Variability in the effects of ocean acidification on coral reef macroalgae

Ecophysiological characteristics of algae and their acclimation potential to environmental stress are often assessed by quenching analyses of chlorophyll fluorescence (qP and NPQ) and changes to the parameters of Photosynthesis vs. Irradiance (P/ E) curves such as , rETRmax and Ek. Using chlorophyll fluorescence in laboratory experiments P/E curves are either measured as Rapid or Steady State Light Curves (RLC, LC). Here, effects of short-term emersion on the photo-response patterns of four brown macroalgae collected from different shore levels (Ascophyllum nodosum, Sargassum muticum, Fucus serratus, Laminaria digitata) were compared by measuring both RLC and LC. When LC were applied, photosynthetic performance was significantly enhanced in A. nodosum and F. serratus upon air exposure as shown by an increase in qP, rETRmax and Ek, whereas short-term emersion had no impact on photosynthetic parameters of S. muticum and L. digitata. NPQmax was significantly higher only in the lower shore species L. digitata during emersion. However relative changes in NPQ as a function of rETR were reduced in A. nodosum, F. serratus and S. muticum, but remained unaffected in L. digitata. Such results were not obtained from the application of RLC. None of the species developed their potential NPQmax as determined from LC. Some artifacts were further generated by RLC for F. serratus and L. digitata resulting in a contrary outcome as photosynthetic performance appeared to be significantly reduced upon exposure to air which was associated with a higher ‘‘NPQmax’’. These results indicate that emersion may not be the main abiotic stressor for intertidal brown seaweeds. Moreover, when photo-response patterns are used as a measure of the extent of stress experienced by a brown macroalgae, the LC protocol should be applied since the application of RLC did not always reveal environmental impacts on photosynthesis.