Vadose zone: Unsaturated zone of soil above the groundwater, extending from the bottom of the capillary fringe all the way to the soil surface. Vector: (i) Plasmid or virus used in genetic engineering to insert genes into a cell, (ii) Agent, usually an insect or other animal, able to carry pathogens from one host to another.

Vegetative: Actually growing state.

Vegetative cell: Growing or feeding form of a microbial cell, as opposed to a resting form such as a spore.

Vesicles: Spherical structures, formed intracellularly, by some arbuscular mycorrhizal fungi.

Viable: Alive; able to reproduce.

Viable but nonculturable: Organisms that are alive but cannot be cultured on laboratory media.

Viable count: Measurement of the concentration of live cells in a microbial population.

Vibrio: (i) Curved, rod-shaped bacterial cell, (ii) Bacterium of the genus Vibrio. Virion: Virus particle; the virus nucleic acid surrounded by protein coat and in some cases other material. Virulence: Degree of pathogenicity of a parasite.

Virus: Any of a large group of submicroscopic infective agents that typically contain a protein coat surrounding a nucleic acid core and are capable of growth only in a living cell.

Volatile: A volatile substance is one that is capable of being evaporated or changed to a vapor at a relatively low temperature. Volatile substances also can be partially removed by air stripping.

Volume resistivity: Or specific resistivity of a material, expressed in W/cm. Resistance to electrical current flow through the bulk of an object. VS/L: Measure of volatile solids, usually expressed as g VS/L/day-grams volatile solids per liter per day.

WAS: Waste activated sludge, mg/L. The excess growth of microorganisms which must be removed from the process to keep the biological system in balance. Wastewater: The used water and solids from a community that flow to a treatment plant. Storm water, surface water, and groundwater infiltration also may be included in the wastewater that enters a wastewater treatment plant. The term "sewage" usually refers to household wastes, but this word is being replaced by the term "wastewater".

Water content: Water contained in a material expressed as the mass of water per unit mass of oven-dry material.

Water-retention curve: Graph showing soil-water content as a function of increasingly negative soil water potential.

Weathering: All physical and chemical changes produced in rock by atmospheric agents.

Weir: A wall or plate placed in an open channel and used to measure the flow of water.

White rot fungus: Fungus that attacks lignin, along with cellulose, and hemicellulose, leading to a marked lightening of the infected wood. Wild type: Strain of microorganism isolated from nature. The usual or native form of a gene or organism.

Winogradsky column: Glass column with an anaerobic lower zone and an aerobic upper zone, which allows growth of microorganisms under conditions similar to those found in nutrient-rich water and sediment.

Woronin body: Spherical structure associated with the simple pore in the septa separating hyphal compartments of fungi in the phylum Ascomycota.

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