Acclimation of kelp photosynthesis to seasonal changes in the underwater radiation regime of an Arctic fjord system

Abstract

In Arctic coastal ecosystems marine macroalgae are subjected to drastic seasonal changes in the environmental conditions, especially in the radiation climate. The seasonal variations in the photosynthetic performance, the photosynthetic pigment content and composition as well as in the relative content of compounds screening ultraviolet radiation (UVR) were studied in young sporophytes of Alaria esculenta, Laminaria digitata, Saccharina latissima and Laminaria solidungula (Laminariales, Phaeophyceae) in relation to changes in the underwater radiation regime.Between May and September macroalgae were collected before and after the break-up of sea ice and during clear and turbid water conditions characteristic for spring, early and mid-summer in the Arctic Kongsfjord (Spitsbergen, Norway).To evaluate the acclimation response of the Laminariales, the contents of photosynthetic pigments and UVR-absorbing compounds as well as the maximum relative electron transport rate (rETRmax), the light saturation point of photosynthesis (Ek) and the photosynthetic efficiency (alpha) were determined immediately after collection. Seasonal variation in the sensitivity of macroalgal photosynthesis to photosynthetically active radiation (PAR) and UVR was studied by measuring the optimum quantum yield of photosynthesis (Fv/Fm) before and during 4 h exposure to different artificial radiation treatments as well as during 20 h of recovery in dim white light.In all species studied, photosynthetic characteristics and pigmentation changed significantly in relation to the prevailing radiation conditions at the natural growth sites. In May, all species were highly susceptible to artificial PAR and UVR and were characterized by low rETRmax and Ek accompanied by high alpha and high contents of chlorophyll a and accessory pigments (fucoxanthin, chlorophyll c). Highest PAR and UVR tolerance was found in June, when all species acclimated to the strongly increased underwater irradiances after the sea ice break-up by an increase in rETRmax and Ek concomitantly with a decrease in alpha and in the photosynthetic pigment content. In addition, the algae showed significantly higher contents of photoprotective beta-carotene, xanthophyll cycle pigments (violaxanthin, antheraxanthin, zeaxanthin) and UVR-screening substances. In contrast, with reduced light availability from July onwards, inverse patterns of these photosynthetic and photoprotective pigments and photosynthetic parameters were observed. These reversible acclimation processes enable the perennial Laminariales to maintain optimal photosynthetic performance throughout the seasons despite of significant changes in solar radiation.