Plant-like protists are named algae. They include single-celled diatoms and multicellular seaweed. Algae are plant-like because they contain chloroplasts and produce food through photosynthesis. However, they lack many other structures of true plants such as roots, stems, or leaves. Some algae also differ from plants in being motile (motility is the ability of an organism to move independently, using metabolic energy) and they may move with pseudopods or flagella.
Algae plays a significant role as the primary producer in aquatic ecosystems. They are the main component of phytoplankton that contribute to the food base of most marine ecosystems. There are three distinct marine macroalgae which are brown (phaeophyta), green (chlorophyta), and red (rhodophyta) algae.
Brown Algae (Phaeophyta)
Brown algae, from the phylum Phaeophyta (meaning "dusky plants"), is the most prevalent type of seaweed. They are found in the waters of both temperate or arctic climates. These algae typically have root-like structures called "holdfasts" that are used to anchor the algae to a surface. They have differentiated tissues, including an anchoring organ, air pockets for buoyancy, a stalk, photosynthetic organs, and reproductive tissues that produce spores and gametes. Some examples of brown algae include sargassum weed, rockweed, and giant kelp, which can reach up to 100 meters in length. There are about 30 kelp varieties and they only grow in salt waters.
Kelp contains many important vitamins and minerals including vitamin K, vitamin A, vitamin C, folate, vitamin E, vitamin B12, vitamin B6, thiamin, riboflavin, niacin, pantothenic acid, iodine, calcium, magnesium, iron, sodium, phosphorus, as well as small amounts of zinc, copper, manganese, and selenium.
Red Algae (Rhodophyta)
There are more than 6,000 species of red algae. Their cell walls consist of cellulose and many different types of carbohydrates. The ability to absorb blue light allows red algae to live at greater depths than either brown or green algae. These algae also contain the pigment phycoerythrin which is the source of their unique colors. Unlike other algae, these eukaryotic cells lack flagella and centrioles. They reproduce asexually by monospores (walled, spherical cells without flagella) that are carried by water currents until germination. Red algae also reproduce sexually and undergo alternation of generations.
Red algae grow on solid surfaces including tropical reefs or attached to other algae. Coralline algae is a subgroup of red algae and is important in the formation of coral reefs. Several types of red algae are used in food additives, and some are regular parts of Asian cuisine. Examples of red algae include Irish moss, coralline (Corallinales), and dulse (Palmaria palmata).
Green Algae (Chlorophyta)
There are more than 4,000 species of green algae. Green algae can be found in marine or freshwater habitats, and some even thrive in moist soils. These algae come in three forms: unicellular, colonial, or multicellular. Multicellular species usually group in colonies ranging in size from four cells to several thousand cells. They have cell walls made of cellulose, and some species have one or two flagella. For reproduction, some species produce non-motile aplanospores that rely on water currents for transport, while others produce zoospores with one flagellum for swimming to a more favorable environment. Phytoplankton is made up of green algae and cyanobacteria, also known as blue-green algae.
Types of green algae include sea lettuce, horsehair algae, and dead man's fingers. Sea lettuce (Ulva lactuca) is commonly found in tidal pools. Codium is the favored food of sea slugs, while the species Codium fragile is commonly referred to as "dead man's fingers."
Dinoflagellates are one-celled aquatic organisms bearing two different flagella. The group is an important component of phytoplankton in all but the colder seas and is an important link in the food chain. Dinoflagellates also produce some of the bioluminescence sometimes seen in the sea. Under certain conditions, several species can reproduce rapidly to form water blooms or red tides that discolour the water and may poison fish and other animals. Dinoflagellates range in size from about 5 to 2,000 micrometres (0.0002 to 0.08 inch). Most are microscopic, but some form visible colonies. Nutrition among dinoflagellates is autotrophic, heterotrophic, or mixed; some species are parasitic or commensal. About one-half of the species are photosynthetic; even among those, however, many are also predatory.
The dinoflagellate cell is banded by a median or coiled groove, the annulus, which contains a flagellum. A longitudinal groove, the sulcus, extends from the annulus posteriorly to the point at which a second flagellum is attached. The nuclei of dinoflagellates are larger than those of other eukaryotes. So-called armoured dinoflagellates are covered with cellulose plates, which may have long spiny extensions; some species lacking armour have a thin pellicle (protective layer). Photosynthetic dinoflagellates have yellowish or brownish plastids (pigment-containing bodies) and may store food in the form of starches, starchlike compounds, or oils.
There are around 16,000 discovered species of Diatoms (class Bacillariophyceae). They are either unicellular or colonial. The silicified cell wall forms a pillbox-like shell (frustule) composed of overlapping halves (epitheca and hypotheca) perforated by intricate and delicate patterns. They have light-absorbing molecules (chlorophylls a and c) that collect energy from the sun and turn it into chemical energy through photosynthesis. Food is stored as oil droplets, and the golden-brown pigment fucoxanthin masks the chlorophyll and carotenoid pigments that are also present.
Diatoms produce 50% of the air we breathe. Through carbon fixation, they remove carbon dioxide (CO2) from the atmosphere. The CO2 is converted to organic carbon in the form of sugar, and oxygen (O2) is released. They also have ranges and tolerances for other environmental variables, including nutrient concentration, suspended sediment, flow regime, elevation, and for different types of human disturbance. As a result, diatoms are vital for assessment and monitoring biotic condition of waters.
1. Libretexts. “8.5: Algae.” Biology LibreTexts. Libretexts, August 15, 2020.
2. Kennedy, Jennifer. "Marine Algae: The 3 Types of Seaweed." ThoughtCo, Nov. 29,
3. Bailey, Regina. "7 Major Types of Algae." ThoughtCo. Sept. 12, 2018.
4. Fondriest Environmental, Inc. “Algae, Phytoplankton and Chlorophyll.”
Fundamentals of Environmental Measurements. 22 Oct. 2014.
5. “What Are Diatoms?” Diatoms of North America, 2020.