Dominique Menu

Photophysiology and daily primary production of a temperate symbiotic gorgonian

Gorgonians are one of the most important benthic components of tropical and temperate areas, and play a fundamental role as ecosystem engineers. Although global warming and pollution increasingly threaten them, the acquisition of nutrients, which is a key process in fitness and stress resistance, has been poorly investigated in such species. This study has thus used an advanced in situ incubation chamber for the first time with gorgonians, to assess the daily acquisition of nutrients and the photophysiology of the Mediterranean symbiotic species, Eunicella singularis. The xanthophyll cycle was assessed in parallel. This work has revealed that E. singularis presents a different functioning than the Mediterranean symbiotic corals. This gorgonian indeed relies on both autotrophy and heterotrophy in summer to optimize its energetic budget, while corals mainly shift to autotrophy for their respiratory needs and tissue growth. In addition, although E. singularis lives in the same depths/locations, and harbours the same symbiont genotype than the corals, the photosynthetic performances of their respective symbionts are significantly different. Indeed, E. singularis acquired 2–3 times less autotrophic carbon from its symbionts than corals, but maintained a positive carbon budget by reducing respiration rates, and by presenting maximal photosynthetic rates throughout the day, suggesting a very efficient light utilization. Almost no photoinhibition was observed under very high light levels, because of the induction of a xanthophyll photoprotection process. These results help understanding why gorgonians often dominate many benthic ecosystems.

Photosynthetic response to light and temperature in Laminaria digitata gametophytes from two French populations

ABSTRACT Given the growing body of evidence on the general decline of kelp beds worldwide, it is crucial to understand the physiological responses of kelp gametophyte stages to environmental parameters. We investigated the physiological responses to light and temperature of gametophytes from two populations of Laminaria digitata in contrasting environments along the French coast of the English Channel. Gametophytes of both populations were highly tolerant of high light through an efficient down-regulation of photosynthesis triggered by the activation of the xanthophyll cycle. Temperature increases promoted photosynthesis and photosystem II showed high resistance to short-term exposure to high temperatures currently encountered in the field. Gametophytes from the two sites displayed some differences in their pigment content and photosynthetic characteristics, but low replication and difference in time of sampling precluded tests of potential local adaptation to the light conditions at each site, as observed in previously published results on adult sporophytes. Gametophytes of L. digitata appeared to be resistant to irradiation and temperature conditions currently experienced in the field, confirming their role in persistence of kelp species under stressful environmental conditions.