A novel and high throughput approach to assess photosynthetic thermal tolerance of kelp using chlorophyll α fluorometry.

Foundation seaweed species are experiencing widespread declines and localised extinctions due to increased instability of sea surface temperature. Characterising temperature thresholds are useful for predicting patterns of change and identifying species most vulnerable to extremes. Existing methods for characterising seaweed thermal tolerance produce diverse metrics and are often time consuming, making comparisons between species and techniques difficult, hindering insight into global patterns of change. Using three kelp species, we adapted a high throughput method - previously used in terrestrial plant thermal biology - for use on kelps. This method employs temperature-dependent fluorescence (T-F 0 ) curves under heating or cooling regimes to determine the critical temperature (T crit ) of photosystem II (PSII), i.e., the breakpoint between slow and fast rise fluorescence response to changing temperature, enabling rapid assays of photosynthetic thermal tolerance using a standardised metric. This method enables characterisation of T crit for up to 48 samples per two-hour assay, demonstrating the capacity of T-F 0 curves for high-throughput assays of thermal tolerance. Temperature-dependent fluorescence curves and their derived metric, T crit, may offer a timely and powerful new method for the field of phycology, enabling characterisation and comparison of photosynthetic thermal tolerance of seaweeds across many populations, species, and biomes.