Difference Between Green Algae And Brown Algae
The common pond scum Spirogyra, sea lettuce (Ulva), and Volvox are just some of the 7,000 species of green algae that make up the phylum Chlorophyta.
The close resemblance of green algae and true plants is no coincidence. Scientists have little doubt that the first plants evolved from green algae. As evidence, green algae and plants use identical photosynthetic pigments—chlorophylls a and b, which produce their characteristic green color, as well as red, orange, and yellow carotenoids. Like plants, all green algae store their food as starch, and many have plantlike cell walls made of cellulose.
Most green algae grow in freshwater—typically as slime on submerged rocks and scum on the water’s surface. These scums and slimes are actually colonies of thousands to millions of microscopic algae such as Spirogyra. Spirogyra is an example of a filamentous green algae. Each organism consists of a single, very long cell, or filament. Each colony consists of a mass of these intertwined unicellular organisms.
Other green algae, such as sea lettuce, or Ulva, grow in salt water. This green seaweed forms delicate, leaflike blades just two cells thick. It is most often seen growing in shallow water along the seashore, where it anchors itself to pilings and rocks.
Still others have made their way onto land, where they survive in wet places, from damp soil and leaves to the melting surface of snow. Several species of green algae have forged symbiotic (mutually beneficial) relationships with fungi to form lichens. A few even grow on the damp parts of animals, such as the fur of tree sloths, or cover the shells of turtles. The sloths and turtles benefit from the green camouflage!
A great many green algae are seldom seen. They are one-celled organisms that lead solitary lives. Some swim through the water like microscopic tadpoles. Others form spinning colonies made of tightly connected individuals.
The classic example of a one-celled green algae is Chlamydomonas, found in stagnant ponds and lakes the world over. Although small, it can move rapidly, darting through the water by beating its two whiplike flagella, or “tails.” A single chloroplast fills most of Chlamydomonas’ body. Near the chloroplast is a red eyespot, or stigma. Chlamydomonas reproduces both sexually and asexually. During asexual reproduction, its nucleus divides twice, producing four identical daughter cells. Sexual reproduction involves the fusion of two individuals belonging to different mating strains.
Volvox is an example of colonial green algae. Each colony is a hollow ball made up of 500 to 50,000 one-celled individuals. The cells share nutrients through openings in their cell walls. Together, the cells spin their colony through the water by each beating its flagella.
The brown algae (phylum Phaeophyta) are multicellular organisms, many of them growing to more than 300 feet (90 meters) in length. Their characteristic color comes from fucoxanthin, a carotenoid pigment. This pigment absorbs light in the green range of the spectrum—namely the wavelengths that are most abundant below the water’s surface. (In contrast, chlorophyll reflects green light, giving plants and green algae their color.) Brown algae store their food as an unusual sugar called laminarin. Their cell walls are a mixture of cellulose and complex sugars.
The best-known brown algae are the mineral-rich kelps so common in northern oceans. Some form enormous floating forests many miles wide and more than 200 feet (60 meters) deep. Many kelp have distinct parts such as holdfast (a rootlike anchor), stipe (a ropelike stem), and blade (a leaflike sheet). The blades and stipes can have air-filled bladders that act as floats. Dense forests of kelp provide lush habitats for many marine animals.
Red algae are the most common seaweed in tropical seas, and also occur in colder waters. Their distinctive colors come from their unique combination of pigments: chlorophyll a, carotenoids, and phycobilins. These pigments are similar to those found in primitive cyanobacteria (blue-green algae), which may be a distant relative.
Red algae always grow attached to rocks and other objects. Unlike other seaweeds, they cannot survive floating freely in the water. This is because they need the tug of tides and currents to move gases in and out of their tissues. Red algae such as dulse (Rhodymenia palmata) and nori (Porphyra species) are important foods. Red seaweeds are also processed into extracts used as gelatin substitutes in puddings, ice cream, and other food products.