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Abstract. Production of hydrogen peroxide has been found in Ulva rigida (Chlorophyta). The formation of H202 was light dependent with a production of 1.2 gmol.g FW l.h q in sea water (pH 8.2) at an irradiance of 700 gmol photons m 2.s 1. The excretion was also pH dependent: in pH 6.5 the production was not detectable (< 5 nmol.g FW -1-h 1) but at pH 9.0 the production was 5.0 lamol-g FW-I'h 1. The production of H20 2 was totally inhibited by 3-(3,4-dichlorophenyl)-l,1-dimethylurea (DCMU). The ability of U. rigida growing in tanks (750 1) under a natural light regime to excrete H202 was checked and found to be seven times higher at 08.00 hours than other times of the day. The H202 concentration in the cultivation tank (density: 2 g FW.1 1) reached the highest value (3 gM) at 11.00 hours. Photosynthesis was not influenced by H202 formation. The H202 is suggested to come from the Mehler reaction (pseudocyclic photophosphorylation). With an oxygen evolution of 120mmol.g FW 1-hl at pH 8.2 and 90 mmol.g FW 1-h l at pH 9.0, 0.5% and 2.7% of the electrons were used for extracellular H202 production.The H202 production is sufficiently high to be of physiological and ecological significance, and is suggested to be a part of the defence against epi-and endophytes.

Extracts from 44 species of seaweed from Gran Canaria (Canary Islands, Spain) were screened for the production of antibacterial and antifungal compounds against a panel of Gram-negative and Grampositive bacteria, mycobacteria, yeasts and fungi. A total of 28 species displayed antibacterial activity, of which six also showed antifungal activity. Asparagopsis taxiformis and Cymopolia barbata were the species with the strongest activities against the broadest spectrum of target microorganisms. All the species with antibacterial activity were active against Gram-positive bacteria, whereas only two species, A. taxiformis and Osmundea hybrida, were active against mycobacteria. The production of secondary metabolites with antimicrobial activities by the macroalgae was also studied under dierent conditions, although no common trend for bioactivity was observed.

Ulva rigida was cultivated in 750 1 tanks at different densities with direct and continuous inflow (at 2, 4, 8 and 12 volumes d_1) of the effluents from a commercial marine fishpond (40 metric tonnes, Tm, of Sparus aurata, water exchange rate of 16 m3 Tm-1) in order to assess the maximum and optimum dissolved inorganic nitrogen (DIN) uptake rate and the annual stability of the 'Ulva tank biofiltering system' . Maximum yields (40 g DW m-2 d-1) were obtained at a density of 2 .5 g FW 1-1 and at a DIN inflow rate of 1 .7 g DIN m-2 d-1 . Maximum DIN uptake rates were obtained during summer (2 .2 g DIN m-2 d-1), and minimum in winter (1 .1 g DIN m-2 d-1) with a yearly average DIN uptake rate of 1 .77 g DIN m-2 d-1 . At yearly average DIN removal efficiency (2 .0 g DIN m-2 d-1, if winter period is excluded), 153 m2 of Ulva tank surface would be needed to recover 100% of the DIN produced by 1 Tm of fish .Abbreviations: DIN= dissolved inorganic nitrogen (NH4 + NO3 + NOz ) ; FW= fresh weight ; DW= dry weight ; PFD= photon flux density ; V= DIN uptake rate

The colour, dietary fibre content and characteristics of the edible marine green algae, Uloa rigida, were investigated in relation with the nitrogen content of the algal culture medium. Colour brightness decreased and total nitrogen of the algae increased in nitrogen-enriched conditions. When nitrogenenriched seaweeds were grown again in normal seawater for 10 days, colour brightness increased and total nitrogen concentration fell. Total dietary fibre content of the wild and nitrogen enriched algae were close (-38.0% DW) but increased markedly when the nitrogen-rich plants were cultured in normal seawater (54.4% after 10 days). Soluble fibre were xylorhamnoglucuronan sulphate (ulvan) with close molar compositions for all the seaweed samples. Insoluble polysaccharides were composed of glucose, xylose, uronic acid, rhamnose and sulphate which molar proportions varied for the different algal samples. The soluble fibre intrinsic viscosity and the water holding capacity of insoluble fibre were not markedly affected by the growth conditions. This report demonstrates that edible seaweed aquaculture provides an alternative or a complement to post-harvest transformations of seaweeds to modify organoleptic and nutritional characteristics.