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Color and structure are important features when identifying and classifying types of algae. Colors include blue-green, brown, red, and green. Structural types of pest algae include planktonic, filamentous, and attached-erect. Microscopic algae impart greenish or reddish colors to water. They also may form a scum that floats on the water surface. Filamentous algae form dense free-floating or attached mats. Attached erect algae, known as Chara and Nitella, resemble flowering plants. This is because they have leaf-like and stem-like structures.

Algae are important food sources for many organisms, including people, but have little medical importance to people or livestock. However, algae give drinking water disagreeable tastes or odors, clog water filtering systems and water cooling towers, foul ponds, pools, waterways, and underwater structures, and interfere with certain operations that use large quantities of water. Large algal blooms will cause fish death because algae deplete oxygen in the water. Some forms of decomposing algae release toxins into water, which may be poisonous to people or livestock.

VIRUSES

Viruses are extremely small organisms that reproduce inside living cells. Con- troversy exists on whether these are actually living organisms. Most can only survive for short periods outside a host plant cell or animal cell. Viruses alter chemical activity within host cells and these changes often cause disease.

Viruses are so small that specialists must use an electron mi- croscope to observe or study them. They have different shapes depending on the type. Structurally, viruses are very simple compared with other living organisms. When viruses invade living cells, they use their own genetic information to alter these cells, so the cells produce more viruses rather than their usual proteins or nucleic acid.

Viruses spread through the air, water, bodily fluids, tissues, and by contami- nated equipment.

SLIMES AND BIOFILMS

Slimes are combinations of fungi, algae, bacteria, and other organisms. Biofilms are a particular type of slime. Slime and biofilm organisms grow onto or penetrate surfaces and produce adhesive substances that make removal difficult. Slimes and biofilms also grow inside empty wooden wine barrels. They can be troublesome in water and sewage systems because they clog pipes. They affect the efficiency of water cooling towers and are pests in certain paper mill operations. They attach to the metal surfaces of gas and oil pipelines and wellheads and cause substantial cor- rosion problems.

MICROBIAL ORGANISMS

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