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Tania Morais


M.Sc.: 1996 - 2000

Present occupation: Aboriginal Liaison Biologist, Species at Risk Program, Environment Canada, Ontario

Thesis: Phosphorus, nitrogen, and carbon nutrition in fresh and starved gametophytes of the red alga Porphyra purpurea (Roth) C. Agardh and occurrence of polyphosphates

Tania Morais Picture Tania Morais working on nori samples at a field site, Bay of Fundy, Canada (photo: Thierry Chopin). .

Abstract: Gametophytes of the red alga Porphyra purpurea (Roth) C. Agardh, a representative of the Bangiophyceae, were directly incubated in enriched seawater [15 µM phosphorus (P) and 25 µM nitrogen (N)] for up to 72 h, or kept under controlled starvation conditions for a few weeks in order to decrease their initial content of tissue total P before being incubated in enriched seawater. The phenomenon of P overcompensation was recorded in P. purpurea under certain conditions, depending on the nutrient prehistory of the plants. Fresh plants were richer in total acid-soluble phosphates, orthophosphates, lipid phosphates, and DNA- and precipitated acid-insoluble phosphates, than starved plants. For the first 6 hours, starved plants has a higher content of Ba-precipitated acid-insoluble polyphosphates than fresh plants; however, at 72 hours, their content was half that of fresh plants.

When detected, Ba-precipitated acid-insoluble polyphosphates and RNA-phosphates were at very low levels. Comparison of the contents of the different phosphorylated fractions between P. purpurea and Chondrus crispus, a representative of the Florideophyceae, considered the more advanced of the two classes of the Rhodophyta, point towards P. purpurea being a much more metabolically active P "pump" than C. crispus, with a higher P turnover rate, in which the orthophosphate fraction is predominant. Polyphosphates, three to four times more abundant in the acid-soluble/low molecular weight fraction than in the acid-insoluble fraction, may accumulate, but may be rapidly reverted to orthophosphates. They, consequently do not seems to be as significant a storage pool as in C. crispus.

Confirmation of the presence of acid-insoluble polyphosphates in the form of cystoplasmic granules was obtained by transmission electron microscopy and energy dispersive X-ray microanalysis. The granules in P. purpurea were much smaller (20 to 110 nm in diameter) than those in C. crispus (around 1 µm, but some larger than 2 µm in diameter). Larger granules (290 to 310 nm in diameter) were also observed. Their surface was more uniformly electron-opaque without the reticulated or globular appearance of typical polyphosphates granules. To our knowledge, this is the first report in algae of such structure, whose occurrence and metabolic role remains enigmatic.