Non-photochemical quenching, a non-invasive probe for monitoring microalgal grazing: influence of grazing-mediated total ammonia-nitrogen

Contaminant invasion is one of the risks associated with microalgal cultivation in open raceway ponds, and the presence of zooplankton predators often results in a pond culture crash. For effective pond management, an early grazer-detection tool is needed to implement timely interventions. Microalgae and their predators co-exist in the pelagic food web and microalgal prey species have evolved to detect and mount physiological responses against the predator. However, algal prey-predator interactions in mass cultivation are poorly understood. This study aimed to understand the causal relationship between prey photosynthetic physiology, particularly non-photochemical quenching (NPQ), and predator infestation. We used the high biomass and glycerol-accumulating green algal species Dunaliella tertiolecta and the dinoflagellate Oxyrrhis marina as the prey and predator, respectively. Measurement of photosynthetic parameters suggests that NPQ levels significantly drop in the grazing culture, which may be related to increased total ammonia-nitrogen (TAN) accumulation by prey cells. This study suggests using an integrated approach by using NPQ and TAN measurements for early detection of predator infestation.