Photosynthetic performance and photoprotection of Cystoseira humilis (Phaeophyceae) and Digenea simplex (Rhodophyceae) in an intertidal rock pool

• The photosynthetic activity of macroalgae is connected with their vertical position in the pool.
• Pigment concentrations showed an increase with depth for both macroalgae.
• Thalli from the shallow level showed higher photoprotective compounds (phenols in C. humilis and MAAs in D. simplex) than thalli from the deep level.
• Species change their photosynthetic and pigment apparatus according to prevailing light conditions.

Rock pools are dynamic and intermittently isolated habitats in the rocky intertidal. In this study, we assessed if the photosynthesis and physiological activity of the brown macroalga Cystoseira humilis and the red macroalga Digenea simplex in a rock pool at Lanzarote Island (eastern Atlantic) was affected due by their vertical distribution. Photosynthetic responses were measured at three depth levels (0.05–1 m, 0.4–2.5 and 3.5 m) through in vivo chlorophyll a fluorescence, in particular the maximal quantum yield (Fv/Fm) as an estimator of the physiological status and photoinhibition, and the electron transport rate (ETR) as an estimator of the photosynthetic capacity. Algal photoprotection and photodamage processes were related to algal zonation; shallow-water thalli had active mechanisms of dynamic photoinhibition compared to algae from the deeper level. The progressive increase of solar radiation during the day caused different responses for each macroalga, where C. humilis showed lower photoinhibition, higher ETR values than D. simplex. Algae from the shallow level had lower pigment content and higher resistance to high solar radiation through the accumulation of photoprotective compounds and higher antioxidant activity (DPPH) compared to thalli from the deeper level. In summary, this study corroborates that algae are vertically distributed inside rock pools according to their adaptive responses to light-induced stress conditions and that photoacclimation occur in a short-term period.