the active and encysted forms. Acidity and alkalinity have been known to affect reproduction and life spans of Euglenozoans. Life spans also greatly differ between each group of Euglenozoans.
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