Homeostatic response of calcium and potassium ions in Microcystis spp. and Chlorella to MC-LR

Cyanobacteria and green microalgae often co-exist during toxic algal blooms, but the influence of microcystins (MCs) on ionic homeostasis of these algae is not well-studied. This study used non-invasive micro-test technology to examine effects of MC-LR (0, 1, 10 and 100 μg/L) on calcium (Ca²⁺) and potassium ion (K⁺) fluxes, physiological parameters related to ATP synthesis and oxidative stress, and growth, in toxic Microcystis aeruginosa, non-toxic Microcystis wesenbergii, and Chlorella vulgaris during a 44-day mono-culture experiment. Gd³⁺ treatments also were set to analyze the relationships between ion homeostasis and growth based on pharmacology experiment results. The growth of tested strains also was explored in a 28-day co-culture experiment. Results revealed that MC-LR caused the greatest percent decrease in cell density in C. vulgaris, with decreases ranging from -14.1 % to -31.9 % at 100 μg/L compared to the control group in the short term. In addition, there was a greater percent decrease in cell density (-2.9 % to -9.6 %) and a 10.8-24.3 % lower percent increase in cell density (depending on MC-LR concentration) in the subsequent incubation phase in M. aeruginosa than in M. wesenbergii in mono-culture experiments. In co-culture experiments, MC-LR significantly decreased the temporal stability of C. vulgaris, which resulted in variable population dynamics, and also caused a greater percent decrease in cell density in M. aeruginosa than in M. wesenbergii on some sampling dates. We propose that the inhibition of C. vulgaris stemmed from the extreme oxidative stress induced by MC-LR, whereas we attribute the inhibition of M. aeruginosa to a decrease in ATP content, originating from MC-LR-induced oxidative stress and decreases in ferredoxin (FDX) content and α-amylase activity. Acute and 8 days MC-LR exposure of between 1 and 100 μg/L caused a decrease in K⁺ flux in the two Microcystis spp. and Ca²⁺ flux in C. vulgaris, with a stronger decrease in K⁺ flux for M. aeruginosa than for M. wesenbergii. Pharmacology experiment results reflected the possible regulatory role of Ca²⁺ or K⁺ in MC-affected physiological indicators. These results revealed that the microbial taxa used in this study respond differently to MC-LR; the stronger self-poisoning observed in toxic Microcystis compared to the non-toxic strain confers a competitive advantage to the latter over other co-existing algae, thereby enhancing our understanding of the ecotoxicity and ecological function of MC-LR.