The ancestor of Plantae was unicellular and may have been similar in general form to the modern glaucophytes (Figure 27.2). These microscopic freshwater algae are thought to be the sister group of the rest of Plantae (see Figure 27.1A). The chloroplast of glaucophytes is unique in containing a small amount of peptidoglycan between its inner and outer membranes—the same arrangement found in cyanobacteria. Peptidoglycan has been lost from the remaining photosynthetic eukaryotes.

In contrast to the glaucophytes, almost all red algae are multicellular (Figure 27.3). Their characteristic color is a result of the accessory photosynthetic pigment phycoerythrin, which is found in relatively large amounts in the chloroplasts of many red algae. In addition to phycoerythrin, red algal chloroplasts contain chlorophyll a as well as several other accessory pigments.

The red algae include species that grow in the shallowest tide pools as well as the photosynthesizers found deepest in the ocean (as deep as 260 m if nutrient conditions are right and the water is clear enough to permit light to penetrate). A few red algae inhabit fresh water. Most grow attached to a substrate by a holdfast.

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Despite their name, red algae don’t always appear red in color. The ratio of two pigments—phycoerythrin (red) and chlorophyll a (green)—depends largely on the intensity of light that reaches the alga. In deep water, where light is dim, algae accumulate large amounts of phycoerythrin and appear red. But many species growing near the surface contain a higher concentration of chlorophyll a and thus appear bright green.

The remaining algal groups in Plantae are the various “green algae.” Like land plants, the green algae contain both chlorophylls a and b and store their reserve of photosynthetic products as starch in chloroplasts. All the groups that share these features are commonly called green plants because both of their photosynthetic pigments are green.

The largest clade of “green algae” is the chlorophytes. There are more than 17,000 species of chlorophytes, most of which are aquatic (some are marine, though more are freshwater forms), although there are a few terrestrial forms that live in moist environments. Chlorophytes range in size from microscopic unicellular forms to multicellular forms many centimeters long and display an incredible variety of shapes and body forms. Surprisingly large and well-formed colonies of cells are found in some unicellular freshwater groups, such as the genus Volvox (Figure 27.4A). Certain cells in these colonies are specialized for reproduction. The cells in these colonies are not differentiated into specialized tissues and organs, as in land plants and animals, but they show vividly how the preliminary step of this great evolutionary innovation might have been taken.

Volvox is a colonial unicellular chlorophyte, but there are also many true multicellular species of chlorophytes. Some of these are filamentous. Others, like species in the genus Ulva (Figure 27.4B), grow into thin, membranous sheets up to 30 centimeters in width.

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Biologists are exploring mass production of several species of chlorophytes as a source of biofuels. One of the biggest limitations in using algae for biofuels relates to the cost of providing an appropriate growth medium for the algae. Growth media typically require fresh water and abundant sources of carbon, nitrogen, and phosphorus. These nutrients are already present in most municipal wasterwater supplies, and the removal of these nutrients by growing algae would help clean the water supply. Investigating Life: Can Chlorella Algae Be Grown in Municipal Sewage Wastewater for Biofuel Production? explores the poential of municipal wasterwater as a growth medium for algae used in biofuel production.