Algae Identification
General Advice & Frequently Asked Questions
If you’ve reviewed the content on our Overview of Algae page, you have probably already realized that algae are extremely diverse! While this makes algae a fascinating group of organisms to study—and lots of fun to look at!—it can also make algal identification feel like a daunting task for beginners. Don’t panic—learning how to identify algae takes time and practice, just like any other skill!
On this page, you’ll find advice on how to use taxonomic keys to algae, as well as answers to commonly asked questions regarding algal identification. Review the “General Advice for Approaching Algal Identification” section of this page to familiarize yourself with the types of features that are commonly mentioned in taxonomic keys and in taxonomic descriptions. For answers to commonly asked questions regarding algal identification, explore the FAQs section of this page.
Table of Contents
General Advice for Approaching Algal Identification
When attempting to identify an unknown algal specimen (i.e., one that you don’t already know the identity of from prior experience), you may be tempted to simply flip through the pages of an identification guide until you find a photo or illustration that looks similar to the specimen. A much more efficient approach, however, would be to use a taxonomic key—that’s why we created our visual guide to common freshwater algal groups, which is essentially a beginner-friendly version of a taxonomic key!
A typical taxonomic key consists of a series of paired statements; the two statements in each pair are mutually exclusive, and the user must determine which one correctly describes their specimen. Each of the statements will direct the user to another specific pair of statements to choose from; this process continues until the user has identified the specimen (at which point the taxonomic key will direct the user to a particular taxon, rather than to another pair of statements in the key). When using a taxonomic key to algae, you will need to assess various morphological features of the specimen in order to move through the subsequent steps—the sets of paired, mutually exclusive statements—of the key. You will also need to assess various morphological features in order to determine whether or not your specimen actually matches the full description of the taxon you were directed to (by the key). Thus, it will be helpful to familiarize yourself with the types of features that are commonly mentioned in taxonomic keys and in taxonomic descriptions:
Traits used to determine which major algal lineage a specimen belongs to
Color
Different algal groups possess different combinations of photosynthetic pigments; these pigments create the characteristic colors typically seen in each group.
For example, cyanobacteria are often blue-green, while diatoms are usually golden-brown.
Cell covering
Many algal groups are characterized by a specific kind of cell covering that can be clearly seen using light microscopy.
For example, diatoms have a siliceous frustule; which is glass-like in appearance and is often intricately patterned with rows of small pores. Many dinoflagellates, on the other hand, possess an “armor” (called a theca) composed of closely adjacent cellulosic plates.
Flagella
The presence or absence of flagella, as well as the number, arrangement, and relative lengths of flagella, are useful characteristics for determining which algal group a specimen belongs to.
For example, cyanobacteria never have flagella, while most Eulgenoids have a single emergent (i.e., visible) flagellum.
Traits used to determine which genus an algal specimen belongs to
Color, cell covering, and flagella (see above).
General vegetative morphology: unicell, colonial, filamentous, etc.
Cell shape and (if you have a stage micrometer) cell size.
(For eukaryotic algae) The number, shape, and arrangement of chloroplasts, as well as the presence and number of pyrenoids.
For colonial algae, make note of colony characteristics, such as:
The overall shape and form of the colony. For instance:
Is it flat and two-dimensional, consisting of a single layer of cells?
Does it form a rectangular or square sheet? Is it linear (but not filamentous)? Is it generally circular in outline?
Is it three dimensional?
Is it spherical? Egg-shaped? Irregular in shape?
For diatom colonies:
Are the cells joined to each other along their valve faces, forming a ribbon- or band-like colony?
Are the cells connected to each other just at one end, forming a zig-zag or star-shaped colony?
Whether the colony consists of many cells or a relatively small number of cells.
The relative orientation and arrangement of cells within colonies:
Are the cells dispersed irregularly throughout the colony?
Do the cells appear to be arranged in a single peripheral layer, as if they are situated on the surface of a colony with a hollow center?
Do the cells appear to be arranged in perpendicular rows?
Do the cells appear to be radiating out from common point in the center where the cells meet at one end?
The relative spacing of cells within colonies:
Are the cells very close together (or even connected/touching), or are they more distant and spaced apart from one another?
Mucilage
Is there a mucilaginous envelope surrounding the entire colony?
Do individual cells or groups of cells within the colony have their own mucilaginous envelopes?
Is the margin of the mucilage distinctly delimited or is it diffuse?
