Taxa specific grazing-induced phytoplankton chlorophyll breakdown pathway: implication in algal bloom monitoring and mitigation programs
Abstract
The roles of biotic factors (grazing/senescence) in chlorophyll a (Chl a) breakdown (CB) to non-fluorescence derivatives in a multistep pathway are frequently reported. In aquatic microalgae, the information on CB-pathway is scarce compared to land plants. The enzymatic activities (e.g. chlorophyllase dephytalation; pheophytinase demetalation) involved in CB vary significantly among phytoplankton species, but how this influences the CB-pathway is unknown, and delineating the same will aid in determining the fate of high microalgal biomass, particularly blooms. The grazing experiments using three species of bloom-forming marine phytoplankton (Skeletonema costatum (Bacillariophyceae), Dunaliella tertiolecta (Chlorophyceae) and Amphidinium carterae (Dinophyceae)) and mixed natural assemblage confirmed that the CB-pathway under grazing-pressure is species-specific and the grazer s feeding habits. In Dunaliella and Amphidinium, with and without chlorophyllase-activity, respectively, higher pheophytin, and lower pheophorbide formation suggested the Chl a pheophytin pheophorbide pathway. Skeletonema with high chlorophyllase activity produced higher pheophorbide suggesting CB through the Chl a-chlorophyllide pheophorbide pathway. In the presence of grazers, the pheophorbide: pheophytin ratio was several times higher in Skeletonema (3-40) than others (0.1-<1) suggesting a potential proxy for determining Chl a chlorophyllide pheophorbide (>1) or Chl a pheophytin pheophorbide pathway (<1) dominance. This study emphasises that using CB pathway indicator to assess bloom fate will advance microalgal bloom research and ecosystem evaluation.
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