Study site.

<p>Map of the Sulejow Reservoir with sampling sites.</p

Response of <i>Daphnia</i>'<i>s</i> Antioxidant System to Spatial Heterogeneity in Cyanobacteria Concentrations in a Lowland Reservoir

<div><p>Many species and clones of <i>Daphnia</i> inhabit ecosystems with permanent algal blooms, and they can develop tolerance to cyanobacterial toxins. In the current study, we examined the spatial differences in the response of <i>Daphnia longispina</i> to the toxic <i>Microcystis aeruginosa</i> in a lowland eutrophic dam reservoir between June (before blooms) and September (during blooms). The reservoir showed a distinct spatial pattern in cyanobacteria abundance resulting from the wind direction: the station closest to the dam was characterised by persistently high <i>Microcystis</i> biomass, whereas the upstream stations had a significantly lower biomass of <i>Microcystis</i>. Microcystin concentrations were closely correlated with the cyanobacteria abundance (r = 0.93). The density of daphniids did not differ among the stations. The main objective of this study was to investigate how the distribution of toxic <i>Microcystis</i> blooms affects the antioxidant system of <i>Daphnia</i>. We examined catalase (CAT) activity, the level of the low molecular weight antioxidant glutathione (GSH), glutathione S-transferase (GST) activity and oxidative stress parameters, such as lipid peroxidation (LPO). We found that the higher the abundance (and toxicity) of the cyanobacteria, the lower the values of the antioxidant parameters. The CAT activity and LPO level were always significantly lower at the station with the highest <i>M. aeruginosa</i> biomass, which indicated the low oxidative stress of <i>D. longispina</i> at the site with the potentially high toxic thread. However, the low concentration of GSH and the highest activity of GST indicated the occurrence of detoxification processes at this site. These results demonstrate that daphniids that have coexisted with a high biomass of toxic cyanobacteria have effective mechanisms that protect them against the toxic effects of microcystins. We also conclude that <i>Daphnia</i>'s resistance capacity to <i>Microcystis</i> toxins may differ within an ecosystem, depending on the bloom's spatial distribution.</p></div

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The cyanobacteria abundance and activity of the antioxidant parameters in <i>Daphnia</i> tissues, measured in August 2012.

<p>(A) The bars represent an average of three replicates (±S.D.) of cyanobacteria abundance (µg dm⁻³); the dark dots indicate the total concentration of microcystins LR and RR (µg g⁻¹), (B) The bars represent an average of seven replicates (±S.D.) of glutathione (GSH) concentrations (nmol/mg protein), (C) The bars represent an average of seven replicates (±S.D.) of lipid peroxidation (LPO) (nmol/mg protein) and (D) The bars represent an average of six replicates (±S.D.) of catalase (CAT) activity (U/mg protein) in <i>Daphnia</i> tissues from the Sulejow Reservoir in 2012. The same letters above the bars indicate that the values did not differ significantly. Each panel of the figure includes the one-way ANOVA test results. Details concerning GSH, LPO and CAT data are presented in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112597#pone.0112597.s001" target="_blank">Tables S1</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112597#pone.0112597.s002" target="_blank">S2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0112597#pone.0112597.s003" target="_blank">S3</a>.</p

Density of <i>Daphnia longispina</i> in the sites of the Sulejow Reservoir.

<p>Density of <i>Daphnia longispina</i> [ind dm⁻³] in the three studied sites: Tresta (TR), Bronisławów (BR) and Zarzęcin (ZA). The sampling was conducted monthly between June and September of 2012, and on September of 2014.</p><p>Density of <i>Daphnia longispina</i> in the sites of the Sulejow Reservoir.</p

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Effects of <i>Daphnia</i> exudates and sodium octyl sulphates on filament morphology and cell wall thickness of <i>Aphanizomenon gracile</i> (Nostocales), <i>Cylindrospermopsis raciborskii</i> (Nostocales) and <i>Planktothrix agardhii</i> (Oscillatoriales)

<p>Grazing is recognized as one of the selective factors shaping the morphology and physiology of cyanobacteria. A recent study has shown that the filamentous cyanobacterium <i>Aphanizomenon gracile</i> strain SAG 31.79 thickened in the presence of <i>Daphnia</i> (Cladocera) and its exudates. The aims of our study were: (1) to determine whether this type of response to <i>Daphnia</i> cues is common for other strains of <i>A. gracile</i>, and other species of filamentous cyanobacteria, (2) to test whether the response is due to nutrients recycled by <i>Daphnia</i>, or kairomone induced, and (3) whether it is related to toxin production. Prior to the experiment, cyanobacterial strains were inspected using chromatographic methods for the presence of two toxins, cylindrospermopsin (CYN) and three homologues of microcystin (MC-RR, MC-YR, MC-LR). HPLC analyses showed that all strains were free of cylindrospermopsin, whereas microcystins were detected only in one strain (<i>Planktothrix agardhii</i>). We then tested whether <i>Daphnia</i> exudates can cause thickening of cyanobacterial filaments, which would suggest the morphological changes in cyanobacterial filaments are caused by recycled nutrients. Cyanobacteria were also exposed to sodium octyl sulphate (a commercially available <i>Daphnia</i> kairomone). Transmission electron microscopy (TEM) was used to check whether <i>Daphnia</i> exudates and sodium octyl sulphate trigger thickening of cyanobacterial cell walls, which would be a defence mechanism against grazing. The TEM analysis revealed no significant effect of either <i>Daphnia</i> exudates or kairomone (sodium octyl sulphate) on the cell wall thickness of cyanobacteria. However, our study showed that <i>Daphnia</i> exudates triggered filament thickening in nostocalean cyanobacteria, while filaments of the oscillatorialean strain <i>P. agardhii</i> did not show this response. It was also demonstrated that sodium octyl sulphate alone can also cause filament thickening, which suggests that this might be a specific defence response to the presence of grazers.</p