For filamentous algae, make note of characteristics such as:
The relative dimensions of cells within the filament(s): are the cells shorter than wide, longer than wide, or isodiametric (about equal in width and length)?
Are the filaments branched or unbranched?
Are the filaments uniseriate (consisting of a single linear chain of cells) or multiseriate (consisting of multiple linear chains of cells, with the linear chains parallel to one another)?
Do both ends of the filament look the same?
Remember: habitat can also help with identification! Many identification guides (including ours) give descriptions of the environments different taxa are commonly found in. Consider the following characteristics of the habitat from which the specimen was collected:
Was the sample collected from a planktonic, benthic, metaphytic, epiphytic, or periphytic habitat?
What type of waterbody was the sample collected from? Was it a lentic or lotic environment?
Keep in mind that identifying genera may be difficult without many specimens (i.e., without either a nearly monospecific natural sample population or an isolated culture). This is because genus-level identification often requires the observation of details which may not be apparent in a single cell, colony, or filament). For example:
For some eukaryotic algae, reproductive structures may need to be observed in order to determine the genus of the organism. Thus, one needs to find fertile/sexually reproducing specimens, which may be unlikely if you observing specimens in a natural sample.
Determining the genus of a specimen based on morphology alone can also be complicated by the fact that various taxa—both among cyanobacteria and among eukaryotic algae—undergo morphological changes during their life cycles. Additionally, some algae simply do not have many unique morphological characteristics (observable with light microscopy) that are useful for determining their generic identity.
If you cannot identify the genus of a specimen, see if you can at least determine which family or order it belongs to!
Algae Identification FAQs
Here, we’ll answer some frequently asked questions about algae identification and address some common sources of confusion for beginners.
I can’t identify whether my specimen is an alga, non-algal protist, or an animal—what should I do?
If you’re having trouble determining which of these three major groups something belongs to, it could be because whatever you’re looking at actually isn’t any of these things! Air bubbles, pollen, bits of dead organic matter (detritus), and various other objects are sometimes mistaken for microorganisms.
See also the “Why do some algal cells appear empty?” topic (also located on this page).
Above: detritus (particles of dead/decayed organic matter)
Above: air bubbles
I can’t figure out which major freshwater algal group my specimen belongs to—what should I do?
If you already tried using our guide to figure out which major algal group your specimen belongs to, try browsing through our taxonomic description pages individually; there are taxa within each major group that can be tricky to identify (specific examples are described under a separate heading on this page). By reading more about each group—and examining more photos of representative genera within each group—you may be able to determine which group your specimen belongs to.
It is also possible that the major group to which your specimen belongs is not included in our guide; the algal groups we have chosen to include are a reflection of the algal communities that NJCWST analyzes, which are mainly planktonic algal assemblages from lakes, ponds, and reservoirs. Thus, some groups of algae that are uncommon in these habitat types are not included in our guide. Information regarding these other algal groups will likely be included in a more comprehensive algal identification guide; NJCWST frequently uses Freshwater Algae of North America: Ecology and Classification (2nd ed), which includes a key to the major groups of freshwater algae.
Are there any morphological features of algae that are difficult to see using light microscopy?
Yes, some morphological details can be difficult to see with light microscopy (LM), especially if you don’t have access to a higher-quality scope with phase contrast and/or differential interference contrast (DIC) capabilities.
In some cases, you may still be able to identify which genus (or at least major algal group) the specimen belongs to, even if you can’t make out a specific feature that’s mentioned in its taxonomic description. However, there will likely be situations where you can’t identify the genus of the specimen, because certain fine details need to be observed in order to distinguish between similar-looking genera.
Structures or features that are sometimes difficult to discern with LM include:
Flagella
Flagella are thin, colorless structures—sometimes they cannot be resolved by a basic light microscope.
There may be situations where you can validly identify a specimen based on the morphological traits that you can make out, even though you cannot observe the flagellum/flagella that the taxon in question is known to have. However, keep in mind that if you can clearly see that a specimen bears flagella, it cannot belong to any taxa known to not possess flagella. Furthermore, if your identification hinges on the presence or absence of flagella, and you are not sure if the specimen truly lacks flagella or if you just cannot see its flagella, don’t make an arbitrary decision to simply identify the specimen as one taxon or the other.
Phacus cells possess a single emergent flagellum; in this image of Phacus, however, no flagellum is visible. The organism can still be identified as Phacus based on other morphological details that are visible (such as the characteristic flattened, leaf-shaped form of the cell).
