Not all photosynthetic organisms found underwater are algae. In some cases, these are aquatic plants and not algae. But, how are they different from each other? Are “algae” and “aquatic plants” two separate categories, like cats and dogs? Or are all algae actually aquatic plants, but certain aquatic plants are not algae? To answer, we first need to know what an algae is and what a plant is. And they are not easy terms to define.
Defining “algae” and “plant”
An algae is a eukaryotic organism —that is, whose cells have a nucleus—, with photosynthetic capacity and unicellular or multicellular organization, but whose cells do not form complex organs, such as roots, stems or leaves. This definition de facto excludes cyanobacteria, formerly classified as cyanophyceae or ‘blue-green’ algae, as they are prokaryotic cells that lack a nucleus. Almost all algae are aquatic organisms.
However. There is no specific and defined group in biology that responds to the term “algae”. On the contrary, there are many groups, separated from each other, that agree with the definition of “algae”. For example, algae are haptophytes, cryptophytes, glaucophytes, orcophytes —which include brown algae and diatoms—, rhodophytes —red algae—, or chlorophytes —green algae—.
So the algae are a heterogeneous set of groups of species not necessarily related to each other from the evolutionary point of view, and, therefore, do not form a defined phylogenetic group. This is a useful definition, but makes no real taxonomic sense.
And regarding the definition of “plant”, the issue is even more complicated. Although the kingdom Plantae traditionally exists, phylogeny-based classifications—those that classify species according to their position on the evolutionary tree, such as classifying members of a family according to their position on the family tree—have blurred the boundaries of this group. Depending on whether it is analyzed more narrowly or more broadly, the Plantae group is today considered as a possible synonym for three phylogenetic groups that are defined: the embryophytes , the green plants, and the archeoplastids .
Embryophytes, in a very strict sense
In 1969, the ecologist Robert Harding Wittaker proposed a revolutionary approach at the time for the classification of living things: the five kingdoms, namely monera, protist, plants, fungi and animals.
Today we know thanks to phylogeny that these five kingdoms do not faithfully represent reality. However, Whittaker’s definition of plants turned out to be relatively valid; He considered “plants” those photosynthetic organisms with cell walls of chlorophyll and whose cells differentiated into tissues that formed organs .
For Whittaker, all algae were part of the kingdom Protista.
Following this definition, plants are made up solely of mosses, liverworts, and vascular plants —lycopods, ferns, horsetails, and seed plants—, as well as related fossil groups.
So organisms such as water lilies , duckweed or species of the genus Posidonia , which have organs made up of tissues, such as leaves and roots, and also, since their evolutionary ancestors are terrestrial plants, would be aquatic plants, but not algae . And no algae would be an aquatic plant.
This classification of Plantae sensu strictissimo agrees with the phylogenetic group Embryophyta .
Green plants, plants in the strict sense
A looser classification than Whittaker’s was proposed by Herbert Faulkner Coperland in 1956 . For him, the term “plant” should include green algae and land plants. Decades before they began to use phylogeny to establish classifications, it was already guessed that land plants must have originated from green algae.
By this definition, water lilies are still aquatic plants and not algae, but green algae would be plants . However, it excludes red algae, brown algae, and all others.
The phylogenetic group with which this classification of the Plantae sensu stricto agrees is called the Viridiplantae .
The archeplastids, plants in the broad sense
If we take the broadest sense in which plants have been defined, we will follow the Cavalier-Smith definition , which defined plants as the group of living beings comprising organisms that acquired chloroplasts by direct symbiogenesis with cyanobacteria.
By this definition, red algae and green algae are plants, just like a pine tree or poplar. However, it excludes other algae such as cryptophytes —whose symbiogenesis was carried out with red algae— or brown algae —with chloroplasts originating from cryptophytes—. These are not plants, whatever definition is taken.
The phylogenetic group with which this classification of Plantae sensu lato agrees would be the one known as Archaeplastida .
aquatic plants and algae
Certain organisms, such as the aforementioned water lilies, duckweed or species of the Posidonia genus , do not fall under the definition of “algae”, because whichever definition of “plant” is chosen, they are organisms with vascular tissues and defined organs such as leaves, stems or roots. These are aquatic plants , and are in no sense algae.
But regarding algae, depending on the definition taken and the group of algae referred to, they can be defined as aquatic plants, or not. If we take the strictest definition, that of Whittaker, no algae is a plant. But if we take the broader Cavalier-Smith definition, red and green algae are plants—and they are aquatic.
In the end, the definition of “plant” is confusing , useful to speak in everyday terms, but cumbersome when approached from an academic perspective, focused on the classification of living beings. In these situations, it is better to avoid talking about “plants”, and instead use the taxonomic groups that are well defined, be it Embryophyta, Viridiplantae or Archaeplastidia.
References:
Cavalier-Smith, T. 1981. Eukaryote kingdoms: Seven or nine? Biosystems , 14 (3-4), 461-481. DOI: 10.1016/0303-2647(81)90050-2
Coperland, HF 1956. The classification of lower organisms . Pacific Books.
Whittaker, RH 1969. New Concepts of Kingdoms of Organisms: Evolutionary relations are better represented by new classifications than by the traditional two kingdoms. Science , 163 (3863), 150-160. DOI: 10.1126/science.163.3863.150
