Abstract

Photosynthetic characteristics of 18 macroalgal species, measured as chlorophyll a (chl a) fluorescence of photosystem II (PSII), pigment contents and susceptibility of photosynthesis to solar radiation, were studied in the field in southern Chile (Niebla, Valdivia) during mid-summer. Photosynthesis versus irradiance curves indicated that algae exhibited sun-adapted characteristics, with saturating points for photosynthesis (Ek) ranging between 50 and 400 ?mol photon m-2 s-1. Under the solar radiation conditions that prevailed during the study period at this locality (daily doses of photosynthetically active radiation, PAR, close to 14 000 kJ m-2 d-1, with instantaneous irradiances exceeding 2000 ?mol photon m-2 s-1 at noon), algae were light saturated for periods (Hsat) between 11 and 14 h d-1. Daily courses of optimal quantum yield (Fv/Fm) of chl a fluorescence of PSII revealed the existence of photoinhibition of 46% at maximum relative to fluorescence measured in the morning. Similarly, the effective quantum yield (?PSII) showed a decrease at noon coinciding with the peak of solar radiation. This reduction in ?PSII was accentuated when algae were maintained in immersion for the whole measuring period; whereas, in algae measured in situ, i.e. subject to varying tidal height, chl a fluorescence decreased less. Although algae are currently exposed to very high doses of ultraviolet (UV) radiation, short-term exposures of 2.5 h around noon using specific cut-off filters did not result in high photoinhibition of Fv/Fm (rates < 25%), reinforcing the idea that these intertidal species are characterized by a marked tolerance to current solar UV radiation. Differences in the rates of photoinhibition of photosynthesis as well as in the photosynthetic characteristics were not related to functional-form group (e.g. thin vs thick morphs). Instead, the photosynthetic performance of algae was determined by their position in the shore: algae growing at mid-/infra-littoral zones had lower Ek and lower photoinhibition than algae from upper locations.