Lightly silicified diatom frustules
The frustules of some diatoms are lightly silicified, which makes them a bit more difficult to see. When viewing live material from natural water samples, the frustules are usually still visible using light microscopy; they are easily overlooked, however, due to their delicate appearance.
Urosolenia and Acanthoceras are two genera of diatoms which are characterized by lightly silicified frustules.
Acanthoceras
Urosolenia
Scales of golden algae
Some golden algae possess an external covering of silica scales; depending on the species, these scales can be easily overlooked without using DIC or phase contrast techniques. For example, the scales surrounding the cells of some Synura species are difficult to see; the scales may be visible upon careful inspection.
Some scaled golden algae, like Mallomonas, also have long siliceous bristles or spines. These bristles/spines help draw attention to the covering of scales that surround the cell(s).
Synura
Mallomonas
Sheaths and mucilage
Some filamentous cyanobacteria possess fine, thin sheaths which may not visible using LM.
Mucilage and mucilaginous stalks are sometimes not visible using LM; staining with Indian ink can help visualize mucilage.
Certain structural details of diatom frustules
Some features of diatom frustules (such as the shape and density of areolae) are difficult or impossible to see without special preparation methods. These methods involve cleaning and mounting the material in a particular mounting medium before viewing.
The “striped” appearance of the cell surface of euglenoids
The cell surfaces of some euglenoids have a conspicuously “striped” appearance; this is due to the pellicle, which is arranged in strips. Though the pellicle is a feature common to all euglenoids, the “striped” appearance of some taxa is subtle or not discernible using light microscopy.
Euglenoids are characterized by other features that can be used to identify them as euglenoids; you can review those characteristics by visiting the Euglenoids page.
Why do some algal cells appear empty?
You will likely encounter “empty” algal cells: in-tact outer coverings (cell walls, frustules, or cellulosic thecae) of cells which no longer contain any cell content. These are non-living remnants of algal cells, and in some cases they may still be identifiable.
Because these cell coverings no longer contain the photosynthetic pigments that make living algal cells so colorful, these empty cells could be mistaken for non-algal protists (which are often colorless). Therefore, it’s a good idea to familiarize yourself with the appearance of these empty algal cells:
Above: empty diatom frustules. These empty frustules are a common occurrence in water samples.
Above: empty dinoflagellate thecae.
How hard is it to differentiate between algae and other types of aquatic microorganisms?
Algae and microscopic aquatic animals are rather easy to tell apart. Algal cells are typically much smaller than rotifers, gastrotrichs, and other microscopic animals; algal cells contain photosynthetic pigments, while microscopic animals are mostly colorless or slightly greyish-brown; microscopic animals also possess numerous anatomical features which are completely lacking in algae, such as a mouth and associated feeding structures, internal organs (associated with the digestive, reproductive, and other systems), and even paired appendages (such as antennae, in crustaceans).
Algae are usually easy to tell apart from ciliates (ciliated non-algal protists), because algae never possess cilia. Other non-algal protists—flagellates and amoeboid protists—can be distinguished from based on cell color: amoeboid protists and flagellates (non-algal) are mostly colorless, while algae possess photosynthetic pigments.
There are some cases where algae could be mistaken for non-algal protists, or vice versa. For example:
Golenkinia (a type of green algae) could be mistaken for a “sun-animalcule” (an axopodia-bearing amoeboid protist). Golenkinia cells have many thin spines, so they bear a slight superficial resemblance to these so-called “sun-animalcules.”
Golenkinia
Paramecium bursaria (a ciliated protist) could be misidentified as a type of algae, because it is densely packed with endosymbiotic green algal cells.
Paramecium bursaria
Which algal groups are the hardest to tell apart from one another?
In general, the freshwater algal groups included in our guide are easy to tell apart from one another based on characteristics like color, cell covering, and flagella (the presence or absence of flagella, and the number and arrangement of flagella). However, there is one major exception: coccoid cyanobacteria and small, non-motile, coccoid green algae can sometimes be difficult to tell apart, especially for beginners. Below, we’ll compare and contrast these two groups of organisms in more detail, and explain how to differentiate between the two.
