Algae

What are Algae?

The term “algae” does not refer to a single group of closely related organisms, but rather describes a collection of multiple unique evolutionary lineages, all of which share some fundamental characteristics. Groups of this nature—containing different lineages which are not united by a common ancestor (that is not also the common ancestor of organisms in other groups)—are called polyphyletic. Different phycologists (scientists who study algae) may accept different definitions of what algae are. For instance, some sources assert that all algae are eukaryotic, and therefore cyanobacteria—which are prokaryotic—are excluded from the group. For the purposes of this site, algae are defined by the unifying characteristics given by Wehr & Sheath (2015): they are photosynthetic organisms which possess chlorophyll a (a photosynthetic pigment) and produce oxygen as a byproduct of photosynthesis; they have no vascular system, and their vegetative structures are simple; they are primarily aquatic; there is no layer of sterile cells surrounding their reproductive structures. Note that we are not excluding prokaryotes, which is to say we are including cyanobacteria. There are exceptions to these criteria, as is often the case when biologists’ definitions are imposed on groups of organisms. The nature of most of these exceptions is beyond the scope of this website, but the one to be aware of is that many types of algae are not strictly aquatic—some even inhabit the soils of deserts! While our focus is on aquatic—specifically freshwater—algae, many taxa can be found in both aquatic and terrestrial habitats.

Within the algae are multiple distinct groups, each with their own defining traits and evolutionary roots. For all groups other than the cyanobacteria, photosynthetic pigments are localized in membrane-bound organelles called chloroplasts. The other algae have cyanobacteria to thank for this sunlight-harvesting machinery: the first eukaryotic cell to acquire a chloroplast did so by engulfing a cyanobacterium! The cyanobacterium and the eukaryotic cell that engulfed it ultimately struck up an endosymbiotic relationship, wherein the cyanobacterium lived and photosynthesized inside of its new eukaryotic partner. Over time, the formerly free-living cyanobacterium lost the ability to survive outside of its eukaryotic host, and came to live only as an organelle within the host cell. In this way, the eukaryote’s chloroplast was acquired by primary endosymbiosis. Currently, just one cyanobacterial primary endosymbiosis is thought to have occurred (Rockwell et al., 2014).

This eukaryotic organism, which acquired chloroplasts via cyanobacterial primary endosymbiosis, is the common ancestor of three groups of algae, collectively known as Archaeplastida: red algae, glaucophytes, and green algae—and, by extension, land plants, which emerged from a lineage of green algae (Rockwell et al., 2014). But how did all the other types of algae—like dinoflagellates, diatoms, and euglenoids, just to name a few—end up with chloroplasts? Unlike the single event that ultimately gave rise to all the members of Archaeplastida, other lineages’ chloroplasts were obtained through a variety of independent events, each of which involved a heterotrophic eukaryote which acquired a photosynthetic eukaryotic endosymbiont (Yamada et al., 2019). For example, in the case of the photosynthetic euglenoids, a heterotrophic euglenoid engulfed a green algal cell and ultimately retained its chloroplast (Triemer & Zakryś, 2015). This is an example of chloroplast acquisition through secondary endosymbiosis: the host cell engulfed a eukaryotic cell, whose chloroplast was derived from the cyanobacterial primary endosymbiosis. Other groups, such as the Cryptomonads, also obtained chloroplasts via secondary endosymbiosis, but with a red algal endosymbiont (Clay, 2015).