EFFECT OF NITROGEN SUPPLY ON PHOTOSYNTHESIS AND CARBONIC-ANHYDRASE ACTIVITY IN THE GREEN SEAWEED ULVA-RIGIDA (CHLOROPHYTA)

Photosynthesis rate and carbonic anhydrase (CA) activity have been stu died in the green seaweed Ulva rigida C. Agardh (Chlorophyta) grown in seawater (SW) and SW supplemented with 40 mu M NH4Cl (N-SW). Higher g rowth and maximal O-2 evolution rates were observed in N-SW- than in S W-grown seaweeds. Western blot analysis of the total homogenates probe d with antibodies raised against small subunits of ribulose-1,5-bispho sphate carboxylase/oxygenase (RuBisCO) showed crossreaction with a 15 kdalton polypeptide in both SW- and N-SW-grown plants, although the ba nd was more intense in N-SW-grown plants. Carbonic anhydrase activitie s in the total homogenate and in the soluble protein fraction were hig her in N-SW-grown plants. Although the pellets from both plants showed a considerable CA activity, the activity of CA in the thylakoid membr anes was undetectable. The low nitrogen concentration is a major envir onmental factor that affects the level of RuBisCO and CA, and therefor e CO2 assimilation in U. rigida.