Green Algae
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Cool Facts About Green Algae!
The chloroplasts found in green algae—along with red algae and a small group of organisms called glaucophytes—were obtained directly through primary endosymbiosis. In this evolutionary event, a free-living cyanobacterium was engulfed by a eukaryotic cell; this eukaryote, now in possession of a cyanobacterial endosymbiont (in the form of a plastid), gave rise to the green, red, and glaucophyte algal lineages (Rockwell et al., 2014).
All land plants arose from a green algal ancestor. Researchers previously suspected that land plants were most closely related to the Charophycea, a group of green algae whose upright, branched shoots resemble the forms of higher plants. Recent evidence, however, suggests that the land plants’ closest relatives are actually the Zygnematophyceae—somewhat surprising, given their much simpler structures (Cheng et al., 2019; Nishiyama et al., 2018).
Different algal groups store energy reserves in the form of different starch-like compounds; in green algae, this storage compound is true starch. Staining cells with iodine turns starch purplish-black, thereby providing one way (albeit slightly more complex) to distinguish green algae from other algal groups!
Green Algal Morphology
Green algae in freshwater habitats are represented by a large number of morphologically diverse genera. In fact, compared to any other taxonomic group, green algae represent the largest proportion of all algal genera observed by NJCWST (as of 2021). All green algae possess chlorophylls a and b as their photosynthetic pigments (Wehr & Sheath, 2015). This gives their chloroplasts a vivid green color, which may help you to recognize green algae—there are some exceptions where the green hue is masked by a reddish coloration, but we here at NJCWST have yet to encounter these taxa. In contrast, many other algal groups’ chloroplasts have a brown or golden-brown color, due to the presence of additional pigments.
Because the degree of morphological variability is so high, we will break down this group into several broad classes: motile (flagellate) green algae, non-motile coccoid green algae, conjugating green algae, and non-conjugating filamentous green algae. There are also a variety of green algae that form macroscopic thalli, which may be membrane-like, blade-like, or plant-like in nature; we will not explore these taxa in detail, because we are focusing (pun intended?) on microscopic algae.
Motile (Flagellated) Green Algae
As you may have guessed, the presence of flagella is the key morphological trait used to distinguish these genera from non-motile coccoid green algae. More specifically, the vegetative cells of this group possess flagella—some non-motile genera do produce flagellated spores, but do not possess flagella in their vegetative stages. Cells often possess a single large chloroplast; one or more pyrenoids may be present (Nakada & Nozaki, 2015). A generally rounded cell shape is common (spherical, oval, ovoid), but is not the only possibility. Both unicellular and colonial forms exist. In colonies, cells may be arranged in three dimensions (for instance, in a spherical colony), or in a single flat layer; cells in colonies may be packed tightly together, or they may be spaced more distantly from one another. When viewing live specimens, you may see unicellular forms whirling about in circles, or colonies tumbling through the water like microscopic soccer balls!
Representative genera commonly observed by NJCWST: Chlamydomonas.
Representative genera occasionally or rarely observed by NJCWST: Pandorina, Eudorina, Tetrabaena.
Non-Motile Coccoid Green Algae
As previously mentioned, the vegetative cells of these taxa lack flagella and are immotile—they can, however, produce flagellated (and motile) cells during certain stages of their lifecycles (Shubert & Gärtner, 2015). This is not a problem for us, as our goal is to help you distinguish green algae from other algal groups, but do be aware of this if you are attempting more detailed identifications (for instance, at the genus level)!
Both unicellular and colonial forms exist. Cells and colonies can take on a diverse array of shapes and/or arrangements, many of them quite beautiful to behold! Cell shapes include spherical, crescent-shaped, or spindle-shaped, among others, and a number of taxa are ornamented with prominent spines. Cells within colonies may have a characteristic, fixed arrangement; Pediastrum, for example, forms snowflake- or starburst-like colonies. In other colonial genera, there is no fixed arrangement, and cells are dispersed irregularly throughout a common mucilage. Number and shape of chloroplasts varies, and pyrenoids may or may not present.
Representative genera commonly observed by NJCWST: Scenedesmus, Desmodesmus, Pediastrum
Representative genera occasionally or rarely observed by NJCWST: Dictyosphaerium, Crucigenia, Gloeocystis, Monoraphidium.
