Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos

ABSTRACT: Microphytobenthos (MPB) inhabiting intertidal flats of estuaries form highly productive diatom-dominated biofilms. The capacity to sustain such high photosynthetic activity under conditions prone to cause photoinhibition is thought to be enabled by efficient photoprotective mechanisms, the main ones being the xanthophyll cycle (XC) and vertical migration (VM). This study compared the photoprotective capacity of 2 MPB communities inhabiting contrasting sedimentary habitats and relying on distinct light responses: epipelic communities, colonizing muddy sediments and using motility (VM) to regulate light exposure; and epipsammic communities, inhabiting sandier sediments and relying solely on physiological photoprotection (XC). The efficiency of physiological photoprotection of the 2 communities was compared regarding Photosystem II (PSII) photoinactivation caused by light stress. Lincomycin was used to distinguish photoinactivation from counteracting repair. Rate constants of PSII photoinactivation (kPI) and repair (kREC) were determined on cell suspensions, based on the light and time dependence of maximum quantum yield of PSII, Fv/Fm, as measured using multi-actinic imaging fluorometry. The results show that motile species, in comparison to epipsammic ones, are inherently more susceptible to photoinactivation (higher kPI), less dependent on the XC for preventing photodamage (smaller increase of kPI induced by nigericin) and more efficient regarding repair capacity (higher kREC). The distinct strategies exhibited by epipelic and epipsammic communities to cope with light stress support the hypothesized trade-off between photoprotective motility and photophysiology. Motile forms have a diminished physiological capacity for preventing photodamage and compensate using VM and a better repair capacity. Non-motile epipsammic forms rely mostly on physiological mechanisms to optimize photoprotection capacity.