High-throughput phenotyping reveals nutrient availability alters thermal performance in two Symbiodiniaceae species.

Nutrient sharing between corals and Symbiodiniaceae is crucial to the survival of coral reefs. However, optimal nutrient conditions for different Symbiodiniaceae species remains unclear, partly due to a lack of high-throughput methods for phenotyping Symbiodiniaceae. We developed a high-throughput phenotyping method for Symbiodiniaceae to grow monocultures of Cladocopium proliferum and Durusdinium trenchii in 96-well plates. We utilised spectrophotometry, light-induced fluorescence transient-fast repetition rate fluorometry, and flow cytometry to characterise physiological responses over 11 days to differing nitrogen and phosphorus conditions (125 nutrient treatments) under two temperatures (250 experimental conditions). In our study, Durusdinium trenchii had higher specificity to growth conditions, requiring high nitrogen concentrations and nitrate proportions to grow. Contrastingly, Cladocopium proliferum was able to grow at all nutrient conditions. Elevated temperature resulted in higher growth rates in both species, with interactions between temperature and nutrient conditions determining photosynthetic health and cell size changes. These findings suggest that Cladocopium proliferum and Durusdinium trenchii differ vastly in their nutritional niches, which could have substantial impacts on both their free-living and symbiotic life stages. The ability to collect high-throughput phenotyping data for different Symbiodiniaceae species may allow us to uncover more of the functional variation that is present within this essential family.