Physiological activity of Porphyra in relation to eulittoral zonation

Eulittoral seaweeds at different tidal elevations are exposed to various frequencies and durations of emergence. Their physiological activities (e.g. nutrient uptake) may be affected by water loss during emersion. We used three Porphyra species from different tidal elevations to test whether species at different vertical elevations on the shore respond differently to the increasingly non-marine environment, in terms of their physiological activities including nutrient uptake, tissue carbon, nitrogen and phycoerythrin contents. Simulated tidal cycles produced water losses of 0%, 40 ± 10% and 90 ± 5% tissue water. Emersion was stressful for all species regardless of their habitat. It was more stressful to nitrate and phosphate uptake for the sublittoral species P. yezoensis than eulittoral species, P. umbilicalis and P. leucosticta. Interestingly, tissue N for thalli that had been emerged and then re-submerged was significantly higher than those of continuously submerged individuals. During exposure, tissue N contents of all species declined but recovered quickly (e.g. within 30 min) after re-submergence. This result suggests that emersion-induced N release may constitute an undescribed biogeochemical pathway linking marine, terrestrial, and atmospheric N reservoirs.