Conjugating Green Algae
This taxonomic class, called Zygnematophyceae, contains some of the coolest-looking green algal species (in our humble opinion). It includes many filamentous and unicellular forms, as well as some colonial types. The class is split into two orders, Zygnematales and Desmidiales. Members of the Zygnematales have smooth cell walls and generally elongated cells (such as ovoid or cylindrical shapes); in filamentous taxa, cells are held tightly together into cylindrical (i.e., like a straw wrapper) filaments. In the Desmidiales, cell walls are ornamented with features like pores, bumps, or spines; cells exhibit a wider variety of shapes, and most are distinctly constricted in the middle, splitting each cell into two “semicells”; some filamentous taxa form stable filaments, while others are only loosely held together. Chloroplast morphology is diverse among the Zygnematophyceae: in some filamentous types, chloroplasts appear like a flat ribbon, sometimes twisted, stretching across the long axis of the cell; in many Desmidiales, each semicell is almost entirely filled by one chloroplast; in Actinotaenium, the chloroplast lobes radiate outward, creating a star-like appearance when observed from a specific viewpoint; and of course, we would be remiss not to mention the characteristic spiraling chloroplasts of Spirogyra (Hall & McCourt, 2015). Pyrenoids may be present.
Beautifully complex forms can be seen amongst the Desmidiales—enough to challenge diatoms’ (unofficial) position as the wonders of the micro-world! Some Staurastrum species bear a striking resemblance to bowtie pasta, while Micrasterias cells look like tiny paper snowflakes. Some species’ semicells are angular and geometric; others are lobed, creating a sort of undulating outline.
But why, you ask, is this group called the “conjugating” green algae? Here is why! The conjugating green algae are characterized by a particular mode of sexual reproduction, called conjugation. During this process, the two partner cells become joined by a conjugation tube, enabling the fusion of their non-motile (lacking flagella) gametes (Permann et al., 2021). Reproduction is mostly through vegetative means, however, and sexual reproduction has yet to be documented in many conjugating taxa (Hall & McCourt, 2015)! Here at NJCWST, we have observed conjugating filaments of Spirogyra, which look very much like tiny ladders: two filaments line up close together and are joined by conjugation tubes along their lengths.
Filamentous taxa, such as Mougeotia and Spirogyra, are capable of forming macroscopic masses. Filaments may be aggregated into slimy, floating mats or billowing, cloud-like growths underwater (a little like neon-green cotton candy).
Representative genera commonly observed by NJCWST: Closterium, Staurastrum.
Representative genera occasionally or rarely observed by NJCWST: Spirogyra, Cosmarium, Euastrum
Non-Conjugating Filamentous Green Algae
In the previous section, we discussed some filamentous forms of conjugating green algae—in this section, we’ll describe non-conjugating filamentous green algae. Our discussion will be limited to microscopic taxa, even though some larger plantlike green algae do have a kind of “filamentous” appearance (with a long central axis and slender, threadlike branches).
“Filamentous green algae” broadly refers to taxa whose cells are arranged into elongated, filament-like growth forms. In many taxa, filaments are uniseriate: comprised of cells arranged end-to-end in a single series, like a chain. The neighboring cells may be directly attached to each other (end-to-end), or they may be only loosely held together in a surrounding mucilaginous sheath. In filaments of the latter type, neighboring cells may not even be directly touching one another (may be called pseudofilaments! In some taxa, cells within filaments are also arranged side-by-side, or in packet-like, three-dimensional clusters of cells (John & Rindi, 2015). In these cases, cells often appear irregularly arranged, and are not uniform in size and/or shape; such filaments may look lumpy or kind of amorphous. We have encountered these taxa only a handful of times; the vast majority of filamentous taxa observed in our samples are of the uniseriate type, so these will be the focus of the remainder of our discussion.
Some filaments are unbranched (as in Ulothrix), while others exhibit varying types and degrees of branching. For example: Rhizoclonium filaments sometimes bear short branches, just one or a few cells in length, while Stigeoclonium may have many branches of various lengths. Similar to higher plants, we can describe branching patterns as opposite, alternate, dichotomous, and so on. In some genera, the cells of branches look different than those of the main filament.
Cell shape varies: some are long and some are short; some are cylindrical and straight-sided, and some have a swollen appearance (these are not the only possibilities). Cells may contain one chloroplast or many, and pyrenoids may or may not be present. Possible chloroplast shapes include netlike (kind of like a chloroplast web), girdle-shaped (like a flattened layer encircling some or all of the inside of the cell), and disklike, among others (John & Rindi, 2015).
