Non-photochemical quenching of chlorophyll fluorescence and operation of the xanthophyll cycle in estuarine microphytobenthos

The induction of non-photochemical quenching of chlorophyll fluorescence (NPQ) and its relationship with the operation of the xanthophyll cycle were studied in estuarine microphytobenthos assemblages. NPQ and xanthophyll cycle operation were characterised by quantifying the induction kinetics and light-response curves of NPQ formation and of the production of xanthophyll cyle pigments diadinoxanthin (DD) and diatoxanthin (DT), on suspensions of benthic microalgae collected during two spring-neap tidal cycles, in summer (July) and in winter (November). The NPQ light response was characterised by a large intraday variability and by the high NPQ values attained under high irradiances. Maximum daily NPQ often reached values above 4.0, and values above 5.0 were observed on 2 days during the November sampling period. Changes in NPQ were generally followed by proportional variations in the DT content, both during exposure to high light and upon transition from darkness to low light, leading to highly significant correlations between NPQ and the DT content or the degree of de-epoxidation, DT / (DD + DT). Higher NPQ values in the dark than under low light were consistently observed. This was due to the generalised increase of Fm′ levels under low light to values higher than the Fm measured in the dark, and was associated to the decrease of the DT content upon exposure to low light. Significant seasonal variability was found regarding the parameters related to NPQ induction kinetics, indicating a higher capacity for NPQ operation in November than in July, both in the dark and as a response to high light. This seasonal variation in the NPQ light response was found to be associated to substantial changes in the taxonomic composition of microphytobenthos assemblages, and was interpreted as resulting from changes in the potential photoprotective response associated to thermal acclimation to winter conditions.