Algal esterase activity as a biomeasure of environmental degradation in a freshwater creek

Abstract

This study investigated the potential for using algal esterase activity of Microcystis aeruginosa and Selenastrum capricornutum as a rapid measure of the biological effects of acid mine drainage (AMD) in a South Australian stream (Australia) also affected by sewage pollution and dry-land salinity. Algal bioassays were based on the non-fluorescent substrate, fluorescein diacetate (FDA) which is metabolised by esterases to the fluorescent product, fluorescein. Esterase activity was interpreted as the mean rate of conversion of FDA to fluorescein and expressed as a percentage of the rate achieved by control algae (%FDAC). Flow cytometry was used to measure the fluorescence of individual algal cells, enabling differentiation of three esterase activity states (low=S(1), normal and stimulated) and calculation of the percentage of algal cells in each activity state relative to that found for control algae (e.g. %S(1)). Algal esterase activity responded rapidly to AMD-affected water but also to increased conductivity (associated with dry-land salinity) and nutrient concentrations (associated with sewage). Exposure to AMD-affected water for 1 h reduced %FDAC by 30-70%, and increased %S(1) by 60-90%, a depression of esterase activity that was maintained over 24 h. A similar depression of esterase activity occurred in both algae exposed to comparatively high-conductivity water (ca. 20 mS cm(-1)) for 1 h but the algae recovered from this 'shock' within 24 h. The %FDAC of S. capricornutum increased from 66 to 158% of control values after a 24 h exposure to nutrient-enriched water sampled downstream from a sewage treatment plant, despite the fact that the alga was grown in nutrient-sufficient culture. The combination of cyanobacterial (M. aeruginosa) and green (S. capricornutum) algal cultures with exposure times of 1 and 24 h was successful in distinguishing between the three types of pollution. Correlation of esterase activity measures with water quality parameters indicated that the clearest and least equivocal biological measure of AMD for the study area was the %S(1) for M. aeruginosa after a 24 h exposure. The use of the flow cytometer to define a low esterase activity state was therefore successful in clarifying the response to AMD-affected water. The study demonstrates the successful application of algal esterase activity bioassays, in combination with flow cytometry, to rapidly assess the toxicity of AMD-affected waters and to differentiate this response from the effects of other pollutants (increased nutrients and conductivity).