The occurrence and ecological importance of dissolved ATP in fresh water

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Abstract
SUMMARY. Dissolved ATP, defined as ATP which passes through 0.2 μm filters, was found in fresh water. During the spring diatom bloom in two eutrophic Danish lakes, concentrations of dissolved ATP varied between 0.1 and 3.8 μgl−1, constituting 14–76% of the total ATP (particulate plus dissolved ATP). The kinetics of the light emission obtained from mixing firefly enzyme with dissolved ATP demonstrated that the major proportion of the dissolved ATP was in fact ATP. Despite some variations, the seasonal changes in dissolved ATP paralleled the changes in the increasing phytoplankton population during the rise of the diatom blooms. The dissolved ATP increased after the diatom peak, indicating that release of ATP from the phytoplankton due to mortality may be a major source of dissolved ATP.Consumption of dissolved ATP was evaluated in uptake experiments using 3H‐ATP. Rates of uptake of 3H‐ATP by micro‐organisms (diameter 0.2–0.6 μm) proved to be close to the rates for 3H‐D‐glucose uptake. The variations in 3H‐ATP uptake during the diatom blooms showed non‐systematic changes and ranged between 1.0 and 15.8% h−1 (mean = 4.9% h−1) of the quantity added. Turnover rates for dissolved ATP varied between 12 and 730 ng l−1 h−1 (mean = 175 ng l−1). These rather high rates of turnover suggest that dissolved ATP is an important compound in the metabolism of freshwater bacteria.