Mechanism of nitrogen assimilation of Porphyra from New England

Date of Completion

January 2008


Biology, Oceanography|Biology, Plant Physiology




Intertidal 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. I used some of the most cosmopolitan species of Porphyra (Rhodophyta, Bangiophyceae, Bangiales) to understand desiccation effects on nitrogen assimilation from coastal New England. Eulittoral and sublittoral species of Porphyra have different strategies for nutrient uptake. Nitrate uptake function recovered quickly after emersion of upper littoral P. umbilicalis , while lower- or sub-littoral P. leucosticta and P. yezoensis, respectively, could not recover their function. Population differences also were found for the upper and lower littoral populations of P. umbilicalis. The upper littoral population of P. umbilicalis was not affected by desiccation, whereas the nitrate uptake and nitrate reductase (NR) activity of the lower littoral population were significantly lower than those of non-desiccated controls post desiccation. Different Porphyra species with different cross-sectional thicknesses, from the same vertical habitats have different growth strategies when they experience periodic desiccation. When Porphyra experienced severe desiccation stress, the thinner species, P. linearis, grew slower (< 40%) as compared to the undesiccated control; whereas the thicker P. umbilicalis was still able to grow at relatively high rates (ca. two-thirds of the control). Tissue nitrogen (N) content and glutamine synthetase (GS) activity were higher in Porphyra acclimated to emersion than thalli that were continuously submerged. The high level of tissue N content and GS activity may be a protective mechanism, enabling Porphyra to elevate desiccation tolerance proteins (dehydrins) and to assimilate N quickly when it becomes available. Tissue N and soluble protein contents in Porphyra declined during emersion but recovered quickly (e.g. within 30 min) after re-submergence, and these two contents showed a positive correlation. This observation suggests that emersion-induced N release may constitute an undescribed biogeochemical pathway linking marine, terrestrial, and atmospheric N reservoirs. This result also suggests that the desiccation induced N release during emersion may have come from the organic nitrogen compounds in P. umbilicalis. The nitrogen isotopic fractionation did not change during emersion, suggesting that N release from Porphyra during desiccation may not be through denitrification, but through an unknown biochemical pathway. ^