Figure 10.37 Cryptosporidium oocysts. (SEM copyright Dennis Kunkel Microscopy, Inc.)

stems, and leaves, although some of the brown algae, especially, have superficially similar structures. Six phyla (Table 10.10) are generally recognized by phycologists (scientists who study algae). The phyla are distinguished mainly on pigments, food storage materials, cell walls, and flagella. For the most part, the phyla are not considered to be closely related; in fact, it is now believed that they evolved separately from different nonphoto-synthetic ancestors.

In Eukarya, photosynthesis occurs within membrane-bound organelles called chloro-plasts. Since the photosynthetically active range (PAR) of light absorbed by algae is roughly from wavelengths of 400 to 700 nm, several different types of pigments are involved. These include various chlorophylls (including a, which is found in all algae, and b, c, or d), caretenoids, and phycobilins, which are associated with proteins to form light-harvesting complexes. Some of these pigments may also provide protection against harmful ultraviolet radiation. It is the combination of photosynthetic pigments present within algal cells that give them their distinctive coloration (e.g., green, red, brown).

Algae are the major primary producers of organic material in most aquatic and marine ecosystems, and hence many other organisms are directly or indirectly dependent on them. Some algae may also be found in or on soil and on terrestrial surfaces (e.g., trees, rocks). On a global level, they are of critical importance because of their production of oxygen and absorption of carbon dioxide.

From the perspective of environmental engineering and science, algae also can be problematic. If waters receive increased amounts of nutrients from wastewater discharges or runoff, excessive growth of algae, cyanobacteria, and plants can occur. This can lead to a number of problems, ranging from physical interference with recreation (or even navigation) to fish kills from oxygen depletion or production of toxins. In fact, this problem, referred to as eutrophication (Section 15.2.6), can even lead to the rapid filling in of a lake or pond, converting it eventually to a wetland and then dry land. On the other hand, algae are also utilized in some wastewater treatment processes to produce oxygen or remove nutrients. Standard Methods (Clesceri et al., 1998), a reference book available

TABLE 10.10 The Algae








Common name


Brown algae

Golden algae/ diatoms


Red algae

Green algae


chl ,-b

chl.-c, xanthophylls



chl -d, phycobilins

chl ,-b

Cell wall



Silica (diatoms)

Cellulose plates



Main storage





a-1,4- and a-1,



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