by Dr. Mark Edwards

Algae are living plants that break the rules for plant classification because they evolved in many different forms—cells, multicellular plants, bacteria and in nearly infinite combinations. While the various species share certain characteristics, different algae, even of the same species, display extraordinary variety in shape, size, structure, composition and color.

A single algal species may change shape, composition and color in a single day based on culture variables such as available light energy, nutrients, temperature and acidity, pH. Similar to all living organisms, when algae are stressed, they switch to survival mode, which changes the speed and composition of cellular metabolism. Stressors may cause algae to store more oil at the expense of proteins or carbohydrates, to use for energy at a later time. Some algae seem to accumulate more oil in order to rise to the top of the water column where they can harvest more solar energy.

The classification of algae into taxonomic groups follows the same rules used for the classification of land plants. Land plant classification came before algae because many nano-sized algae species could not be seen prior to advanced microscopes. The major algal groups are distinguished on the basis of pigmentation, shape, structure, cell wall composition, flagella characteristics, products stored and method of propagation.

Algae display so many variations, even within each species, that they express exceptions to nearly every classification rule. Interestingly, many species can change the way they propagate based on ambient conditions. When conditions are good, they propagate sexually. When conditions degrade, they are able to use one or more asexual methods such as cell division, fragmentation or spores.

The ability to see minute differences in algal cells with the electron microscope has changed classifications substantially since the 1960s. Classification changes continue as new differentiators are discovered.

Algae are differentiated from other plants because they generally:

  • Display the ability to perform photosynthesis with the production of molecular oxygen, which is associated with the presence of chlorophyll a, b or c;
  • Do not have specialized transport tissues or organs consisting of interconnected cells that move nutrients and metabolites among different sites within the organism;
  • Reproduce sexually or asexually to produce gametes that generally are not surrounded by protective multicellular parental tissue.

Land plants evolved from algae about 500 million years ago and evolved specialized cells for absorbing and moving nutrients and for reproduction. Algae are distinguished from the higher plants by a lack of true roots, stems or leaves. Some seaweed, such as kelp, appear to have leaves, but they are pseudo leaves made up of the same cellular structure as the rest of the plant. Scientists believe macroalgae — seaweeds — developed in parallel evolution with land plants.

Algal species culture collections are available at The University of Toronto, U.C. Berkeley, University of Texas, University of Copenhagen, the Scottish Marine institute, The Chinese Academy of Sciences , the University of Prague and the World Federation of Culture Collections. Most collections with provide composition and culturing information, culture sales, descriptive details and pictures. The excellent collection at the University of Texas run by Professor Jerry Brand offers a wide set of searchable parameters. The Algal Image Laboratory run by Dr. Rex Lowe at Bowling Green provides digital images of algae at no charge for educational purposes.

Many species are single-celled and microscopic including phytoplankton and other microalgae while others are multicellular and may grow as tall as trees such as kelp. Phycology, the study of algae, includes the study of prokaryotic forms known as blue-green algae or cyanobacteria. Some algae also live in symbiosis with lichens, corals and sponges. The basic single-celled organism, algae, has the general appearance illustrated in the figure.

Algae Cell

Algae Cell

Eukaryotic green algae (Greek for “true nut”) plants are structured like a nut with a shell protecting their genetic material, which is arranged in organelles. Green algae create discrete structures with specific functions and have a double membrane-bound nucleus or nuclei. The prokaryotic cells of blue-green algae, cyanobacteria, contain no nucleus or other membrane-bound organelles.

Algae can be lively little critters even though they are not animals. Many can swim, such as dinoflagellates that have little whip-like structures called flagella, which pull or push them through the water. Some algae squish part of their body forward and crawl along solid surfaces. A few algae can even form eye buds that can detect light, which is critical for their energy supply.

Other species are made of fine filaments with cells joined from end to end. Some clump together to form colonies while others float independently. Seaweeds may grow in nearly any shape such as cones, tubes, filaments or circles. Algae form many more shapes than land plants and may change the shape or structure to adapt to local conditions. Major steps in cell complexity occurred with the evolutionary progression from a virus to bacterium and then from the prokaryotic cells of bacteria to the eukaryotic cells of algae. Cell walls enable algae to protect itself from the surrounding environment, typically water and pressure, called osmotic pressure.

Algal Cell Walls

Algal Cell Walls

Cell walls regulate osmotic pressure produced by water trying to flow in or out of the cell through its semi-permeable membranes due to a differential in the solution concentrations. Algae typically possess cell walls constructed of cellulose, glycoproteins and polysaccharides. Some species have a cell wall composed of silicic (silicon) or alginic acid.

