Abstract
BackgroundSensitivity differences to chemical pollutants in different phytoplankton species may potentially shape the community structure of phytoplankton. However, detailed information supporting the understanding of sensitivity variations between phytoplankton species is still limited.ResultsTo investigate sensitivity differences between the cyanobacterium Microcystis aeruginosa, and the green alga Chlorella sp. to paraquat, multiple physiological parameters were measured and compared through acute and chronic toxicity assays. Early photosynthetic responses during acute toxicity assays showed that paraquat affects Photosynthesis System II energy fluxes in M. aeruginosa within 3 h of exposure, but not in Chlorella sp. After 5 h of cumulative exposure, an EC50 based on the maximum quantum yield for primary photochemistry of 0.54 mg L−1 was achieved and remained more or less constant, while the EC50 values for Chlorella fluctuated around 44.76 ± 3.13 mg L−1 after 24 h of exposure. During chronic 96 h exposure to paraquat, differences in antioxidant enzyme activities, reactive oxygen species (ROS) levels, and ultrastructure were observed in both M. aeruginosa and Chlorella sp. An increase in the intracellular levels of ROS and the number of plasma membrane damaged cells was observed in M. aeruginosa in the 0.2, 0.5, and 1.0 mg L−1 treatments (p < 0.01), but not for Chlorella. In addition, at an exposure level of 1.0 mg L−1, extensive disruption of cell structure was observed in M. aeruginosa. Conversely, little disarrangement of organelle structure was found in Chlorella sp.ConclusionThese results confirm that paraquat is more toxic to M. aeruginosa than to Chlorella sp. The sensitivity differences between these two species (one a prokaryote and the other a eukaryote) to paraquat might be partially explained by the differences in cell structure (cell wall and photosynthetic structure), the enzymatic antioxidant system, and the physiological vulnerability. The multiple physiological endpoint analysis approach used in the current study provides more detailed information for understanding the mechanisms of sensitivity variation between these phytoplankton species.
Highlights
Sensitivity differences to chemical pollutants in different phytoplankton species may potentially shape the community structure of phytoplankton
The growth of M. aeruginosa was lower at 0.5 and 1 mg L−1 during the acute exposure stage (24 h, Fig. 1a), while for Chlorella sp., the declining growth curve was observed at 30 mg L −1 until 72 h (Fig. 1b)
A concentration-dependent decrease in the fluorescence yield was observed in Chlorella sp. at concentrations higher than 0.5 mg −1 with the lowest yield found in cultures exposed to 30 mg L −1 (Fig. 2b)
Summary
Sensitivity differences to chemical pollutants in different phytoplankton species may potentially shape the community structure of phytoplankton. Understanding the physiological basis of sensitivity variation of different species provides the knowledge to explain the adverse outcome pathways of these chemical pressures at the ecological level. Parameters associated with oxidative stress, DNA damage, and cell apoptosis (the mitochondrial membrane potential parameter) have been shown to be highly sensitive endpoints in unicellular algae exposed to different concentrations of prooxidant pollutants [10, 11]. In comparison with single endpoint-based assays (i.e., growth inhibition of phytoplankton), multiple endpoint assays can offer a more comprehensive understanding of the risk presented by toxicants by providing important insights into their modes of action [8]. In the present study, multiple cellular level endpoints were used to study the relative sensitivity of two different phytoplankton species to the photosynthesis inhibitor—paraquat (1, 1′-dimethyl-4, 4′-bipyridilium dichloride)
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