Dynamics of Fucus serratus thallus photosynthesis and community primary production during emersion across seasons: canopy dampening and biochemical acclimation

The brown alga Fucus serratus forms dense stands on the sheltered low intertidal rocky shores of the Northeast Atlantic coast. In the southern English Channel, these stands have proved to be highly productive, particularly during emersion periods. Here, we studied the dampening effect of the canopy cover, associated with physiological and biochemical acclimation processes, that allows this species to withstand emersion stress. The F. serratus community primary production and the photosynthetic performance of thalli were concurrently followed in situ, throughout the midday emersion period, in different seasons and under various weather conditions. In addition, thallus samples were taken at various tidal stages to determine their content in biochemical compounds involved in photoprotective and antioxidant mechanisms. Under high light and temperature, the F. serratus community exhibited high aerial production rates (sometimes exceeding 1 g C m-2 h-1) that never decreased to less than 59% of the initial value during the emersion period. Under mild weather conditions, photosynthesis in thalli at the top of the canopy (measured as the relative electron transport rate) varied in response to changing incident light. Under harsh weather conditions (i.e. high light and temperature), the effective quantum yield of photosystem II (PSII) dramatically decreased in thalli at the top of the canopy, but remained high in thalli at the bottom of or within the canopy. Due to self-shading, photosynthesis was light-limited in thalli in the lowest layer of the canopy, but was effective in thalli in the intermediate layers. Photoinhibition was observed in thalli at the top of the canopy (as a dramatic decrease in the optimal photosynthetic quantum yield Fv/Fm), but not in thalli beneath the canopy. At the end of the emersion period, Fv/Fm was strongly correlated to the relative water content of thalli. The findings from our simultaneous analysis of biochemical and photosynthetic parameters suggest coordination between the xanthophyll and the ascorbate-glutathione cycles that varies with season. An accumulation of hydrogen peroxide was nevertheless observed once, indicating that oxidative stress is nonetheless possible under particularly harsh conditions.