Red algae, for example, are a large group of about 10,000 species of mostly multicellular, marine algae, including seaweed. These include coralline algae, which live symbiotically with corals, secrete calcium carbonate and play a major role in building coral reefs. Red algae such as dulse (Palmaria palmata) and laver (nori or gim) are a traditional part of European and Asian cuisine and are used to make other products such as agar, carrageenans and other food additives.

The broad algae classification includes:

  • Bacillariophyta — diatoms
  • Charophyta — stoneworts
  • Chlorophyta — green algae
  • Chrysophyta — golden algae
  • Cyanobacteria — blue-green
  • Dinophyta — dinoflagellates
  • Phaeophyta — brown algae
  • Rhodophyta — red algae
Diatoms, stoneworts and dinoflagellates

Diatoms, stoneworts and dinoflagellates

Green algae evolved with chloroplasts, which enables photosynthesis and greatly enhances available O2. Blue-green algae have received most of the recent research because many scientists trained in bacteria research have begun studying the commercial value of this plant, classified as both a blue-green algae and bacteria; cyanobacteria.

Prochlorococcus, a blue-green algae may be the smallest organism on Earth, only 0.6 microns (millionths of a meter), but it is one of the most abundant organisms on the planet. A single drop of water may contain more than 100,000 of these single-celled organisms. Sallie Chisholm at MIT studies Prochlorococcus and says that trillions of these tiny cells make up invisible forests and provide about half the photosynthesis in the oceans.

Algae Classification

Taxonomic GroupChlorophyllCarotenoidsStorage products
Bacillariophytaa, cβ-carotene, ± -carotene rarelyfucoxanthinChrysolaminarin oils
Chloro phycophyta (green algae)a, bβ-carotene, ± -carotene rarely carotene and lycopene, luteinStarch, oils
Chrysophycophyta (golden algae)a, cβ-carotene, fucoxanthinChrysolaminarin oils
Cyanobacteria (blue green algae)a, cβ-carotene, phycobilins
Phaeco phycophyta (brown algae)a, cβ-carotene, ± fucoxanthin, violaxanthinLaminarin, soluble carbohydrates, oils
Dinophyta (dinoflagellates)a, cβ-carotene, peridinin, neoperididnin, dinoxanthin, neodinoxanthin.Starch, oils
Rhodo phycophyta (red algae )a, rarely dβ-carotene,zeaxanthin, ± β caroteneFloridean starch, oils

The green often associated with algae comes from chlorophyll but algae also contain pigments of many colors, especially cyan, red, orange, yellow, blue and brown. Some varieties are colorless. Green algae appears green because green is the only color of light it does not absorb. Red algae absorb a full spectrum of colors and reflect red. Red algae can grow deeper in the oceans than most other species because they are equipped to absorb the blue light that penetrates deep in the ocean.

Algae use pigments to capture sunlight for photosynthesis but each pigment reacts with only a narrow range of the spectrum. Therefore, algae produce a variety of pigments of different colors to capture more of the sun’s energy. Algae channels light into chlorophyll a, which converts light energy into high-energy bonds of organic molecules.

Green, Blue and Red Algae

Green, Blue and Red Algae

Algae provide color to herbivores that feast on them. Algae give the greenish cast to the white fur of the well-known giant sloth. Algae live in the hollow hairs of polar bears and provide the pink pigment for flamingos, which they consume in both shrimp and algae. Similar algal carotenoids give the pink pigmentation to salmon.

Arizona’s Palo Verde nuclear power plant attracted a pink flamingo to its cooling ponds several years ago. The poor bird turned white and created worldwide press speculation about possible radiation leaks. Fortunately, a biologist figured out the ponds lacked sufficient beta-carotene in the algae to sustain the bird’s pink coloration. The flamingo flew to another pond with algae and quickly regained its pinkness.

Algae may grow in symbiosis with fungus to create lichen – the colorful rough material on the sunny side of rocks and trees. Algae and the fungus share a mutual dependence as the algae produces food for both plants and in exchange, gets water and minerals from the fungus. The fungus also provides critical protection against desiccation – drying and dying in the sun.

The use of algae-lichen plants for pigments and dyes pre-dates Julius Caesar. The classic red color of Roman tunics came from pigments extracted from lichens known as urchilles. Roman women valued the plant and used it as rouge to give their faces more color. Nearly all modern cosmetics contain algae components to improve color, emulsification and/or moisture retention.