The disruption of calcium and hydrogen ion homeostasis of submerged macrophyte Vallisneria natans (Lour.) Hara caused by microcystin-LR

Aquatic plants play an important role in maintaining lake water status and ecosystem stability, but the effect of the cyanotoxin microcystin (MC) on ion homeostasis in aquatic plants and the resulting adverse consequences remains unclear. This study used non-invasive micro-test technology to detect the effect of MC-LR on homeostasis of calcium (Ca2+) and hydrogen ions (H+) in Vallisneria natans (Lour.) Hara, and examined the relationship between ion homeostasis and physiological indicators. Results showed that 1) MC-LR was enriched in V. natans tissues, with greater absorption in roots than in leaves, and 2) MC-LR induced a sustained and dose-dependent Ca2+ efflux from leaves and recoverable Ca2+ efflux from roots. Although H+-ATPase of leaves and roots was activated by MC-LR, the effluent of H+ from roots and influent of H+ into leaves was enhanced. By affecting the homeostasis of Ca2+ and H+, MC-LR directly or indirectly affected accumulation of nutrients essential for maintaining normal growth: accumulation of nitrogen, magnesium, phosphorus, calcium, iron, and zinc decreased in leaves; calcium, magnesium, and zinc decreased in roots; and potassium showed an increase in both leaves and roots. Microscopy revealed MC-LR results in leaf swelling and reduced accumulation of protein and starch, presumably due to changes in nutrient processes. In addition, efflux of Ca2+ and reduced accumulation of transition metals resulted in decreased ROS levels in leaves and roots. The disruption of ionic homeostasis in aquatic plants can be caused by as small a concentration as 1 mu g/L MC-LR, indicating potential ecological impacts caused by microcystin need greater attention.