Representative genera observed by NJCWST: Stigeoclonium, Ulothrix, Rhizoclonium, Radiofilum, Chaetophora, Oedogonium
Ecology of Green Algae
Green algae can be found in all sorts of freshwater habitats. However, habitat types differ in which green algal groups they tend to support. Coccoid green algae are more common in nutrient-rich standing waters (such as lakes and ponds), with non-flagellated coccoid forms becoming more abundant in the plankton when temperature and light levels are higher. Plantlike green algae—filamentous types bearing branches and other plant-like structural features—are usually found attached to hard surfaces (like rocks), in standing as well as flowing waters (such as streams). Filamentous conjugating types commonly form floating mats or masses in stagnant waters, such as small ephemeral pools, or amongst aquatic vegetation in the near-shore zones of lakes. Desmids are typically abundant in boggy habitats (higher-acidity, lower-nutrient wetland habitats) amongst aquatic plants or filamentous green algae (Wehr & Sheath, 2015; Hall & McCourt, 2015).
Image Gallery
Click the arrows next to each genus listed below to view photos.
Actinotaenium
Actinastrum
Ankistrodesmus
Botryococcus
Chaetophora
Chlamydomonas
Closterium
Coelastrum
Cosmarium
Crucigenia
Desmodesmus
Dictyosphaerium
Elakatothrix
Eudorina
Gloeocystis
Kirchneriella
Klebsormidium
Micractinium
Monoraphidium
Oedogonium
Oocystis
Pandorina
Pediastrum
Pleurotaenium
Scenedesmus
Selenastrum
Sorastrum
Sphaerocystis
Spirogyra
Staurastrum
Stauridium
Stigeoclonium
Tetrastrum
Ulothrix
Zygnema
References
Cheng, S., Xian, W., Fu, Y., Marin, B., Keller, J., Wu, T., Sun, W., Li, X., Xu, Y., Zhang, Y., Wittek, S., Reder, T., Günther, G., Gontcharov, A., Wang, S., Li, L., Liu, X., Wang, J., Yang, H., … Melkonian, M. (2019). Genomes of Subaerial Zygnematophyceae provide insights into land plant evolution. Cell, 179(5). https://doi.org/10.1016/j.cell.2019.10.019
Hall, J. D., & McCourt, R.M. (2015). Conjugating Green Algae Including Desmids. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
John, D.M. & Rindi, F. (2015). Filamentous (Nonconjugating) and Plantlike Green Algae. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
Nakada, T. & Nozaki, H. (2015). Flagellate Green Algae. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
Nishiyama, T., Sakayama, H., de Vries, J., Buschmann, H., Saint-Marcoux, D., Ullrich, K. K., Haas, F. B., Vanderstraeten, L., Becker, D., Lang, D., Vosolsobě, S., Rombauts, S., Wilhelmsson, P. K. I., Janitza, P., Kern, R., Heyl, A., Rümpler, F., Villalobos, L. I., Clay, J. M., … Rensing, S. A. (2018). The chara genome: Secondary complexity and implications for Plant Terrestrialization. Cell, 174(2). https://doi.org/10.1016/j.cell.2018.06.033
Permann, C., Herburger, K., Felhofer, M., Gierlinger, N., Lewis, L. A., & Holzinger, A. (2021). Induction of conjugation and zygospore cell wall characteristics in the alpine Spirogyra mirabilis (Zygnematophyceae, Charophyta): advantage under climate change scenarios? Plants (Basel, Switzerland), 10(8), 1740. https://doi.org/10.3390/plants10081740
Rockwell, N.C., Lagarias, J.C., & Bhattacharya, D. (2014). Primary endosymbiosis and the evolution of light and oxygen sensing in photosynthetic eukaryotes. Frontiers in Ecology and Evolution, 2, 1-13.
Shubert, E. & Gärtner, G. (2015). Nonmotile Coccoid and Colonial Green Algae. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.
Wehr, J. D., & Sheath, R. G. (2015). Introduction to the Freshwater Algae. In J. D. Wehr, R. G. Sheath, & J. P. Kociolek (Eds.), Freshwater Algae of North America: Ecology and Classification (2nd ed). Waltham, MA: Elsevier.