Bridging land and sea perspectives of photosynthetic thermal tolerance in a climate change context

Abstract

Temperature is one of the most critical drivers of species evolution and distribution. Due to climate change, it has become increasingly variable and extreme, profoundly impacting terrestrial and marine ecosystems. Rising heatwaves, fluctuating temperatures, and variable rainfall affect land ecosystems, while marine systems face ocean warming and more frequent marine heatwaves, causing species range contractions and community restructuring. Sessile photosynthetic organisms in both environments must adapt to climate-induced temperature changes to survive. My thesis investigates the physiological mechanisms behind species' thermal tolerance to identify those at risk and support conservation efforts.

My work aims to elucidate the physiological mechanisms behind photosynthetic organisms' responses to temperature extremes using scalable metrics crucial for high-throughput research. It investigates various methods for assessing thermal tolerance, including chlorophyll a fluorescence to measure photochemical efficiency. Insights gained from genomic clusters helped confirm the performance of genetically resilient individuals for future restoration efforts.

The empirical work includes four data chapters. The first focuses on the heat and cold thermal tolerance of Australian angiosperm juveniles, revealing that both extremes narrow their thermal tolerance range. The photobiology tools developed informed a second study, adapting terrestrial high-throughput techniques for seaweed research, marking a significant advancement in phycology and enabling broader ecological questions relevant to seaweed resilience.

I then examined the thermal tolerance of Phyllospora comosa across a latitudinal gradient, showing correlations with genetic structure. Warm-edge populations, though highly tolerant, had low genetic diversity, while central populations were key genetic repositories. The final chapter assesses seasonal variation in thermal plasticity in two temperate canopy-forming seaweeds, Ecklonia radiata and Phyllospora comosa, highlighting critical times and places of vulnerability. Both species adjust their thermal tolerance in response to seasonal sea surface temperatures, with Phyllospora displaying stable respiration and Ecklonia showing potential energy deficits in warmer periods.

These findings highlight the link between thermal tolerance and respiratory responses in seaweeds, offering insights into their resilience amid changing climates. By applying lessons from terrestrial systems to marine ecology, this research enhances our understanding of plant thermal stress responses and emphasizes the importance of interdisciplinary approaches. My PhD lays the foundation for future studies on the thermal biology of seaweeds and other species facing climate challenges.