LAUSR

Abstract In bivalve hatcheries, microalgae production accounts for a large proportion of hatchery operation costs. A reliable supply of good quality algae is essential for optimal output. Two commercially important algal species, Tisochrysis lutea (T-Iso) and Chaetoceros calcitrans were grown in batch culture under optimal and high-pH conditions for 7 and 5 days respectively. A suite of parameters were compared to identify those that could be considered as early markers of algal stress. One day after inoculation of both algal species, carbon dioxide (CO2) addition was removed to cause the high pH treatment condition and cultures were monitored to determine any changes in algal health. A variety of indicators of algal function including photosynthetic parameters by Pulse Amplitude Modulation (PAM) fluorometry and morphological and functional changes by flow cytometry were determined in parallel during the pH stress to correlate them and identify parameters that could be used as an early indicator of decreasing algal health. The pH in the sub-optimal treatments was significantly higher compared with controls from 1 day after the removal of CO2 addition. At the same sampling time (Day 2), in the high-pH treatment, the PSII photosynthetic efficiency in the light measured by PAM fluorometry and, chlorophyll fluorescence and reactive oxygen species production measured by flow cytometry were significantly lower for T-Iso, compared with controls. In sub-optimal cultures of C. calcitrans, relative algal size, complexity, chlorophyll and neutral lipid content (as measured by flow cytometry) were higher as early as Day 2, compared with controls. Additionally, the bacterial abundance associated with T-Iso and C. calcitrans cells was increased in the high pH treatment compared with controls, from Day 6 and Day 5 respectively. These responses to a subtle negative change in the culture environment were detected 24 h prior to a change in cell density and indicate the promise of PAM fluorometry and flow cytometry as tools to provide a rapid, sensitive and reliable assessment of microalgal health in a hatchery setting. Such advances in algal health monitoring will improve bivalve production systems.