Complex Dynamics of Adaptation in a Nonaxenic Microcystis Culture: 1. Effects of Dinitrophenol on Population Growth

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Chronic exposure to toxicants is a selective pressure affecting populations and also the interactions between populations. Nonaxenic cultures of the blue-green alga Microcystis aeruginosa were used to investigate the ecological dynamics and the effect of preexposure to 2,4-dinitrophenol (DNP) on the tolerance toward subsequent DNP inputs. It was predicted that preexposure would induce an increased tolerance to further inputs. This should cause a higher population growth rate under a given DNP exposure, a broader tolerance range (the range of concentrations over which population growth can be sustained), a higher EC50, and a lesser variability in growth rates, over the range of experimental exposure concentrations. DNP reduced Microcystis growth proportionally to exposure concentration. Light, inorganic carbon, and DNP were likely limiting factors for algal growth. Heterotrophic bacteria presumably used the dead cells and the exudate of living algae as substrates. Some unexpected effects occurred, such as an apparent increase in dissolved DNP in the medium following its initial decline and fluctuations of the bacterial population. The hypotheses were verified as concerns the effect of preexposure on tolerance. Changes were apparent in the EC50 and in the breadth of the tolerance range. Moreover, the variability of preexposed populations, in terms of algal growth rate, over the range of exposure concentrations, was smaller than that of non-preexposed populations. Such a decrease in variability may reduce the potential of a population to resist further stresses.