Biometric and physiological responses of Egeria densa Planch. cultivated with toxic and non-toxic strains of Microcystis.

Cyanobacterial blooms are becoming increasingly common in aquatic environments around the world, mainly due to eutrophication and climate change. Cyanotoxin-producing strains (e.g., microcystins (MC) producers) may be present in these blooms, affecting the growth of other aquatic organisms, such as aquatic macrophytes. In this study, we evaluated the morphometric and physiological responses of the aquatic macrophyte Egeria densa to the exposure to a toxic strain of Microcystis aeruginosa (MCs producer) and a non-toxic Microcystis panniformis (non-MC producer). The effects of Microcystis strains on E. densa growth and biomass were verified for five weeks (Experiment 1) and physiological responses were evaluated for 14days (Experiment 2). Prolonged exposure of E. densa to the MC producing strain reduced growth, accompanied by the inhibition of shoot and root emission. Both Microcystis strains caused a decrease in the content of photosynthetic pigments, like total chlorophyll and chlorophyll a and b, accompanied by an increase of carotenoids. At the beginning of the MC-producing strain exposure, E. densa showed an increase in the activity of the anti-oxidative enzymes superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), accompanied by an increase in the levels of malondialdehyde, indicating lipid peroxidation. During the 14th day of exposure, the activity of antioxidant enzymes remained similar to the control, suggesting that E. densa has an efficient anti-oxidative system to control the reactive oxygen species produced in response to the stress caused by microcystins. However, when prolonged exposure occurred, possible damage to proteins may have affected the growth and development of E. densa. No changes were observed in the enzymatic activity of the plants exposed to the non-MC producing strain, suggesting that this cyanobacterial strain do not cause significant damage to the development of E. densa. These results are important for understanding the anti-oxidative defense mechanisms of aquatic macrophytes when coexisting with an MC producing strain.