Development of a chronic microalgal bioassay to assess contaminated sediment

Charlotte E. Watson

Abstract

Aquatic ecosystems can be exposed to a variety of contaminants from a range of sources including those of anthropogenic origin. Sediments can act as both reservoirs and future sources of these contaminants. To assess the impacts of these contaminants in the environment bioassays can be used to determine their effects on factors such as the photosynthesis and growth rate of organisms. Contaminants such as metals can bioaccumulate in the food chain and an understanding of the interactions of metals with microalgae is crucial as they are significant primary producers and form the base of the aquatic food chain.

Biofilms contain benthic microalgae and are located at the sediment-water interface and the study aimed to determine the effectiveness of the Phyto-PAM EDF in measuring benthic microalgal cell density (biomass) in sediments. The pulse-amplitude modulated (PAM) fluorometer is a non-destructive technique which uses chlorophyll a fluorescence as a proxy for microalgal biomass. In this study the fibre-optics-emitter-detector unit Phyto-PAM EDF was trialled for measurements on sediment surfaces in an attempt to measure growth in laboratory-grown biofilms over time using the test species Entomoneis cf punctulata and Nitzschia closterium.

Although there was an initial trend of increasing chlorophyll a minimum fluorescence over time, growth only doubled over a week and was not in line with the OECD mandate of an increase in cell concentration by a factor of at least 16 within 3 days. It was proposed that the laboratory-grown biofilms were not structurally forming properly and that the Phyto-PAM EDF works best with mature microphytobenthic communities collected in situ and allowed to colonise over a period of time.

The study optimised the conditions for a method for the solvent extraction of chlorophyll from sediments and used chlorophyll a content as a measure of microalgal cell density (biomass). The optimal duration of the bioassay was found to be up to 96-h.

The ability of the chronic microalgal bioassay to demonstrate toxicity in contaminated field sediments was also assessed. It was found that the bioassay demonstrated sufficient sensitivity in the response of inhibition of growth in Nitzschia closterium to the possible contaminant of elevated porewater zinc concentrations.