ABSTRACT: Here we examine the relationship between the photosynthetic performance of a wide range of Mediterranean marine macrophyte species (5 angiosperms and 18 macroalgae of widely different size and growth form), and several descriptors of the photosynthetictissue such as thickness, chlorophyll a (chl a) and nutrient (C, N, P) content. The photosynthetic performance of the plants is described by their photosynthesis-irradiance curves, either as continuous functions, or as the individualparameters defining the curves. Our results demonstrate a strong relationship between the photosynthetic efficiency (alpha) measured at low light and the photosynthetic rate at saturating light (P_max), and show alpha to be closelyrelated to differences in tissue thickness and chl a concentration among the marine macrophytes examined. The interaction between thickness and chl a concentration in the regulation of alpha is parallel to their interaction in the regulationof light absorption properties. Tissue thickness and pigment concentration not only explained variation in individual photosynthetic parameters (P_max and alpha), but were also able to summarise differences in the response ofphotosynthesis to irradiance. The strong relationship between tissue thickness and photosynthetic performance lend support to the important adaptive role attributed to the thickness of marine macrophyte tissues in the past. This importance of thicknessextends beyond the regulation of plant productivity to the resistance to grazing, mechanical damage and the longevity of the tissue. Considering previous demonstrations of the functional importance of thickness for phytoplankton and for land plants, wesuggest the existence of a general relationship between tissue thickness and the photosynthetic performance of photosynthetic organisms.