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 photosynthetic tissue 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 individual parameters defining the curves. Our results demonstrate a strong relationship between the photosynthetic efficiency (a) measured at low light and the photosynthetic rate at saturating light (Pmax), and show a to be closely related 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 a is parallel to their interaction in the regulation of light absorption properties. Tissue thickness and pigment concentration not only explained variation in individual photosynthetic parameters (Pmax and a), but were also able to summarise differences in the response of photosynthesis 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 thickness extends 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, we suggest the existence of a general relationship between tissue thickness and the photosynthetic performance of photosynthetic organisms.