ALBERT

Description: Kelp forests are important habitats in the strongly environmentally and seasonally variable Arctic. There is a critical lack of knowledge about how seasonal conditions and climate change scenarios influence survival and reproduction of kelp early life stages. To better understand the regulation of kelp life cycle processes in this harsh environment we focused on the physiological performance and reproductive success of early life stages in Alaria esculenta and Laminaria digitata from Kongsfjorden, Spitsbergen. Gametophyte growth and survival during Arctic winter and subsequent sporophyte recruitment under spring conditions were investigated. Winter conditions (2°C, complete darkness) halted gametophyte growth and prevented the onset of gametogenesis in both species. The gametophytes of L. digitata but not A. esculenta became fertile after returning to spring conditions, suggesting that sporogenesis, sexual reproduction and recruitment in A. esculenta must occur successively during summer/autumn while in L. digitata a new generation of sporophytes could develop from overwintering gametophytes. The effects of simulated canopy shading (offering protection against extreme irradiance stress, particularly as sea ice retreats), present-day and projected Arctic summer seawater temperatures, and nutrient levels on gametophyte survival, fertility and sporophyte recruitment success were also investigated in both species. A. esculenta gametophytes had greater survival and reproductive success than L. digitata, except under very low light (simulating dense canopy). In contrast, shading was required for reproductive success in L. digitata gametophytes. Predicted summer temperatures of 9°C reduced sexual reproduction in both species. Interactions observed between these environmental drivers probably reflect species-specific seasonal patterns of survival and reproduction. These differences between kelp species in response to abiotic factors and light levels (simulated canopy shading) suggest that climate change could alter community structure in the Arctic through effects on sexual reproduction and sporophyte recruitment success.

Description: Many organisms have endogenous clocks that synchronize biological processes with environmental changes, leading to optimized development and reproduction. However, certain environments, like the Arctic, pose a special challenge to circadian clocks, particularly due to extreme seasonal changes in daylength, ranging from permanent sunlight to complete darkness. Kelps seem to be well adapted to the variable environmental conditions characteristic of this region. However, daylength might affect kelp species that use circadian rhythms to control the timing of daily egg release from female gametophytes. We aimed to investigate how daylength and light intensity affect gametogenesis and reproductive success of summer-reproducing kelp species (using Alaria esculenta as a model). As daylength and temperature co-vary most of the year, we also investigated the thermal resilience of the sporophytes developed under different daylengths to understand if there is a cross-tolerance between light doses and temperature tolerance. Although continuous daylight, characteristic of Arctic summers, enhanced gametogenesis and increased gametophyte vegetative growth, and thereby the number of potential reproductive gametophyte cells, sporophyte production was higher under long (16 h light:8 h dark) and intermediate (12:12 h) days. Sporophyte growth was triggered by changing daylength from short to long days, suggesting a synchronization with annual daylength variation. High daily light doses during reproduction and early development improved subsequent sporophyte survival at high (sub)lethal temperatures, indicating cross-tolerance between light and temperature. Reproductive success in Arctic A. esculenta was hampered under continuous light, and we hypothesize that this might result from disturbance of synchronized egg release and subsequent fertilization.