T

taxon (plural, taxa)- A group into which related organisms are classified.

taxonomy- Study of scientific classification and nomenclature.

teichoic acids- All wall, membrane, or capsular polymers containing glycerophosphate or ribitol phosphate residues.

teleomorph- Sexual stage in reproduction in which cells are formed by the process of meiosis and genetic recombination.

temperate virus- Virus which upon infection of a host does not necessarily cause lysis but whose genome may replicate in synchrony with that of the host.

terminal electron acceptor- External oxidant (often oxygen) that accepts the electrons as they exit from the electron transport chain.

test- Hard external covering or shell.

texture- See soil texture.

thallus- Vegetative body that is not differentiated into tissue systems or organs.

thermophile- Organism whose optimum temperature for growth is between 45 and 85¼C.

Ti plasmid- Conjugative tumor-inducing plasmid present in the bacterium Agrobacterium tunefaciens which can transfer genes into plants.

topsoil- (i) Layer of soil moved in cultivation. (ii) The A horizon. (iii) Presumably fertile soil material used to topdress roadbanks, gardens, and lawns.

toxin- Microbial substance able to induce host damage.

trace gas- Gas other than nitrogen and oxygen in the atmosphere, particularly those gases that are active in the chemistry or radiation balance of the atmosphere.

transcription- Synthesis of an RNA molecule complementary to one of the two strands of a DNA double-stranded molecule.

transduction- Transfer of host genetic information via a virus or bacteriophage particle.

transfer RNA (tRNA)- Type of RNA that carries amino acids to the ribosome during translation.

transformation- Transfer of genetic information into living cells as free DNA.

transgenic- Describes genetically modified plants or animals containing foreign genes inserted by means of recombinant DNA techniques.

translation- Synthesis of proteins using the genetic information in mRNA as a template.

transposable element- Genetic element that can to move (transpose) from one site on a chromosome to another.

transposition- Movement of a piece of DNA around the chromosome, usually through the function of a transposable element.

transposon- Transposable element of which, in addition to genes involved in transposition, carries other genes; often confers selectable phenotypes such as antibiotic resistance.

transposon mutagenesis- Insertion of a transposon into a gene; this inactivates the host gene leading to a mutant phenotype and also confers the phenotype associated with the transposon gene.

tricarboxylic acid cycle (TCA cycle, citric acid cycle, Krebs cycle)- Series of metabolic reactions by which pyruvate is oxidized completely to carbon dioxide, also forming NADH, which allows ATP production.

trichome- Row of cells which have remained attached to one another following successive cell divisions. Trichomes are formed by many cyanobacteria and by species of Beggiatoa.

trophic level- Describes the residence of nutrients in various organisms along a food chain ranging from the primary nutrient assimilating autotrophs to the predatory carnivorous animals.

U

uronic acid- Class of acidic compounds of the general formula HOOC(CHOH)nCHO that contain both carboxylic and aldehydic groups, are oxidation products of sugars, and occur in many polysaccharides; especially in the hemicelluloses.

V

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 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.

vesicular-arbuscular mycorrhiza- See arbuscular mycorrhiza.

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.

W

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

water potential- See soil water potential.

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.

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.

wilting point- See permanent wilting point.

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.

X

xenobiotic- Compound foreign to biological systems. Often refers to human-made compounds that are resistant or recalcitrant to biodegradation and decomposition.

xerophile- Organism adapted to grow at low water potential, i.e., very dry habitats.

Y

yeast- Fungus whose thallus consists of single cells that multiply by budding or fission.

Z

zoospore- An asexual spore formed by some fungi that usually can move in an aqueous environment via one or more flagella.

zygospore- Thick-walled resting spore resulting from fusion of two gametangia of fungi in the phylum Zygomycota.

zygote- In eukaryotes, the single diploid cell resulting from the union (fusion) of two haploid gametes.

zymogenous flora- Refers to microorganisms, often transient or alien, that respond rapidly by enzyme production and growth when simple organic substrates become available. Also called copiotrophs.