Photochemical changes during rehydration of an intertidal cyanobacterial mat exposed to variations in salinity and light intensity.

This study focuses on a cf. dominated mat community from the intertidal zone of the Laguna Ojo de Liebre, Baja California Sur. In this environment, the mat is desiccated for several days between spring tides. While the mats were desiccated, photosynthetic activity was absent but recovered rapidly (~3 h) upon rehydration. It has been shown previously that the rate of photosynthetic recovery is dependent on both light intensity and salinity. In the current study, photosynthetic recovery was measured based on chlorophyll fluorescence using pulse amplitude modulated (PAM) fluorometry. Upon the addition of water, photosystem II (PSII) complexes recovered the capacity for reaction centre excitation. However, these functional centres were initially closed. Respiratory activity early in recovery probably reduced the plastoquinone pool through the shared use of part of the photosynthetic transport chain, thus temporarily blocking electron transport downstream of PSII. The time that PSII complexes remained closed increased with light intensities above saturation. This condition is potentially damaging to the cyanobacteria since the exposure of closed PSII centres to high light intensities can lead to the production of singlet oxygen. After this initial lag period, PSII centres opened rapidly indicating an increase in the flow of electrons from PSII to PSI. The rate of photosynthetic recovery appeared to be limited primarily by the relatively slow return of functional PSII. Photosynthetic recovery rates were slower in salinities greater than those that naturally occur in the intertidal zone.