Photo-regulation in microphytobenthos from intertidal mudflats and non-tidal coastal shallows

Abstract

The study investigated seasonal changes in the photo-regulatory mechanisms of microphytobenthos found in intertidal mudflats (Aiguillon Bay, the Atlantic, France) and non-tidal sandy coastal shallows (Puck Bay, the Baltic, Poland) based on photosynthetic pigment characteristics and the estimates of photosynthetic parameters obtained through oxygen evolution measurements. The intertidal communities consisted of motile diatom species typical of epipelon. The non-tidal microphytobenthos was composed of epipsammic species mostly belonging to four taxonomic groups chiefly contributing to the assemblage biomass, namely cyanobacteria, euglenophytes, green algae and diatoms (comprising mainly small-sized species). The epipelon was low light acclimated as shown by the lower values of photoprotective/photosynthetic (PPC/PSC) carotenoids and diatoxanthin/diadinoxanthin (Dt/Dd) ratios. In contrast, the epipsammon exhibited features of high light acclimation (high PPC/PSC and Dt/Dd ratios). In both microphytobenthos types, the photosynthetic capacity (Pm) showed the same seasonal variation pattern and there were no statistically significant differences between the investigated sites in corresponding seasons (P > 0.05). In both assemblage types, the photosynthetic efficiency at limiting irradiance (α) decreased over time. The epipelon had higher α compared to the epipsammon. Seasonal changes of the photoacclimation index (Ek) estimated for the epipelic communities reflected variations observed in Pm, whereas in the epipsammon an increasing trend in Ek values was observed. Ek was always higher for the epipsammon when comparing analogous seasons, which further corroborated low and high light acclimation in the epipelic and epipsammic communities, respectively. The presence of the photoinhibition parameter (β) in the epipelon and the lack of it in the epipsammon suggested that the latter was resistant to high irradiance and the physiological mechanisms were sufficient to protect it from photoinhibition. In the epipelon, a downturn in photosynthetic rates showed that it was susceptible to high light intensities, suggesting that physiological photoprotective mechanisms must be supported by behavioural photoacclimation in order to avoid damaging light influence.