PHOSPHORUS-LIMITED GROWTH IN THE SEASONAL WINTER RED ALGA PORPHYRA-LEUCOSTICTA THURET IN LE-JOLIS

The effect of N- and P-enrichment on the growth rate of the gametophyt ic phase of Porphyra leucosticta in culture was studied to assess the hypothesis that growth of the alga is N-limited at the start of the gr owing season (mid-winter) and P-limited at the end (late winter-early spring). Porphyra leucosticta was cultivated at sixteen combinations o f different concentrations of NO3- (0, 0.6, 3 and 6 mM) and PO43- (0.5 , 10 and 50 mu M) Relative growth rate (RGR) was measured after two we eks at 15 degrees C, 85 mu mol m(-2) s(-1) and 16:(8) over bar, L:(D) over bar photoperiod. Experiments were carried out at the beginning (F ebruary) and the end (April) of the seasonal cycle of the alga. Growth rate of the alga was not affected by nutrient enrichment at the start of the seasonal cycle, suggesting the lack of a severe nutrient limit ation in February (overall mean 4.0 +/- 1.0% d(-1)). In April, P-enric hment produced a significant increase in growth rate at 10 and 50 mu M phosphate (overall mean 4.7 +/- 1.0% d(-1)) with respect to lower pho sphate concentrations (0 and 5 mu M), but N-enrichment nor the N x P i nteraction did not affect the RGR. These data suggest that the growth was P-limited in April. Significant similar RGR values were obtained i n the control and in nutrient enrichment treatments in February (all d ata pooled) and in 10 and 50 mu M phosphate in April, indicating that growth was nutrient-saturated in natural conditions at the start of th e seasonal cycle.