Edited by Katyanne M. Shoemaker
When we walk along the beach and see seaweed, we associate it with terrestrial plants. Afterall, scientific evidence strongly suggests that plants evolved from green algae in the Paleozoic Era. However, they are quite quite different in many ways. Algae, like terrestrial plants, are eukaryotic organisms (the cell has several organelles including a nucleus surrounded by a membrane) and photosynthetic autotrophs (produce their own food through photosynthesis).
The word algae comes from Latin and means "marine plant," but you must be aware that not all algae live in the water. Some live in terrestrial environments associated with fungi, in a mutually beneficial relationship, or symbiosis, forming so-called lichens.
One thing is important to keep in mind: while the plants belong to a single Kingdom, the Plantae, the term "algae" encompasses many distinct taxonomic groups in the Kingdom Protista, including the Stramenopila (brown algae and diatoms), Rodophyta (red algae) and Chlorophyta (green algae) (Nybakken & Bertness, 2005).
Thus, due to the complexity and constant taxonomic changes of these organisms, we will not go into details of classification of this polyphyletic group (they do not share a common ancestor) called "algae," but we will focus on its general characteristics.
Lichen on a rock. By: Wikipedia public domain.
Algae have several forms of stuctural organization. They can be found in unicellular forms such as diatoms and dinoflagellates, or as multicellular filamentous forms. They can form colonies that are physically united, and their organization can be defined between amorphous colonies that do not have defined numbers of cells, or those that present complex organization in number of cells and defined forms. They can also take planktonic or benthic forms (learn more about these forms here). The stalk may be divided into cells, or it may not and instead take a tubular shape (cenocytic). Among these various forms, it is common to hear the term “microalgae” when they are microscopic, and “macroalgae” when they are visible to the naked eye.
Usually, the macroalgae are confused with plants. One of the main characteristics that differentiates macroalgae from plants is their structure. They may appear similar, but the macroalgae do not have specialized organs and tissues, and they are not vascularized. They also do not have the capacity to form a structure with flowers, leaves, roots or a stem. The multicellular algae just have a stalk to support its filaments.
a) Multicellular algae, by Stef MaruchCC BY-SA 2.0 via Commons Wikimedia; b) Unicellular algae (dinoflagellate), by Pxhere CC BY 2.0; c) Multicellular algae, by Ronile Pixabay; d) Unicellular algae, by Prof. Gordon T. Taylor via Commons Wikimedia public domain.
Now, what about underwater plants? Are all of those green things in the aquarium algae? No! An example of an aquatic plant is Elodea, a common waterweed which is widely used to decorate aquariums and artificial aquatic environments. This plant belongs to the group of Angiosperms, of the Kingdom Plantae.
This kingdom is comprised of vascular and avascular photosynthetic organisms, that is, with or without the presence of vessels that are responsible for the conduction of mineral salts and water. Vascularization is also responsible for the presence or absence of the reproductive parts; in the case of Angiosperms these reproductive parts generate flowers, leaves and fruit.
The leaves of the submerged aquatic plants are generally very thin and stubby, allowing them to support turbulence and oscillations of the water, without tearing. The leaves of the aquatic plants also have a permeable surface, which aids in internal circulation of the air.
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