Rosalie J. Harris1, Callum Bryant1, Melinda Coleman2,3,4, Verónica F. Briceño1, Pieter A. Arnold1, Andrea Leigh5, Adrienne B. Nicotra1
1Division of Ecology & Evolution, Research School of Biology, The Australian National University, Canberra, ACT, Australia. rosalie.harris@anu.edu.au, ph: 0422762767
2New South Wales Fisheries, National Marine Science Centre, 2 Bay Drive, Coffs Harbour, NSW 2450, Australia.
3Southern Cross University, National Marine Science Centre, 2 Bay Drive, Coffs Harbour, NSW 2450, Australia.
4Oceans Institute and School of Biological Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.
5University of Technology Sydney, School of Life Sciences, PO Box 123, Broadway, NSW 2007, Australia.
Foundation seaweed species are experiencing widespread declines and localised extinctions due to increases in sea surface temperature. Characterising temperature thresholds is critical 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 two species: Phyllospora comosa and Ecklonia radiata across their latitudinal range size – Port Macquarie to Tasmania –, we employed a new method to measure the photosynthetic thermal tolerance of these species acclimated to warm and cold-water temperatures. This new method employs temperature-dependent fluorescence (T–F0) curves under heating regimes to determine the critical temperature (Tcrit), 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. We found large differences in critical temperatures, with a 10ºC difference between populations at their northern vs southern limit, matching the thermal latitudinal gradient and the decrease of heat tolerance with latitude described for animals and terrestrial plants. Temperature-dependent fluorescence curves and their derived metric, Tcrit, may offer a timely and powerful new method for the field of phycology, enabling characterisation and comparison of photosynthetic thermal tolerance of kelp across many populations, species and biomes.