All biosynthetic and catabolic reactions within cells require the participation of ATP. ATP is sensitive to phosphatases and does not persist in soil in a free state. It should be ideal for determining the amount or activity of life within soil, sediment, or aquatic systems. The substrate luciferin, an aromatic N- and S-containing molecule, reacts with ATP and luciferase in the presence of Mg2+ to yield an enzyme-luciferin-adenosine monophosphate intermediate. This, in the presence of O2, breaks down to produce free adenosine monophosphate, inorganic P, and light. The light emitted is measured by a photometer or scintillation counter and plotted against ATP content to form a standard curve. If pure luciferase and luciferin rather than firefly tails are used, the light output is extended and constant. Useful extracting reagents include combinations of anions (phosphate) and cations that adequately lyse cells, inhibit adsorption of the ATP to soil surfaces, and inhibit the numerous soil ATPases. Most extraction reagents inhibit the luciferase reaction to some extent; therefore, the solution is diluted as much as possible before measurement. A mixture consisting of phosphoric acid, urea, dimethyl sulfoxide, adenosine, EDTA, and polyoxyethylene-10-laryl ether is now often used. Total adenylates (ATP, ADP, and AMP) can be measured after extraction with dimethyl sulfoxide and a 0.01 M Na3PO4/0.02 M EDTA buffer by the HPLC (high-performance liquid chromatography) method. During microbial growth, the C:ATP ratio can vary from 1000:1 to 40:1. In the resting state, the ATP:C:N:P:S ratios are usually 1:250:40:9:2.6. ATP measures of both biomass and activity are influenced by the soil P content. This method can be used most successfully to characterize soils whose microbial population is mainly in the resting state at excess or constant P levels. As such, it is a rapid and sensitive technique. The measurement of ATP content relative to AMP and ADP plus ATP, (ATP + 0.5 X ADP)/(AMP + ADP + ATP), gives a measure of the adenylate energy charge of the soil biota.
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