Climate stress priming of juvenile Southern California giant kelp (Macrocystis pyrifera) to thermal extremes

Climate change threatens food production across the globe, creating challenges for food systems. Aquaculture, including seaweed production, is expanding while being threatened by global climate stressors, including increasing extreme events. Marine aquaculture is especially vulnerable to heatwaves, which can rapidly raise temperatures above the physiological limits of some organisms. While several interventions to increase resilience to climate impacts are being explored, priming has emerged as a possible adaptation for seaweeds that maintains genetic diversity but hardens individuals to stressors later in life. California has a developing seaweed sector while also experiencing some of the most extreme marine heatwave conditions on record. We explore temperature impacts and priming - exposing an earlier stage of an organism to a mild stressor to prepare the individual for future stress - on giant kelp Macrocystis pyrifera, an important foundation species along the West Coast of the United States. Our experiments focused on the juvenile sporophyte stage on miniaturized spools, from approximately one week before outplanting size to one week after. First, we determined the reaction norms of M. pyrifera in waters ranging from 5 to 30{degrees}C at the outplanting stage. Next, we explored how priming (heat + or - nutrients) in a hatchery setting prepares M. pyrifera for outplanting to a marine heatwave. To assess experimental outcomes, we took measures of growth, survival, photosynthetic function (Fv/Fm), and carbon and nitrogen assimilation via isotopes. We found temperatures above 20{degrees}C had significant negative impacts on all metrics of performance during the juvenile sporophyte stage. Further, we determined heat priming in conjunction with hatchery level (+) nutrients resulted in overall increased performance when exposed to a marine heatwave. These findings support the continued exploration of priming as a tool for climate resilience and can inform current hatchery practices for aquaculture practitioners looking to improve crop outcomes for this species.