Complex dynamics of adaptation in a nonaxenic microcystis culture. 1. Effects of dinitrophenol on population growth.

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 EC(50), 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 EC(50) 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.

Complex dynamics of adaptation in a nonaxenic microcystis culture. 2. Computer simulation of dinotrophenol effects.

A hypothesis was modeled to account for complex 20-day dynamics in a culture of blue-green algae Microcystis and heterotrophic bacteria exposed to 2,4-dinitrophenol (DNP). In trials with little or no added DNP, a limiting factor (light or CO(2)) may cause algal density to fluctuate after 14 days of increase. Such factors may be unimportant at levels of DNP that restrict photosynthesis. Bacterial growth may be limited by organic substrate, and bacteria may be more resistant to DNP than blue-green algae. Hence, at intermediate levels of DNP, substrate provided by increased algal death stimulates bacterial growth more than DNP retards it, causing a bacterial peak. Sorption of DNP to cells may cause the DNP decline. Greater growth and slower DNP decline in experiments with preexposed organisms indicate lower DNP sorption affinity in preexposed cells. Bacterial assimilation of DNP-containing substrate may cause the reappearance of DNP. The model reproduced the fluctuation in algal density after growth was limited and better growth and lower DNP decline with preexposed organisms. Reappearance of DNP occurred, but was not obvious. Bacterial dynamics were least well reproduced. Changes in bacterial constants most affected output. Despite model inadequacies, probable aspects of toxicant action in nature have been revealed. Ecological relationships among populations of different species and genetic differences among individuals may have led to lower than expected toxicity, adaptation, and even growth stimulation. Responses of single species tested in isolation may be inadequate to predict toxicant impact.

Influence of the energy relationships of trophic levels and of elements on bioaccumulation.

A concern of ecotoxicology is to predict to which trophic levels in biocenoses bioaccumulation of compounds or of elements occurs. Transformity, a measure of the energy required to produce and maintain a component or a flow resulting from an energy transformation process, may help predict bioaccumulation potential. This notion derives from two concepts. First, common substances are more likely to be processed by the biosphere. Moreover, the uptake of rare ones from the physical environment by organisms of low trophic levels makes them less unusual to organisms of high trophic levels, which may evolve a capability of processing them. Second, transformity expresses energy relationships between parts of a system. Substances that require more energy to form or concentrate are also the more unusual. The hypothesis was formulated that there is a correlation between the rarity, complexity, and energy required for concentrating a substance, and thus its transformity, and the transformity of the trophic level to which it bioaccumulates. This hypothesis was tested for a set of elements with published data on their biogeochemistry and bioaccumulation and on energy transfers between trophic levels in ecosystems. The transformities of the elements were calculated from the energy required by the biosphere for maintaining a difference in concentration compared to its physical environment. Transformities of corresponding trophic levels were calculated from the energy driving the energy flows. There was a significant rank correlation between the transformity of elements and that of trophic levels. This may be an important generalization in ecotoxicology because it may lead to the possibility of predicting bioaccumulation tendency.