Predicting heat tolerance in sugar kelp juvenile sporophytes via gametophyte heat stress testing

Warming sea temperatures are threatening global kelp populations, including sugar kelp (Saccharina latissima), one of the most commonly farmed kelp species. For the future of kelp aquaculture and restoration, we must identify individuals with natural adaptations to cope with heat. If heat tolerance of adult kelp can be predicted by assessment at the early-life gametophyte stage (prior to fertilization), then we can accelerate breeding of heat-tolerant strains. We developed a high-throughput method for assessing gametophyte physiological stress under heat and conducted an experiment to generate predictions of heat-tolerant strains. Gametophyte heat tolerance was assessed for 93 distinct genotypes by exposing them to temperatures representing current (12°C) and future high (24°C) temperatures in the Gulf of Maine, U.S.A., and measuring their photosynthetic performance (as chlorophyll a fluorescence) using a plate-reading spectrofluorometer. Individual tolerance to heat, which was determined by comparing post-heat to baseline fluorescence values, ranged from 4% to 100%. We then conducted a heat stress experiment with juvenile sporophytes made by crossing male and female suspected heat-tolerant gametophytes, and male and female suspected intolerant gametophytes. Juvenile sporophytes from 7 unique crosses were exposed to control (12°C) and warming (up to 22°C) temperatures and their growth and percent degradation were monitored. Our results indicate that we can identify heat-tolerant gametophytes that, when crossed, yield juvenile sporophytes that have greater growth under heat stress than sporophyte progeny of predicted intolerant gametophytes. These results indicate the potential to create heat-tolerant sporophytes by knowing only the gametophyte performance under heat stress.