Photosynthetic
Euglenoids

Morphology of Photosynthetic Euglenoids

The chloroplasts of photosynthetic euglenoids are similar in color to those of green algae: a yellowish, vibrant green hue, like spring green. In contrast to the green chloroplasts, a reddish-orange spot—commonly called the eyespot—is visible at one end of the cell. Flagella emerge from this same end (the anterior end) of the cell; even though all taxa have at least two flagella, most genera have only one that actually emerges from the cell, and thus they appear to have just one flagellum (Triemer & Zakryś, 2015).

Characteristic of photosynthetic euglenoids is their particular type of cell surface: called a pellicle, it consists of a series of proteinaceous strips arranged underneath the plasma membrane (Kostygov et al., 2021). As a result of the pellicular strips, a striped or striated pattern is often visible on the cell surface using a light microscope. The form and arrangement of the pellicular strips influences how flexible the cell surface is: some cells are rigid, while others are very flexible (Triemer & Zakryś, 2015). Taxa with more flexible cells can change shape in an almost fluid-like manner (such flexible cells with no fixed shape are called metabolic), contracting and extending their forms, tumbling over themselves to move along. Cells vary in shape, but are often tapered to what looks like a pointed “tail” at one end. Most taxa are unicellular, and do not form colonies—the exception is the genus Colacium (Triemer & Zakryś, 2015). 

Euglenoids store carbohydrates in the form of a compound called paramylon. In all taxa, cells possess small, oval-shaped grains of paramylon (Triemer & Zakryś, 2015). Many taxa also possess more prominent paramylon bodies, which may be shaped like discs, rings, or rods, and usually appear colorless or whitish.

Some euglenoid cells are surrounded by a shell-like structure called a lorica. Loricas are typically yellowish-brown or orange-brown, and some are extremely dark brown—unfortunately, this means you can’t make out all those interesting cellular features we just spent three paragraphs describing! Don’t freak out: loricate taxa may not look very euglenoid-esque at first glance, but they are fairly easy to recognize once you know what they are. The shape of the lorica may be spherical, oblong, or vase-like, and it may be ornamented with bumps and/or spines. The flagellum emerges through an opening at one end; this opening may have a rim- or collar-like structure around it (this sometimes looks a bit like the neck of a bottle). Trachelomonas is one of two loricate genera (the other being Strombomonas), and is very common in NJCWST water samples.