Economic importance of algae

Algae use chlorophyll and sunlight to make food by means of photosynthesis.

Besides making food, algae also give off oxygen. Oxygen is a by-product of photosynthesis. Many animals depend on algae for both food and oxygen. Algae are important to animals for this reason. In fact, some people believe that if there were no algae (both one-celled and many-celled ones), there would not be enough food and oxygen for all the animals on the earth.

Some people depend on algae for food. In Japan, almost everyone eats a food known as nori. It comes from laver, a kind of red algae. The Japanese eat 130 000 tons of this seaweed each year. It is eaten in soups and biscuits. People in Korea, Ireland, Great Britain, and New Zealand also eat it.

Have you ever eaten algae? You might be surprised to find out that you have. In the many countries, algin is used to thicken foods such as ice-cream. Algin comes from brown algae. Some ingredients that may be found in food are gelose, agar, carrageenan, nori (red algae), kombu (brown algae).

Algae are important in other ways too. Some are used as fertilizers. Some are a source of iodine. Some are used to make certain drugs. Others can be used to make paints and soap.

Scientists who study bacteria use algae, too. They grow bacteria on jellylike agar. It comes from red algae.

Algae and products made from algae have a long history of usefulness. Dulse, a red alga, has been used as food for people and domestic animals for centuries. The algae now most widely used for food is nori, grown chiefly by coastal villagers in Japan. Nori is dried into sheets that are used in soups and biscuits and as a flavoring in many foods.

Although most algae are low in protein, they contain concentrated minerals and some starch. In many countries, dried algae are ground into a powder and added to animal feed as a mineral supplement. Algae yield valuable extracts. Agar, an extract taken from red algae, is a jellylike substance that is used for laboratory cultures in which bacteria are grown.

Carrageenin is another extract from red algae that is used to keep small particles in suspension in many foods. For example, carrageenin prevents chocolate from separating out in chocolate milk. Carrageenin is a common ingredient in jams and jellies, instant coffee, honey, wine and ice cream.

Fungi

Fungi are eukaryotic organisms; most species are multicellular. The cell walls of most fungi contain a hard substance called chitin. Chitin is found only in fungi and in the hard outer skeletons of insects.

The body of a typical fungus consists of many individual filaments called hyphae. Hyphae contain cytoplasm and one or more nuclei. Hyphae secrete enzymes that digest food. The fungus then absorbs the nutrients from the food through its cell walls.

Intertwined hyphae form the body of the fungus, or mycelium. Most of a fungus lives under the sub­strate, or material in which the fungus is growing. The visible part contains the spore-producing structures and is called the fruiting body. Saprophytic fungi feed on dead matter. Parasitic fungi feed on living organisms.

Nutrition. Fungi are nonmotile organisms that obtain food by decomposing organic matter. Fungi were once considered plants, but studies later revealed that fungi have none of the characteristics that plants possess. Unlike most plants, fungi lack chloroplasts and cannot carry out photosynthesis. Neither do fungi have many animal characteristics. Because of their unusual combination of traits, fungi are classified in a separate kingdom.

Terrestrial molds

About 600 species of terrestrial molds make up the class Zygomycetes. The fuzzy part of the mold actually consists of the specialized hyphae that produce tiny spores in structures called sporangia. These hyphae are called sporangiophores.

The common black bread mold, Rhizopus stolonifer, shows the development of a typical terrestrial mold. An airborne spore that lands on a piece of bread forms hyphae, which branch out over the bread’s surface. These surface hyphae, called stolons, form short extensions that penetrate into the bread. These extensions are called rhizoids. Rhizoids anchor the mold to its food supply, secrete digestive enzymes and absorb the nutrients.

Sexual reproduction occurs in terrestrial molds when contact occurs between hyphae from two genetically different molds, called mating strains. The mating strains are called plus (+) and minus (-) rather than male and female, because they have identical shapes and functions. The tip of each mating hypha contains nuclei that fuse and form a diploid zygote. In unfavorable conditions the zygote may become a durable zygospore. In favorable conditions the zygote undergoes meiosis and produces spores that form new hyphae. Terrestrial molds and other fungi do not go through alternation of-generations.

