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The humic acid-like substances released from Microcystis aeruginosa contribute to defending against smaller-sized microplastics
Summary
Researchers studied how the cyanobacterium Microcystis aeruginosa responds to polystyrene microplastic exposure over 17 days at different particle sizes and concentrations. They found that the algae released humic acid-like substances as part of their extracellular secretions, which helped defend against smaller microplastic particles. The study suggests that algae have adaptive mechanisms to cope with microplastic stress, but these defense responses vary depending on particle size.
Microplastics (MPs) are ubiquitous in freshwater ecosystems, but knowledge of their effects on extracellular polymeric substance (EPS) produced by algae is poorly understood. The components in specific EPS fractions of Microcystis respond when exposed to MPs is also still unclear. In this study, the responses of Microcystis aeruginosa under polystyrene (PS) microplastic exposure were studied over 17 days of cultivation, using 0.1 μm and 1.0 μm sized PS at three concentration gradients (1, 10 and 100 mg/L). Results indicate that algal growth significantly increased using the 0.1 and 1.0 μm PS at a high concentration (100 mg/L) on day 17, with growth rates of 74.71% ± 0.94% and 35.87% ± 1.23%, respectively. All tested PS had a maximum inhibitory effect on the photosynthesis on day 5, but the inhibition of photosynthetic activity by 0.1 μm PS alleviated after 13 days of exposure, indicating recovery of microalgae from the toxic environment. The two PS sizes at 100 mg/L concentration triggered EPS release in the latter stage of the experiment; meanwhile, fluorescence EEM analysis showed that smaller-sized PS (0.1 μm) at various doses noticeably increased humic acid-like substances in tightly bound EPS (TB-EPS) fractions on day 17. Our findings showed that EPS release and humic acid-like substances secretion of Microcystis likely can resist MPs exposure. The results provide new insights into the toxicity mechanism of MPs on freshwater microalgae, as well as understanding the ecological risks of microplastics.