Abiotic. Pertaining to substances or objects that are of nonbiologic origin; used especially in reference to organic matter produced via chemical reactions in the absence of living systems.

Acetic acid. An organic compound, the two-carbon carboxylic acid CH3COOH.

Acetogenesis. Formation of acetic acid (e.g., from alcohol by bacterial fermentation).

Acetyl-coenzyme A (acetyl-co A). An organic complex involved in enzymatic acetyl transfer reactions, CH3COSCoA.

Acritarch. A “phytoplankton-like” microfossil of unknown or uncertain biologic relationships, unicellular or apparently unicellular, organic-walled, commonly spheroidal, smooth (e.g., sphaeromorphs) or spiny (e.g., acanthomorphs); “Acritarcha” is an artificial group composed of such microfossils.

Actinolite. A mineral of the amphibole group, Ca2(Mg,Fe)5Si8O22(OH)2.

Adenosine triphosphate (ATP). An organic compound, adenosine 5′-triphosphoric acid, C10H16N5O13P3, a coenzyme involved in the transfer of phosphate bond energy.

Adiabatic. In thermodynamics, pertaining to a relationship of pressure and volume when a gas or fluid is compressed or expanded without either giving or receiving heat.

Aerobe. An organism able to live in the presence of free (uncombined) molecular oxygen; aerobes are termed “obligate” if they can exist and grow only in the presence of O2.

Aerotaxis. The locomotory movement of an organism in response to an environmental gradient of molecular oxygen, either toward a higher oxygen concentration (“positive aerotaxis”) or away from a higher oxygen concentration (“negative aerotaxis”).

Akinete. Specialized reproductive cells (resting spores) occurring singly or in uniseriate groups, commonly adjacent to heterocysts, in various types of (predominantly filamentous) cyanobacteria.