Efficiency of photoprotection in microphytobenthos: role of vertical migration and the xanthophyll cycle against photoinhibition

Abstract

The capacity of microphytobenthos to withstand the variable and extreme conditions of the intertidal environment, prone to cause photoinhibition, has been attributed to particularly efficient photoprotection. However, little is known regarding the capacity of this protection against photoinhibition or the mechanisms responsible for it. The present study quantified the photoprotective capacity and the extent of photoinhibition under excess light, estimated the contribution of vertical migration and the xanthophyll cycle to overall photoprotection, and evaluated the effects of photoacclimation. A new experimental protocol combined (1) chlorophyll fluorescence imaging, for the simultaneous measurement of replicates and experimental treatments, (2) specific inhibitors for vertical migration and for the xanthophyll cycle, to quantify the relative contribution of each process, and (3) recovery kinetics analysis of photosynthetic activity during light stress-recovery experiments, to distinguish rapidly reversible photochemical down-regulation from photoinhibition. The results show a high photoprotective capacity in 2 study periods, May and October, with photoinhibition rates below 20%. A clear change in photoacclimation state was observed, with acclimation to lower irradiances in autumn associated with higher susceptibility to photoinhibition. In May, vertical migration and the xanthophyll cycle provided comparable protection against photoinhibition; in October, the former predominated. The sum of their contributions was ~20% in both months, suggesting that other processes also contribute to photoprotection.KEY WORDS: Microphytobenthos · Photoinhibition · Photoprotection · Xanthophyll cycle · Vertical migration · Non-photochemical quenching · Chlorophyll fluorescence · Diatoms Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 67: [161][162][163][164][165][166][167][168][169][170][171][172][173][174][175] 2012 damaging to the photo synthetic apparatus when acting individually, the combined effects of all of these factors likely coalesce in the photoinhibition of photosynthesis of benthic microalgae. Of particular importance is the exposure to direct sunlight, which can result in excessive reductant pressure and in the formation of intracellular reactive oxygen species (ROS; Roncarati et al. 2008, Waring et al. 2010. High levels of ROS cause the permanent inactivation of photosystem II (PSII) protein D1, negatively im pacting photosynthetic yield and primary productivity (Nishiyama et al. 2006).Despite these harsh conditions, microphytobenthos of intertidal flats typically exhibit high growth rates, forming dense and diverse sedimentary biofilms, and are recognized as a major contributor to ecosystemlevel carbon fixation and primary productivity (Underwood & Kromkamp 1999). Furthermore, an apparent lack of photoinhibition in microphytobenthic biofilms has been repeatedly reported (Blanchard & Cariou-LeGall 1994, Kromkamp et al. 1998, Underwood 2002, Blanchard et ...

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