Comparison of growth and nitrate uptake by New England Porphyra species from different tidal elevations in relation to desiccation

Desiccation stress can determine the upper distribution limits and may enhance the uptake of nitrate and ammonium of eulittoral algal species. Upper shore species may exhibit greater stimulation of nitrate uptake following desiccation and achieve maximum uptake at higher desiccation levels. The objective of this study was to determine whether Porphyra species from different vertical elevations respond differently to the desiccation stress, in terms of growth and nitrate uptake. A eulittoral species (Porphyra umbilicalis) and a sublittoral species (P. amplissima) were compared in the present study. Samples were exposed to air for 0, 30 min (40 10% water loss) and 2 h (90 5% water loss), after an initial 4 h light period every day. Desiccation was more stressful to the sublittoral species, Porphyra amplissima, than to the eulittoral species, P. umbilicalis. When tissues were exposed for 2 h daily, P. amplissima lost weight over a 24 h day, while the growth rate of P. umbilicalis dropped by only 30% compared with that of continuously submerged blades. Nitrate uptake rate of sublittoral P. amplissima was only 73% (40 10% water loss) and 62% (90 5% water loss) of that of continuously sub-merged tissue. Nitrate uptake rates of P. umbilicalis were not significantly affected by desiccation. These results suggest that species in the eulittoral zone, which have longer exposure times, have a higher time-use efficiency than the sublittoral species in terms of nitrate uptake. This indicates a possible correlation between nitrate uptake and observed vertical distribu-tion patterns.