Thermal threshold for fertilisation and gametophyte survivorship of the giant kelp Macrocystis pyrifera

Macrocystis pyrifera is a globally important and widely distributed kelp species, but one that is acutely susceptible to the impacts of climate change and, in many regions of the world, is in decline. Rising ocean temperature is a key driver of decline and is linked to the loss of the adult sporophyte stage of the kelp when temperatures persist above 17-20°C for extended periods of time. What is not well understood is the effect of temperature on the early developmental processes that occur from the spore to the embryonic sporophyte stage. To examine this, gametophytes collected from populations spanning the thermal range of M. pyrifera in Aotearoa New Zealand were exposed to 9 temperature levels from 10.5 to 23.8°C for a duration of 20 d as they underwent maturation and fertilisation. A temperature bottleneck was identified between 18.8 and 23.6°C, where fertilisation was inhibited; populations from warmer latitudes had a higher thermal threshold for successful fertilisation. A total of 30% of all gametophytes survived at the maximum tested temperature of 23.6°C, although they did not undergo fertilisation. This suggests that the microscopic stages of M. pyrifera likely exhibit greater thermal tolerance than the adult sporophyte stage, which is in agreement with similar studies. This study greatly enhances our understanding of the complex life cycle of M. pyrifera and emphasises the urgent need for extensive conservation initiatives to safeguard this ecologically crucial species in a changing ocean.