Fresh water

Chlorophyll b; carotenes; xanthophylls

Fats; starchlike carbohydrates

Lacking, but elastic pellicle present

One to three anterior flagella

Asexual only by binary fission

a rock or some other firm substrate. Unlike a root, it is not used to obtain water and nutrients for the organism. Nutrients and water surround the organism and do not need to be drawn up from the soil. The stalk of an alga, known as the stipe, usually has leaflike structures or blades attached to it. The blades are the principal photosynthetic portion of an alga, and some also bear the reproductive structures. Many large algae have gas-containing bladders or floats that help them maintain their blades in a position suitable for obtaining maximum sunlight.

Cell Walls

Algal cell walls are rigid and for the most part composed of cellulose, often associated with pectin. Some multicellular species of algae such as some red algae contain large amounts of other compounds in their cell walls. Compounds such as carrageenan and agar are harvested commercially and commonly used in foods as stabilizing compounds. As described earlier, agar is also used to solidify growth media in the laboratory. It is useful because although it melts at 100°C, it stays in a liquid state at relatively low temperatures (45°C-50°C) so that nutrients can be added, and yet is solid at room temperatures to act as a growing surface in a Petri dish. ■ agar, p. 84 ■ Petri dish, p. 84

Diatoms are algae that have silicon dioxide incorporated into their cell walls. When these organisms die, their shells sink to the bottom of the ocean, and the silicon-containing material does not decompose. Deposits of diatoms that formed millions of years ago are mined for a substance known as diatomaceous earth, used for filtering systems, abrasives in polishes, insulation, and many other purposes.

Eukaryotic Cell Structures

As is true in all eukaryotes, algae have a membrane-bound nucleus. The genetic information is contained in a number of chromosomes that are tight packages of DNA with their associated basic protein. These chromosomes are enclosed in a nuclear membrane. ■ nucleus, p. 75

In addition, algae have other organelles in their cytoplasm such as chloroplasts and mitochondria. Chloroplasts contain chlorophyll as well as other light-trapping pigments such as carotenoids and phycocyanin. Photosynthesis occurs in the chloroplast. Respiration and oxidative phosphorylation occur in the mitochondria. ■ chloroplasts, p. 75 ■ mitochondria, p. 75

Algal Reproduction

Most single-celled algae reproduce asexually by binary fission, as do most bacteria (figure 12.5). The major difference between prokaryotic and eukaryotic fission involves events that take place with the genetic material within the cell. Recall from chapter 3 that in prokaryotic fission, the circular DNA replicates and each daughter cell receives half the original double strand of DNA plus a newly replicated strand. In eukaryotic organisms with multiple chromosomes, after the DNA is replicated, the chromosomes go through a nuclear division process called mitosis. This process ensures that the daughter cells receive the same number of chromosomes as the original parent.

Some algae, especially multicellular filamentous species, reproduce asexually by fragmentation. In this type of reproduction, portions of the parent organisms break off to form new organisms (see figure 12.5), and the parent organism survives.

Sexual reproduction also regularly occurs in most algae. During the process known as meiosis, haploid cells with half the chromosome content are formed. These cells are called gametes and when they fuse together they form a diploid cell with a full complement of chromosomes known as a zygote. Gametes are often flagellated and highly motile. Many algae alternate between a haploid generation and a diploid generation. Sometimes, as is the case with Ulva (sea lettuce), the generations look physically similar and can only be told apart by microscopic examination. In other cases, the two forms look quite different.

Paralytic Shellfish Poisoning

Although algae do not directly cause disease in humans, they do so indirectly. A number of algae produce toxins that are poisonous to humans and other animals. Several dino flagellates of

Binary fission

Binary fission


Figure 12.5 Binary Fission Is an Asexual Reproduction Process in Which a Single Cell Divides into Two Independent Daughter Cells

Fragmentation is a form of asexual reproduction in which a filament composed of a string of cells breaks into pieces to form new organisms.

Figure 12.5 Binary Fission Is an Asexual Reproduction Process in Which a Single Cell Divides into Two Independent Daughter Cells

Fragmentation is a form of asexual reproduction in which a filament composed of a string of cells breaks into pieces to form new organisms.

Figure 12.6 The Dinoflagellate Cymnodinium Scanning Electron Micrograph (4000x)

the group Pyrrophyta cause red tides, or algal blooms in the ocean. Red tides were reported in the Bible along the Nile and today seem to be spreading worldwide. In the warm waters of Florida and Mexico, red tides result from an abundance of Gymnodinium breve (figure 12.6). This dinoflagellate discolors the water about 17 to 75 km from shore and produces breve-toxin, which kills the fish that feed on the phytoplankton. It is unclear why algae suddenly grow in such large numbers, but it is thought that sudden changes in conditions of the water are responsible. The runoff of fertilizers along the waterways and coastlines may also be the cause of red tides. In addition, an upwelling of the water often brings more nutrients as well as the cysts that are resting, resistant stages of the G. breve from the ocean bottom to the surface. When these cysts encounter warmer waters and additional nutrients, they are released from their resting state and begin to multiply rapidly. Persons eating fish that have ingested G. breve and thus contain brevetoxin may suffer a tingling sensation in their mouths and fingers, a reversal of hot and cold sensations, reduced pulse rate, and diarrhea. The symptoms may be unpleasant but are rarely deadly, and people recover in 2 to 3 days.

Red tides caused by the dinoflagellate of the genus Gonyaulax are much more serious. Gonyaulax species produce neurotoxins such as saxitoxin and gonyautoxins, some of the most potent non-protein poisons known. Shellfish such as clams, mussels, scallops, and oysters feed on these dinoflagellates without apparent harm and, in the process, accumulate the neuro-toxin in their tissues. Then when humans eat the shellfish, they suffer symptoms of paralytic shellfish poisoning including general numbness, dizziness, general muscle weakness, and impaired respiration. Death can result from respiratory failure. Gonyaulax species are found in both the North Atlantic and the North Pacific. They have seriously affected the shellfish industry on both coasts over the years. At least 200 manatees died along the coast of Florida in the spring of 1996 as a result of red tide poisoning.

Another dinoflagellate, Pfiesteria piscida, usually is found as a non-toxic cyst or ameba in marine sediments. This organism changes when a school of fish approaches. The fish secrete a chemical cue that alerts the Pfiesteria to transform into a flagellated

304 Chapter 12 The Eukaryotic Members of the Microbial World zoospore. It then releases two toxins. One stuns the fish and the other causes its skin to slough away. The Pfiesteria then enjoys a meal on the fish's red blood cells and sexually reproduces. The toxins are so potent that researchers working with them in a laboratory were seriously affected. This organism now must be studied using the same precautions that are used to study the virus that causes AIDS.

Another algal toxin found in some species of diatoms has more recently been recognized to cause paralytic shellfish poisoning. This poison is domoic acid and is most often associated with mussels and other shellfish and crabs that accumulate the poison. Persons eating the shellfish suffer nausea, vomiting, diarrhea, and abdominal cramps as well as some neurological symptoms such as loss of memory.

State agencies constantly monitor for algal toxins, and it is wise to check with the local health department before harvesting shellfish for human consumption. Algal toxins may be present even when the water is not obviously discolored. Cooking the shellfish does not destroy these toxins.

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