Microplastics interfere with mixotrophic Ochromonas eliminating toxic Microcystis.

Microplastics with different sizes exist widely in fresh waters, which may affect the interspecific dynamics between predator and prey. The flagellate Ochromonas gloeopara can efficiently eliminate Microcystis aeruginosa and degrade microcystins, which shows great potential for controlling harmful Microcystis. In order to evaluate the effects of microplastics on O. gloeopara eliminating Microcystis, we designed an experiment of O. gloeopara feeding on Microcystis under different sizes (0.07 and 3 µm) and concentrations (0, 0.4, 0.8, 1.6, and 2.0 mg L-1) of microplastics. The results showed that maximum abundance of M. aeruginosa decreased significantly with addition of microplastics, regardless of the size and concentration. O. gloeopara can ingest the microplastics and suffer from their adverse effects. The maximum abundance of O. gloeopara decreased with enhancing concentrations of 3 µm microplastics during the process of O. gloeopara eliminating M. aeruginosa, whereas 0.07 µm microplastics did not affect the growth of O. gloeopara obviously. During the period of exposure under microplastics, clearance rate of O. gloeopara on M. aeruginosa decreased with the increasing concentrations of microplastics. Specially, 3 µm microplastics had a stronger reduction on clearance rate of O. gloeopara. The time to M. aeruginosa extinction was prolonged with the increasing concentrations of microplastics in both sizes. Comparatively speaking, 3 µm microplastics had a stronger delayed effect on the removal of Microcystis. These findings suggest that microplastics can interfere with protozoa eliminating toxic Microcystis, which may aggravate their adverse impacts on aquatic ecosystem.

High temperature promotes the inhibition effect of Zn2+ on inducible defense of Scenedesmus obliquus.

Morphological defense is assumed to be an effective anti-grazer strategy in phytoplankton. Scenedesmus obliquus, a globally widespread freshwater chlorophyte, can form colonies in response to the infochemicals of herbivorous zooplankton and survive in coexistence with grazers. However, the inducible defense response is often disturbed by abiotic or biotic factors, especially under the increasing global warming and environmental pollution. In this study, two nonlethal environmental factors, namely, elevated temperature and environmentally relevant Zn2+ concentrations, decreased colony formation of S. obliquus induced by Daphnia grazing infochemicals. Elevated temperature (30 °C) reduced the inducible colony size and shortened the maintenance time of defensive colonies. Decreased colony size was detected with increased Zn2+ concentration. Colony formation was inhibited even at low Zn2+ concentration (0.131 µmol L-1), which neither retarded growth nor affected photosynthesis. Warming promoted the inhibition effect of Zn2+ on inducible colony formation of S. obliquus. Warming also enhanced Zn2+ toxicity, which caused the growth rate of S. obliquus to be hindered by high Zn2+ concentrations at elevated temperature. Specially, S. obliquus which formed inducible colonies under the condition of Daphnia infochemicals had higher tolerance to Zn2+ toxicity and thus likely exerted protective effects against heavy metals. The results indicated the combined effects of global warming and heavy-metal pollution result in more severe impact on the inducible defense of S. obliquus.

Magnesium depletion suppresses the anti-grazer colony formation in Scenedesmus obliquus.

In aquatic ecosystems, many phytoplankton species have evolved various inducible defense mechanisms against the predation. The expression of these defenses is affected by environmental conditions such as nutrient availability. Here, we investigated the anti-grazer colony formation in Scenedesmus obliquus at different magnesium concentrations (0-7.3 mg L-1 Mg2+) in the presence of zooplankton (Daphnia)-derived infochemicals. Results showed that at adequate Mg2+, S. obliquus formed high proportions of multi-celled (e.g., four- and eight-celled) colonies, resulting in significantly increased number of cells per colony in response to Daphnia filtrate. On the other hand, in Mg2+-deficient treatment, the proportion of multi-celled colonies decreased, together with reduced algal growth rate and photosynthetic efficiency. Finally, the treatment without Mg2+ strongly suppressed the formation of large colony (mainly eight-celled colonies), whereas the algal growth rate was comparable to that in Mg2+ sufficient treatment. Despite the inhibition of colony formation, the time reaching the maximum number of cells per colony was not affected by the Mg2+ concentration, which generally took three days in all groups. Our results indicate that Mg2+ deficient/absent environments significantly reduced anti-grazing colony formation but not the algal growth, suggesting strong dependability of this morphological defensive trait to magnesium fluctuation in S. obliquus.

Chlorophytes prolong mixotrophic Ochromonas eliminating Microcystis: Temperature-dependent effect.

Cyanobacterial blooms, caused by eutrophication and climate warming, exert severely negative effects on aquatic ecosystem. Some species of protozoans can graze on toxic cyanobacteria and degrade microcystins highly efficiently, which shows a promising way to control the harmful algae. However, in the field, many different species of algae coexist with Microcystis and may affect protozoans eliminating Microcystis. Therefore, in this study, we assessed the impacts of chlorophytes, a type of beneficial algae for zooplankton and common competitors of cyanobacteria, on flagellate Ochromonas eliminating toxin-producing Microcystis at different temperatures. Our results showed that Ochromonas still eliminated Microcystis population and degraded the total microcystins with the addition of chlorophytes, although the time of eliminating Microcystis was prolonged and temperature-dependent. Additionally, in the grazing treatments, chlorophytes populations gradually increased with the depletion of Microcystis, whereas Microcystis dominated in the mixed algal cultures without Ochromonas. The findings indicated that although chlorophytes prolong mixotrophic Ochromonas eliminating Microcystis, the flagellate grazing Microcystis helps chlorophytes dominating in the primary producers, which is significant in improving water quality and reducing aquatic ecosystem risks.