Differentiating between nonmotile coccoid green algae and coccoid cyanobacteria
Within both of these groups, there are taxa that are very morphologically simple—that is, taxa which essentially just look like small round cells with no distinguishing cell surface features, no flagella, and no particularly unique vegetative growth form (e.g., simple unicells or irregular colonies or agglomerations of cells). The coloration of coccoid cyanobacteria can be greyish-green or olive-green, and therefore their coloration may appear fairly similar to that of green algae. Additionally, in some green algal cells, the form of the chloroplast is such that the cell content appears homogenously pigmented throughout (i.e., it is difficult to tell that the photosynthetic pigments are contained within a chloroplast). Thus, you may experience some difficulty when trying to determine whether a specimen is a cyanobacterium or a green alga—as you gain experience with algal identification, however, cyanobacteria and green algae will become rather easy to tell apart.
Here are some helpful tips for distinguishing between morphologically simple coccoid green algae and cyanobacteria:
Examine the color of the specimen.
The green color of green algae tends to be more of a vivid grass-green color. Cyanobacteria are often blue-green (hence the common name, “blue-green algae”), but they can also be simply green. When cyanobacteria are green—as opposed to blue-green—it is usually more of a muted greyish-green or olive-green color.
Green algae, shown above, typically have a grass-green color.
Cyanobacteria, shown above, may be greyish-green or olive-green. Cyanobacteria almost never have the grass-green color seen in green algae.
Carefully examine the specimen to try and determine whether or not the cell contains one or more chloroplasts. This can be a bit difficult for beginners, so let’s take some time to discuss chloroplasts in more detail:
Chloroplasts—as you may recall from the Overview of Algae page—are membrane-bound organelles that contain photosynthetic pigments. Chloroplasts are found in eukaryotic algae, and they are not found in cyanobacteria (cyanobacteria are prokaryotes). Thus, in eukaryotic algal cells, photosynthetic pigments are not distributed homogeneously throughout the volume of the cell—instead, they are contained within one or more sub-cellular structures (these structures are the chloroplasts).
In some green algae, it is easy to tell that the photosynthetic pigments are contained in chloroplasts, and not distributed homogeneously throughout the cell’s content. Here are some examples:
Above: Closterium. Here, we can see two chloroplasts. Each chloroplast is green, and each one appears to fill about half of the cell’s volume. There is a colorless region in between the two chloroplasts (this is where the nucleus is). There are also two colorless regions visible at the ends of the cell (one colorless region at either end).
Above: Spirogyra. Here, the chloroplast appears as a spiraling green band; the rest of the cell’s volume appears colorless.
Above: Dictyosphaerium. Here, the chloroplast is a green, cup-shaped structure; the rest of the cell’s volume appears colorless.
In all of the images shown above, we can easily recognize that the cells possess chloroplasts because we can see that the photosynthetic pigments (which are responsible for the green color of the chloroplasts) are contained in distinct sub-cellular structures within the otherwise colorless cell.
In some green algae, it can be harder to recognize that the photosynthetic pigments are contained within one or more chloroplasts. Here are some examples:
Above: Desmodesmus.
Above: Gloeocystis.
In the examples above, it is less obvious that the photosynthetic pigments are contained within chloroplasts, because we cannot tell that the rest of the cell’s volume is colorless—instead, the entirety of the cell’s volume appears green to us.
Pyrenoids can help you recognize the presence of chloroplasts in eukaryotic algae cells, even when the entire volume of the cell appears green-colored.
Pyrenoids (pictured below) are structures associated with the chloroplasts of many (but not all) green algae; when viewed with LM, they look like raised circular bumps. If pyrenoids are present, then the cell must be a chloroplast-containing (and thus eukaryotic) alga.
To review:
Green algae and cyanobacteria can both be green. However, green algae are usually vivid grass-green, while cyanobacteria are usually greyish-green to olive-green.
Keep in mind that many cyanobacteria do not have a green color—cyanobacteria are often distinctly blue-green, and many planktonic species appear dark brown, due to the presence of aerotopes.
Green algal cells always have one or more chloroplasts, while cyanobacteria never have chloroplasts.
Chloroplasts are often visible as distinct, green-colored, subcellular structures within the otherwise colorless volume of the cell.
Chloroplasts can also appear to fill the entire volume of the cell (rather than appearing as distinct subcellular structures within the otherwise colorless volume of the cell). In these cases, the presence of pyrenoids can help distinguish green algae from cyanobacteria. Pyrenoids, which look like raised circular bumps, are structures that are associated with the chloroplasts of many (but not all) green algae.