Ecophysiology of Tropical Rhodophytes. Ii. Microscale Acclimation in Photosynthesis1

Parameters of photosynthesis vs. irradiance curves varied markedly between tissues from microsites along the < 10‐cm axes of the tropical intertidal red algae Ahnfeltiopsis concinna (J. Ag.) Silva et DeCew and Laurencia mcdermidiae (J. Ag.) Abbott. Differences in photosynthetic performance between tissues from canopy and understory microsites indicates that L. mcdermidiae exhibited an expected sun‐to‐shade acclimation but over the space of < 10 cm. Respiration, Ic, Ik, and Pmax values were significantly lower in tissues from the understory relative to tissues from the canopy of L. mcdermidiae, while photosystem I (PS I) sizes (PSU I) were significantly higher in tissues from understory microsites. Quantum efficiency was unchanged. Ahnfeltiopsis concinna, in contrast, exhibited higher α in tissues from understory rather than canopy microsites. The values of Pmax for tissues from the canopy of A. concinna were not higher than the understory, while PSU O2(PS II size) of tissues from canopy microsites were unusually higher than those of understory microsites. These characteristics suggest a high degree of irradiance stress in tissues from the canopy of A. concinna, the highest tidal alga in Hawaiian coastal zones. Acclimation to high photosynthetically active radiation and ultraviolet irradiance levels especially in tropical regions appears to be an essential mechanism(s) for stress resistance and persistence of intertidal algae. Algal turfs acclimate at microscales in part fostered by their dense stands that create sharp irradiance gradients as well as adjust physiologically to canopy irradiance levels as mechanisms for optimal photosynthetic performance and stress tolerance.