Effects of thermal history on the heat shock response of bull kelp ( <i>Nereocystis luetkeana</i> )

Abstract In 2013, a marine heatwave called the “Blob” caused northern California bull kelp ( Nereocystis luetkeana ) populations to decrease significantly. The loss of this foundation species motivated recent investigations on kelp thermal tolerance to understand how warming events might impact their physiological performance. We tested the effect of acclimation temperature on bull kelp by measuring the protein abundance of a molecular chaperone, heat shock protein 70 (Hsp70). Blades were collected from two sites (Big River and Russian Gulch) and acclimated for 7-days at two temperatures (13 and 17 °C) before tissues were subjected to a 1-h heat shock (13, 17, 20, 23, and 26 °C). Results showed a significant difference between acclimation temperature with 20 % greater total Hsp70 protein abundance in bull kelp acclimated to the warm treatment, while site was only marginally different. Heat shock temperature had no effect on total Hsp70 protein abundance. This study is the first to report about the heat shock response of bull kelp and found that thermal history of an organism is an important factor in determining whether an organism can mount a heat shock response. This information can help understand bull kelp’s tolerance to future marine heatwaves.