Toxic effect of norharmane on a freshwater plankton community

Recently, some aquatic cyanobacteria and bacteria were found to produce norharmane and other β-carboline analogs. Some studies revealed bioactivity of these compounds against cyanobacteria and microalgae in batch monocultures. However, the toxic effects of these compounds on natural plankton community have not been explored yet. In the present study, laboratory microcosm experiments were conducted to test the possible allelopathic effects of norharmane on freshwater plankton community (<150 μm). The results showed that norharmane reduced the growth (cell number) of most plankton groups. The inhibitory effect of norharmane on the growth of different plankton groups was concentration and time-dependent. The highest concentration of norharmane (120 μM) caused complete inhibition of the growth of most plankton species on the first day of incubation, whereas the complete growth inhibition of these species occurred on the fifth day of incubation at the lowest concentration (0.6 μM). Based on the median inhibition concentration (IC50) for one and five-day exposure to norharmane, cyanobacteria (IC50 = 0.03–4 μM), flagellate chrysophytes (IC50 = 0.5–1.5 μM) and ciliates (IC50 = 0.4–2.7 μM) seemed to be more sensitive to norharmane than chlorophyta (IC50 = 0.6–5 μM), dinoflagellates (IC50 = 0.7–4 μM) and diatoms (IC50 = 2–8 μM). This study suggests that toxic effects, mediated via the excretion of norharmane by aquatic organisms could cause significant changes in the plankton communities leading to the dominance of norharmane producing species and enhancement of the eutrophication in aquatic environments.