We describe in the present study a quick and reliable method based on chlorophyll a fluorescence to assess putative algicidal effect of different microalgal extracts. We couple microalgal production under chemostat cultivation mode to continuously produce a given microalgae species (e.g. Dunaliella salina in this study) at a stable physiological state to ease comparison between extracts tested; with a non-destructive method based on chlorophyll a fluorescence. Pulse amplitude modulated (PAM) fluorometry was used to assess over time the effect of different microalgal crude extracts on the efficiency of the photosystem II (PSII) of a tested microalgae (Dunaliella salina).

• Microalgal production at stationary phase in stirred closed photobioreactor (PBR) operating in continuous have stable photophysiological parameters, which is a prerequisite to compare the impact of different algicidal compounds.

• The combination of both methods, allows to quickly assess the algicidal effect of diverse microalgal (crude) extracts on the PSII efficiency of a tested microalgae.

• The method may be used to identify and isolate algicidal molecules affecting algal PSII using a bio-guided isolation protocol.