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A method for cultivation of Gracilaria secundata Harv. in plexiglass cylinders and in tanks with submerged light sources is described. Growth rates up to 47% d" 1 were recorded in the System, compared to growth rates of 6% d' 1 in a spray System and less than 3% d -1 in tank cultivation of G. secundata. The advantage of the System described is the greater availability of light to the seaweeds.-'s-1 ). Growth of G. secundata was not light saturated up to the highest photon irradiance used (1450 μΕηι Self-shading followed a hyperbolic function with increasing biomass and had a great impact on the growth rates of the seaweed. At a distance of 15 cm from the light source 90% of the irradiance was absorbed at a plant density of 3 kg fresh-weight per m~3.The water in the tanks was agitated by injection of compressed air at the bottom. The constant rubbing of the seaweeds against each other and on the walls, together with recirculation of the water through filters, kept the System free from epiphytes and debris. In this way the cultures were stable, thus allowing growth under optimal conditions without any infection by bacteria or epiphytes. High growth rates could be achieved for long periods of time. An annual yield of 80 kg dry-weight per m~3 of G. secundata was estimated after pilot-scale studies.Ectocarpus siliculosus (Dillw.) Lyngb. and Ceramium rubrum (Huds.) C. Ag. also showed high potentials for growth in the System. Enteromorpha intestinalis (L.) Link and other species that easily form dense mats are not suitable for the cultivation technique described.

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The large subunit of ribulose-l,5-bisphosphatecarboxylase/oxygenase (RuBPCase, EC 4.1.1.39) in Gracilaria secundata Harv. was localized, by use of immuno-gold labelling technique and transmission electron microscopy, to chloroplasts and crystalline inclusions. Changes in amounts of the RuBPCase large subunit were observed and followed during a nitrogen starvation experiment with the seaweed. These changes are discussed in relation to changes in nitrogen content of the alga.