Rapid effects of diverse toxic water pollutants on chlorophyll a fluorescence: Variable responses among freshwater microalgae

Abstract

Chlorophyll a fluorescence of microalgae is a compelling indicator of toxicity of dissolved water contaminants, because it is easily measured and responds rapidly. While different chl a fluorescence parameters have been examined, most studies have focused on single species and/or a narrow range of toxins. We assessed the utility of one chl a fluorescence parameter, the maximum quantum yield of PSII (Fv/Fm), for detecting effects of nine environmental pollutants from a range of toxin classes on 5 commonly found freshwater algal species, as well as the USEPA model species, Pseudokirchneriella subcapitata. Fv/Fm declined rapidly over <20 min in response to low concentrations of photosynthesis-specific herbicides Diuron® and metribuzin (both <40 nM), atrazine (<460 nM) and terbuthylazine (<400 nM). However, Fv/Fm also responded rapidly and in a dose-dependent way to toxins glyphosate (<90 μM), and KCN (<1 mM) which have modes of action not specific to photosynthesis. Fv/Fm was insensitive to 30–40 μM insecticides methyl parathion, carbofuran and malathion. Algal species varied in their sensitivity to toxins. No single species was the most sensitive to all nine toxins, but for six toxins to which algal Fv/Fm responded significantly, the model species P. subcapitata was less sensitive than other taxa. In terms of suppression of Fv/Fm within 80 min, patterns of concentration-dependence differed among toxins; most showed Michaelis–Menten saturation kinetics, with half-saturation constant (Km) values for the PSII inhibitors ranging from 0.14 μM for Diuron® to 6.6 μM for terbuthylazine, compared with a Km of 330 μM for KCN. Percent suppression of Fv/Fm by glyphosate increased exponentially with concentration. Fv/Fm provides a sensitive and easily-measured parameter for rapid and cost-effective detection of effects of many dissolved toxins. Field-portable fluorometers will facilitate field testing, however distinct responses between different species may complicate net Fv/Fm signal from a community.