The mechanism of inorganic-carbon (Ci) accumulation in the red seaweed Gracilaria tenuistipitata Zhang et Xia has been investigated. Extracellular and intracellular carbonic-anhydrase (CA) activities have been detected. Photosynthetic O2 evolution in thalli and protoplasts of G. tenuistipitata were higher at pH 6.5 than at pH 8.6, where HCO 3 (-) is the predominant form of Ci. Dextran-bound sulfonamide (DBS), a specific inhibitor of extracellular CA, reduced photosynthetic O2 evolution at pH 8.6 and did not have any effect at pH 6.5. After inhibition with DBS, O2 evolution was similar to the rate that could be supported by CO2 from spontaneous dehydration of HCO 3 (-) . The rate of photosynthetic alkalization of the surrounding medium by the algal thallus was dependent on the concentration of Ci and inhibited by DBS. We suggest that the general form of Ci that enters through the plasma membrane of G. tenuistipitata is CO2. Bicarbonate is utilized mainly by an indirect mechanism after dehydration to CO2, and this mechanism involves extracellular CA.

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.

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Mechanisms of carbon assimilation were investigated in thalli and protoplasts of Ulva rigida by measuring HCO 3 (-) -dependent O2 evolution at pH 6.5 and 8.6. In thalli, dextran-bound azetazolamide (DBAZ), a specific inhibitor of extracellular carbonic anhydrase (CA), inhibited the rate of O2 evolution at pH 8.6 when HCO 3 (-) was the only available form of inorganic carbon (Ci) in the medium. At pH 6.5 when CO2 is accessible, DBAZ did not affect photosynthetic O2 evolution. Inhibition of total CA activity (extracellular and intracellular) by ethoxyzolamide (EZ) inhibited photosynthesis at pH 6.5 and 8.6. During illumination of thalli the medium was alkalized at a rate which increased with increasing light. This alkalization decreased during inhibition of extracellular CA by DBAZ. Protoplasts at pH 6.5 exhibited a higher rate of O2 evolution than in pH 8.6. Addition of CA to protoplasts at pH 8.6 increased the rate of O2 evolution, whereas EZ was inhibitory at both pH 6.5 and 8.6, and DBAZ did not affect photosynthesis at either pH. We suggest that both extracellular and intracellular CA are present and that Ulva rigida assimilates HCO 3 (-) by an indirect mechanism. A theoretical scheme for carbon utilization is suggested.

Explants of Grateloupia dolphora were cultivated in Provasoli Enriched Seawater culture medium (PES) supplemented with glycerol (O. 1.0.3,0.5 or 0.8 m01 1-') or carbohydrates (0.1 or 0.3 m01 1-' mannose.glucose and galactose) and agar (3, 8, 15 g 1-'). The osmolality of the medium was adjusted by dilution of the seawater (70 or 100%, v v). The increase in fresh weight of explants cultivated in liquid medium with glycerol(0.3 m01 1-') and without glycerol was compared. Al1 experiments were carried out in the light, except for one assay in which the explants were cultivated in the dark. Glycerol was an effective carbon source for the vegetati\.e propagation of G. dorvphora in solid and liquid media. Mannose, glucose and galactose al1 had no effect on growth or morphogenesis of the explants. In solid media the main effect oí' glycerol was as a rnorphogenetic inductor, with PES70 (70% seawater) + 0.1 or 0.3 m01 1-' glycerol + 3 or 8 g 1 -' agar the best formulation. An increase in the concentration of agar in glycerolcontaining medium reduced the morphogenetic capacity of the explants, which developed into compact cell masses. The effects of glycerol were observed only in explants cultivated under light.Abbreviations: PES. Provasoli Enriched 100% Seawater; PES70, Provasoli-Enriched 70% ( v~) seawater in distilled water.