Non-photochemical quenching, a non-invasive probe for monitoring microalgal grazing: an early indicator of predation by Oxyrrhis marina and Euplotes sp.

Abstract

Microalgae are major primary producers in aquatic environments, and many synthesize an array of industrially important biomolecules such as pigments, lipids and proteins. As a result, there is considerable interest in and effort into growing algae for industrial purposes. An economical method to produce a large amount of biomass is the use of the open-raceway pond cultivation platform. However, the nature of the cultivation and the nutrient-rich profile of such cultures attract contaminants such as zooplankton predators, which can result in a sudden culture crash. For effective pond management, an early indicator of grazer presence is needed to implement timely interventions. Currently available tools are offline, time-consuming and dependent on predator concentration. This study explores grazing-mediated changes in photosynthetic parameters during infestation of Dunaliella tertiolecta cultures by the heterotrophic dinoflagellate Oxyrrhis marina and the ciliate Euplotes sp. A significant reduction in non-photochemical quenching levels 24–48 h prior to the crash was observed in both bulk and single-cell prey samples. An increasing rate of grazer ingestion correlated with decreasing non-photochemical quenching levels as the culture progressed towards the crash. The reductions in the non-photochemical quenching levels were consistent in grazing cultures at different prey cell concentrations. Although the maximum photosynthetic yield remained unaltered, maximum relative electron transport rates were enhanced and the light-harvesting efficiency, alpha, reduced in comparison to controls. We suggest that, along with traditional methods, non-photochemical quenching monitoring could be used as a part of integrative pest management to minimize predator outbreaks.