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Photosynthetic activity in thalli of Laminaria saccharina (Lamouroux) was followed in situ in the upper subtidal zone of a Northern Brittany rocky shore (Roscoff, France), using a submersible pulse amplitude modulated (PAM) fluorometer. Two fluorescence parameters, the effective quantum yield of photosystem II (Φ PSII) and the relative electron transport rate (rETR), were estimated at various stages of a tidal cycle from 10:00 to 18:30 h, and different light conditions due to variations of water depth and position of the sun. The Φ PSII decreased strongly during the ebb tide, essentially due to a drop in the maximal fluorescence level for light-adapted samples (Fm'). This was the result of increas- ing non-photochemical quenching (NPQ). Algae totally recovered during the rising tide, indicating that no significant photosynthetic damage occurred at ebb tide. L. saccharina responded to high light stress with photoprotective processes such as the xanthophyll cycle. The de-epoxidation ratio (DR) (i.e. conversion of violaxanthin into antheraxanthin and zeaxanthin) increased during the ebb tide and decreased during the rising tide. However, in spite of the development of a photoprotective mechanism, the overall photosynthetic activity (rETR) declined strongly at the highest irradiance level. This result indicates that primary production levels have been overestimated in the past.

Laminaria saccharina photosynthesis measured in situ: photoinhibition and xanthophyll cycle during a tidal cycle