Water molds

More than 500 species of water molds make up the class Oomycetes. Most are water-dwelling saprophytes. Water molds are the only fungi whose cell walls consist mainly of cellulose rather than of chitin. Unlike all other fungi, water molds produce distinctly male and female gametes. The differentiation of gametes into distinctly male (sperm) and female (egg) forms is called oogamy.

A water mold called Phytophthora infestans ruined potato crops in Ireland between 1845 and 1847. Potatoes were Ireland’s main crop, and the resulting famine caused the death of more than 2 million people. Other water molds cause disease of fish, a common problem in aquariums.

Slime molds

The slime molds are difficult to classify. In body form, slime molds resemble protozoa, but their method of reproduction somewhat resembles a fungus. The body of a slime mold is usually a brightly colored, jellylike network of cytoplasm called a plasmodium. The plasmodium has no cell walls but many nuclei. It creeps by amoeboid movement over the ground and dead logs. Like an amoeba, the plasmodium engulfs microscopic prey and digests the food in food vacuoles.

When its food supply runs short, the plasmodium separates into tiny mounds of cytoplasm that develop into fungus like sporangia. The spores produced in these sporangia may be scattered to new areas where food is more plentiful. Slime mold spores grow into either flagellated or amoeba-like cells. A new plasmodium develops when two flagellated cells fuse or when a mass of the amoeba-like cells join and form the larger organism.

Club fungi

The class Basidiomycetes contains about 25,000 species, including mushrooms, shelf fungi, puffballs, rusts, and smuts. Basidiomycetes are called club fungi because they produce spores on club-shaped, microscopic structures called basidia. The basidia develop within the fruiting body.

Mushrooms.The most common club fungi are mushrooms. What you think of as a mushroom is the spore-producing structure of an underground mycelium. The mushroom first develops as a tight mass of hyphae called a button. A stemlike structure known as the stipe pushes the button above ground. There the button opens into a cap, the fruiting body of the mushroom. The underside of the cap contains thin sheets of tissue, or gills, to which the basidia are attached. Within each basidium are two haploid nuclei that fuse to form a diploid nucleus. This nucleus undergoes meiosis, and the resulting haploid nuclei produce four basidiospores. Each basidipspore is capable of developing into a new mushroom.

Some kinds of mushrooms are edible, but others are poisonous. Because some poisonous mushrooms resemble harmless ones, you should never eat wild mushrooms. For example, even a small portion of the "destroying angel", Amanita bispo-rigera, can be lethal. A toxin in this mushroom damages the liver so severely that death results.

Rusts and Smuts. Rusts and smuts are parasitic club fungi that cause severe damage to cereal and vegetable crops. Rusts and smuts produce spores, but not in mushroom like fruiting bodies. The mycelia spread throughout the host plant, destroying the plant’s cells while producing billions of basidiospores.

The life cycle of wheat rust involves two alternate hosts. In the spring, the fungal spores infect young wheat plants. In the summer, a second cycle of spore production infects barberry plants. The disease can be controlled by destroying all barberry bushes growing near wheat fields.

Corn smut produces large deformed growths on ears of corn. The life cycle of corn smut involves only one host. Corn smut can be eliminated by burying or burning the infected plants before the fungi produce spores.

Sac fungi

About 30,000 species belong to the class Ascomycetes. Members of this class include the gourmet delicacies morels and truffles as well as the single-celled yeast used in making bread. Ascomycetes are called sac fungi because sexually produced spores form in an ascus, or "little sac". An ascus begins to develop when two gametes or two mating strains fuse. Nuclei divide as the hyphae grow, resulting in a row of haploid ascospores within the ascus. In sac fungi and club fungi, hyphae are divided by cross walls. Nuclei and cytoplasm flow through pores in these cross walls as the hyphae grow.

Yeasts

Yeasts are unusual sac fungi. They contain chitin and reproduce sexually by forming ascospores, but they are unicellular and do not form hyphae. Yeasts also reproduce asexually by budding. Many types of yeasts grow most rapidly in environments with high sugar content. In bread dough, yeast cells feed on carbohydrates. As yeast cells grow, they produce carbon dioxide gas by fermentation. The process of fermentation causes bread dough to rise and creates the bubbles in beer.