An Introduction to
Algae & Other Aquatic Microorganisms
This page provides a brief overview of three broad groups of aquatic microorganisms: algae, animals, and non-algal protists. Read on to find out how these three groups can be distinguished from one another based on characteristics like size and coloration.
Algae
Taxonomic Classification
Algae are far from representing a monophyletic group: a group that includes all the descendant lineages of one common ancestor, and no other lineages. Instead, this is a grouping that is based on some common characteristics, and not based on evolutionary relationships. While it’s true that if you trace the tree of life back far enough, you would find a single common ancestor that ultimately gave rise to all of these organisms, you’d also find that “algae” does not include all of the descendants of that common ancestor—we’ve just cherry-picked certain groups that fulfill some unifying criteria, and given the name “algae” to that collection of groups.
Because of this taxonomic unruliness, it’s hard to make generalizations that apply to all, or even most, algae. In fact, one could argue that it isn’t even accurate to say that “all algae are photosynthetic”! This is because some groups—such as Dinoflagellates and Euglenoids—contain representatives that are non-photosynthetic. You could argue right back: “if algae are defined by a set of criteria which includes being photosynthetic, then aren’t all algae by definition photosynthetic!?” Luckily for us, this website is not intended to serve as a treatise on the taxonomy of microorganisms. We will cover only photosynthetic (algal) taxa, and our descriptions of algal groups will reflect this.
Characteristics
Algae are Colorful!
Algae possess specialized pigments that enable them to capture energy from sunlight. These photosynthetic pigments come in a variety of colors, including bright green, golden-brown, and red. Due to the presence of these pigments, algal cells tend to be pretty colorful. In all groups except for cyanobacteria, these pigments are associated with structures called chloroplasts. The size, shape, and number of chloroplasts that are present in each cell varies among taxa—some algal cells may appear to be completely filled with bright pigmentation, while in others the pigments are confined to distinctly visible structures within the cell.
Size
Individual algal cells are all microscopic, though colonies may reach macroscopic size.
Form
Algae assume a very wide variety of forms. The overall structure and appearance of individual cells may be relatively simple—spherical to rod-shaped cells, lacking any ornamentation—or surprisingly complex, as is the case with many desmids (a group of green algae) and diatoms.
Many algae do not spend their lives as solitary individual cells. Instead, they form colonies or filaments, the shape, organization, and structural complexity of which varies widely. Simple colonies include gelatinous agglomerations of haphazardly-arranged cells; more complex colonies may be characterized by a particular number and arrangement of cells. Simple filaments consist of identical cells lined up end-to-end, and filaments are uniform in size and shape from one end to the other; more elaborate types may be extensively branched and can include multiple specialized cell types.
Movement
Many algae are not motile, or display motility only during a certain part of their life cycle. Many types are motile, however, and those that can move do so via a variety of different mechanisms. For example, some algae are propelled through the water by beating their flagella, while others creep or glide along surfaces.
These photos illustrate the colorful nature of algae!
Animals
Taxonomic Classification
Animals are members of the kingdom Animalia. Unlike the chaotic collection of unrelated life-forms that is “algae,” the animal kingdom is a monophyletic group. All animals are characterized by a set of fundamental biological traits, including eukaryotic cell structure and multicellularity.
Freshwater assemblages of microscopic animals include representatives of distinctly different animal lineages. Thus, as with algae, it is still difficult to make general statements about morphology (at least about any features we can see with a light microscope) that would apply to all species. But don’t worry, we’re going to try to make some of those general statements anyway!
Types of microscopic aquatic animals observed by NJCWST include rotifers, gastrotrichs, and crustaceans.
Characteristics
Color
The types of microscopic animals listed above have mostly minimal coloration or appear essentially colorless. Muted shades of brown, greyish-brown, or greyish-pink may be present. Internal anatomical features may be visible within otherwise colorless bodies.
Size
In general, an individual animal is larger than an individual algal cell. Algal colonies, however, may be comparable in size to microscopic animals.
Form
Animals, by definition, are multicellular organisms. In almost all animals, cells are organized into different tissue types. These specialized tissue types are further organized into reproductive, digestive, nervous, and other body systems. Some of the anatomical features associated with these bodily systems may be visible, such as eyes, eggs or egg sacs, intestines, and feeding structures. Crustaceans like water fleas and copepods also possess miniaturized versions of some more familiar animal attributes, like legs and antennae.
Movement
All of these organisms are capable of movement. Many move their entire bodies, either along surfaces or through the water; others live attached to surfaces (sessile), but move certain body parts, like feeding structures.
Larval crustacean
Rotifer
Non-Algal Protists
Taxonomic Classification
“Protist” is something of a taxonomic orphanage: unclaimed eukaryotes can be found here. To put it in more scientific terms, eukaryotes that are neither plants nor animals nor fungi are referred to as protists. There is a significant amount of biological diversity contained within this unnaturally-defined group, both phylogenetically and morphologically speaking.
Notice that this section is titled “Non-Algal Protists”—that’s because many kinds of algae are also considered protists (not our prokaryotic friends, the Cyanobacteria!). The descriptions given here will apply to protists that are not also considered algae.
Groups of non-algal protists observed by NJCWST include ciliates, flagellates, and amoeboid protists.
Characteristics
Color
These organisms are often colorless, though this is not always the case.
Colorless protists sometimes contain variously colored inclusions (these may even be algae that they have eaten!), and it is also possible for the organisms themselves to be pigmented, as is the case with the turquoise-hued Stentor coeruleus (Slabodnick & Marshall, 2014). Paramecium bursaria is one example of a non-photosynthetic protist that actually houses live algae inside of itself. As a result, this ciliate takes on an overall bright-green color, though a closer look reveals that this is due to the presence of many algal endosymbionts. Some amoeboid protists are enclosed within a shell-like structure called a test, which may be colored. For instance, Arcella are enclosed within yellowish-brown tests.
Still, many non-algal protists do take on a colorless appearance—which is not the case with live algal cells.
Size & Form
Very wide variety of forms. Protists are unicellular (though some form colonies), and the majority are microscopic in size. At about 1 mm in length, Stentor coeruleus is unsually large (Slabodnick & Marshall, 2014).
Movement
Many protists are motile, and different types of protists exhibit different mechanisms of movement. Some use the beating of hair-like structures—flagella or cilia—to generate movement; others seem to flow over surfaces in a fluid manner, reaching out with projections of the cell to pull themselves along (Encyclopaedia Brittanica, 2019).
Ciliate
References
Encyclopaedia Brittanica. (2019, November 12). Protist. In Encyclopedia Britannica. Retrieved December 19th, 2021 from https://www.britannica.com/science/protist
Slabodnick, M. M., & Marshall, W. F. (2014). Stentor coeruleus. Current Biology, 24(17), R783–R784. https://doi.org/10.1016/j.cub.2014.06.044