We all know plants like pines, oaks or beech. We all know seaweed such as sea lettuce, wakame or others used in Japanese cuisine. But do we really know what plants and algae are? Well, as we might well suppose, the answer is no. All the organisms mentioned above are evolutionarily related and belong to the same group, that of plants. However, algae are also called another group of organisms less known and less evolutionarily related, cyanobacteria, belonging to the kingdom of bacteria. If you want to know what are the similarities and differences between algae and plants.
Evolutionary history to better understand plants and algae
Molecular analysis of ribosomal RNA from living beings at the end of the 1970s allowed its division into three large domains: Bacteria, Archaea and Eukarya. The first two, Bacteria and Archaea, are formed by prokaryotic organisms and the third by eukaryotic organisms. The Eukarya domain harbors a great biological diversity that includes unicellular and multicellular organisms, autotrophs and heterotrophs .
The first eukaryotic organisms according to the Endosymbiosis Theory (Lynn Margulis, 1967) come from the fusion of two prokaryotic organisms , one of which had the ability to breathe oxygen, which gave rise to mitochondria. Later, this primitive eukaryote would engulf a photosynthetic cyanobacterium, which became integrated into the cell and gave rise to the current chloroplasts of plant cells .
The knowledge of the origin of plant cells and subsequent phylogenetic analyzes have allowed us to know the kinship that exists between plant species and how and when they have diversified. These analyzes suggest that green algae and land plants, red algae (rhodophytes), and a small group of freshwater unicellular algae (glaucophytes) are descended from a common ancestor . This common ancestor would be the first eukaryote to acquire chloroplasts about 1.5 billion years ago. This monophyletic group is called the Plantae or Archaeplastida.
The first of all is to explain a series of concepts widely used in evolutionary biology and taxonomy that help us better understand the kinship relationships between species: monophyletic, paraphyletic and polyphyletic groups. A monophyletic group is one that includes a common ancestor and all its descendants. A paraphyletic group does include the common ancestor but not all descendants, and a polyphyletic group includes species with different ancestors.
Having understood this, we are going to see what a plant is and what an alga is . For plants to be a monophyletic group, terrestrial plants, called Embryophytes, Glaucophytes, Rhodophytes and Chlorophytes must be included. The terrestrial plants or embriofitas include nonvascular plants and bryophytes (liverworts and mosses), and vascular as ferns and seed plants (gymnosperms and angiosperms). However, terrestrial plants would be a paraphyletic group by not including all the descendants of the ancestor that originated them.
In contrast, the concept of “alga” broadly includes all photosynthetic eukaryotic organisms other than land plants. In this way the algae would form a paraphyletic group. But they are also algae, or rather we call them that, cyanobacteria , prokaryotic photosynthetic organisms, which have a very different origin from the rest of the other algae (Glaucophytes, Rhodophytes and Chlorophytes). In this way, what we call algae would correspond to a polyphyletic group without any evolutionary or taxonomic meaning.
Therefore, we could conclude that plants or the monophyletic group Plantae includes all terrestrial and aquatic plants (Embryophytes), and some algae (Chlorophytes, Rhodophytes, Glaucophytes). And on the other hand, that the algae group is formed by several photosynthetic eukaryotic descendants of the original ancestor of the Plantae group and by cyanobacteria, photosynthetic prokaryotic organisms.
Well, once we know what plants and algae are, we are going to see their similarities and differences. In this article, eukaryotic algae will be discussed, since cyanobacteria are less related organisms and for this we should better talk about similarities and differences between prokaryotic and eukaryotic organisms.
We start with the similar points between these two types of living things. These are the main similarities between plants and algae :
- They have chloroplasts with two membranes. The existence of the two membranes suggests that in this group the organelles that enable photosynthesis evolved from an endosymbiotic event between a primitive eukaryotic ancestor and photosynthetic cyanobacteria. The chloroplasts of plant cells have chlorophyll.
- Chlorophytes, Rhodophytes, Glaucophytes, and Embryophytes store starch as a reserve carbohydrate.
- Cell mitochondria often have flattened ridges. Mitochondria are the organelles where cellular respiration takes place , a process by which the cell consumes oxygen and organic matter in exchange for energy.
- Cell walls are made up of cellulose polysaccharides.
- They carry out photosynthesis . Thanks to solar energy they fix CO2 and produce oxygen and organic matter that they will need to carry out cellular respiration and obtain energy.
- They are autotrophs , that is, they make their organic matter from inorganic. Concept related to photosynthesis.
- Both algae and plants can live in aquatic and terrestrial environments .
Finally, we indicate the main differences between plants and algae . Before, we return to highlight what has been explained in the definition. The terrestrial plants , understood as embriofitas, fomarian paraphyletic while algae , including cyanobacteria, would form a polyphyletic group. Therefore, we are going to see the differences between Embryophytes (Terrestrial Plants) and algae.
- Differences in the level of structural complexity . All embryophytes are multicellular while algae can be multicellular or unicellular , like glaucophytes.
- Embryophytes, as their name suggests, go through an embryonic stage in some of the phases of their life, while this does not happen in algae. In embryophytes, during their development, the embryo develops and gives rise to a diploid multicellular sporophyte.
- Embryophytes can develop specialized reproductive structures , such as flowers .
- Although algae and embryophytes share photosynthetic pigments, some such as phycobilins are unique to algae such as Rhodophytes and Glaucophytes, although these can also be found in cyanobacteria.
- Although we have commented that both algae and embryophytes share the same habitats , it is true that algae generally inhabit aquatic environments and plants have adapted much better to the terrestrial environment.
- Algae do not have true tissues , tissue being understood as a group of specialized cells that perform a specific function. In the case of cells, the tissues of the leaves, stems and roots are different.