Deciding what is and is not algae is complicated as the term encompasses organisms as different as the microscopic diatom to the plant-like giant kelp. Taxonomists, the scientists in charge of classification, often argue over how an organism should be classified, which makes understanding even harder for the layperson.
A General Definition of Algae
At the Tree of Life web project, David J. Patterson describes algae in the article “Algae: Protists with Chloroplasts” as photosynthetic eukaryotes that are not plants. He puts algae in the kingdom Protista which is generally thought of as a catch-all kingdom for organisms that don’t fall into the other more well-defined kingdoms.
In general algae lack the specialized roots, stems, and leaves of plants. Algae also tend to have a greater variety of pigments. The Center for Environmental and Molecular Algal Research or CEMAR holds that algae was once the name for aquatic photosynethetic organisms considered to belong to “lower” orders. However, recent molecular research shows algae to exist on many branches of the tree of life. Thus, scientists who study algae, or phycologists, have had a difficult time deciding on a modern definition for algae. However, algae is still a commonly used term and several types of algae are widely recognized.
Seaweeds Are Algae, Probably
Most taxonomists say seaweeds are algae and not plants. The University of Queensland’s Reef Education Network (REN) lists three main groups of algal seaweeds. These are Red Algae, Green Algae, and Brown Algae which respectively fall into the Phyla Rhodophyta, Chlorophyta, and Phaeophyta. Sea lettuce (Ulva lactuca) is a type of Green Algae while kelp such as Ecklonia radiatais a type of Brown AlgaeHowever, these classifications are in flux. While REN classifies Green Algae as a Protist, Richard M. McCourt et al. say in the Tree of Life article “Green Plants” that Green Algae belong to the Kingdom Plantae.
Microscopic Algae are Phytoplankton
Maryland’s Department of Natural Resources say on their webpage “Chesapeake & Coastal Bay Life” that algae make up the most well known group of phytoplankton. Phytoplankton are microscopic, photosynthetic, and form the base of most aquatic food chains. Diatoms and Dinoflagellates are two classes of algal phytoplankton. Phytoplankton are well known for their seasonal algal blooms which can sometimes cause dangerous red tides. Although they look very different, molecular studies show that Diatoms are closely related to Brown Algae.
An Algae Definition into the Future
For those wondering, “What is Algae?” there is no simple answer. Algae come in a vast variety of shapes, sizes and colors. Taxonomists themselves have no clear answer to “What is Algae?” and the definition continues to be somewhat fluid. Algal organisms exist in various branches of the tree of life under different kingdoms. However, there are several main types of algae still recognized including the seaweeds and phytoplankton organisms.
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