Abstract
One of the most important anthropogenic impacts on freshwater aquatic ecosystems close to intensive agriculture areas is the cumulative increase in herbicide concentrations. The threat is especially relevant for phytoplankton organisms because they have the same physiological targets as the plants for which herbicides have been designed. This led us to explore the evolutionary response of three phytoplanktonic species to increasing concentrations of two herbicides and its consequences in terms of growth and photosynthesis performance. Specifically, we used an experimental ratchet protocol to investigate the differential evolution and the limit of resistance of a cyanobacterium (Microcystis aeruginosa) and two chlorophyceans (Chlamydomonas reinhardtii and Dictyosphaerium chlorelloides) to two herbicides in worldwide use: glyphosate and diuron. Initially, the growth rate of M. aeruginosa and D. chlorelloides was completely inhibited when they were exposed to a dose of 0.23 ppm diuron or 40 ppm glyphosate, whereas a higher concentration of both herbicides (0.46 ppm diuron or 90 ppm glyphosate) was necessary to abolish C. reinhardtii growth. However, after running a ratchet protocol, the resistance of the three species to both herbicides increased by an adaptation process. M. aeruginosa and D. chlorelloides were able to grow at 1.84 ppm diuron and 80 ppm glyphosate and C. reinhardtii proliferated at twice these concentrations. Herbicide-resistant strains showed lower growth rates than their wild-type counterparts in the absence of herbicides, as well as changes on morphology and differences on photosynthetic pigment content. Besides, herbicide-resistant cells generally showed a lower photosynthetic performance than wild-type strains in the three species. These results indicate that the introduction of both herbicides in freshwater ecosystems could produce a diminution of primary production due to the selection of herbicide-resistant mutants, that would exhibit lower photosynthetic performance than wild-type populations.
Highlights
One of the main environmental threats resulting from the impact of human activities characterizing the Anthropocene is the loss of biolog ical diversity, resulting in extinction rates of species up to a hundred times faster than the value estimated over geological time (Pimm et al, 2014)
It must be highlighted that because each replicate per species evolved as an independent population, different numbers of generations were required for each species to reach to the same cell concentration as control cultures, especially at the highest herbicide concentrations (Fig. 4)
Three isolates of M. aeruginosa, were able to grow at 40 ppm glyphosate after different numbers of generations, whereas another isolate of the same strain was unable to survive at this con centration (Fig. 4)
Summary
One of the main environmental threats resulting from the impact of human activities characterizing the Anthropocene is the loss of biolog ical diversity, resulting in extinction rates of species up to a hundred times faster than the value estimated over geological time (Pimm et al, 2014).
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