Multibiological‐level responses of freshwater phytoplankton to pesticide stress

Axenic batch cultures of freshwater algae were used to study the molecular, cellular and population effects of the organophosphorus insecticide fenitrothion on freshwater phytoplankton. The unicellular chlorophytes Chlamydomonas segnis, Chlorella pyrenoidosa, Scenedesmus obliquus, Ankistrodesmus falcatus and Selenastrum capricornutum were exposed to three treatment levels (0.1, 1.0 and 10.0 mg/L) of fenitrothion. The monitored response parameters represent three levels of biological organization and included total cellular protein, carbohydrate, RNA and DNA levels, cell dry weight, cell volume, growth estimates and cell size distribution. Growth curves revealed differential algistasis among the species following 24 h exposure to fenitrothion, with induction of an extended lag phase in all cases. Levels of 10.0 mg/L elicited reductions in the maximum growth rate and standing crop of up to 62 and 96%, respectively. Biomolecular indicators of phytotoxicity were manifested in the significant augmentation of macromolecular fractions. This biochemical accrual together with elevated dry weight and reduction in cell density was a common response pattern. It was postulated that fenitrothion prevented normal mitotic divisional processes from occurring. Uninhibited biomolecular synthesis resulted in an accumulation of macromolecules and subsequent cell weight augmentation. Modal cell volumes increased and normalized cell distribution spectra displayed a shift toward larger cells in treated algal populations.