In situ photosynthetic activity and xanthophylls cycle development of undisturbed microphytobenthos in an intertidal mudflat

The photosynthetic performances of microphytobenthos as well as the xanthophylls cycle development were monitored directly in situ on undisturbed intertidal mudflats in the Authie Bay (Northern France) during the two daylight emersion periods of the same day. The effective quantum yield of PSII was measured regularly using the PAM fluorescence technique at the same three locations on the sediment, allowing the estimation of the relative electron transport rate (rETR) and the establishment of two light curves. The photosynthetic responses of microalgae at a given light level differed for one emersion period to the other. The maximum rETR (rETRm) calculated from the afternoon light curve were significantly higher than in the morning. Pigments analyses show evidence of the progressive conversion of diadinoxanthin (DD) to diatoxanthin (DT) with increasing light. We demonstrate for the first time, with in situ measurements, the involvement of the xanthophylls cycle in the photoacclimation process in microphytobenthos at high irradiances.