SEASONAL CYCLE OF THE GAMETOPHYTIC FORM OF PORPHYRA-UMBILICALIS - NITROGEN AND CARBON

Seasonal variations in environmental factors (irradiance, seawater concentration of NO3-, NO2- and NH4+, temperature) and in physiological variables of Porphyra umbilicalis (L.) Kutzing (intracellular NO3-, NO2- and NH4+ concentrations, nitrate reductase activity, amino acids, soluble proteins, total C, total N, C:N:P ratio) were determined during winter 1991-92 at a site in Lagos (Malaga, southern coast of Spain). In the western Mediterranean Sea, the leafy thallus of P. umbilicalis occurs only during winter (mid-December to mid-March). Maximal % cover (80 %) was reached in the first part of February. Conditions of low temperature (14.7 +/- 0.6-degrees-C), short days (10 h daylight) and low global solar irradiance (2.5 to 3.4 kWh m-2) prevailed during the period of increase in % cover. The period of decrease in % cover was characterized by long days (12 h), higher irradiance (3.4 to 5 kWh m-2), and slightly higher water temperature (15.5 +/- 1.34-degrees-C). NO3- concentration in seawater ([NO3-]e) at the study site ranged from 1 to 6.7 muM, [NO2-]e was always lower than 1 muM, and [NH4+]e, the largest source of inorganic nitrogen, ranged from 6.8 to 17.6 muM. Intracellular concentration of NO3- ([NO3-]i) ranged from 2.8 to 51.3 mmol l-1 cell water (11.7 to 210.6 mumol g-1 dry wt), which represents a concentration factor of up to 4.2 x 10(4) with respect to the seawater. [NO2-]i ranged from 0.2 to 0.7 mmol l-1 cell water and [NH4+]i from 0.016 to 2.2 mmol l-1 cell water. Nitrate reductase (NR) activity varied widely during the winter cycle (between 1.2 and 9.4 mumol NO2- g-1 fresh wt h-1). It was found to be a very sensitive parameter for indicating when NO3- is being used as the N source for growth. Therefore, low NR activity levels coincided with the highest [NO3-]i and were more or less independent of [NO3-]e when the alga was using NH4+ as N source. A large increase in NR activity was detected when [NO3-]i decreased sharply. Cysteine was the predominant amino acid, amounting to between 83.5 and 97.2 % of total amino acid content (5 to 28.3 mumol g-1 fresh wt). No correlation was found between total amino acid and soluble protein content. Soluble protein content showed the same seasonal variation as total N and total C, and varied inversely to the C:N ratio. Winter changes in the Redfield ratio, from 258:20:1 (24 December 1991) to 495:38:1 (16 March 1992) (in atoms), suggest that primary production of P. umbilicalis is limited by P rather than N on this coast.