The effects of increased CO2 levels (10,000 microl l(-1)) in cultures of the green nitrophilic macroalga Ulva rigida C. Agardh were tested under conditions of N saturation and N limitation, using nitrate as the only N source. Enrichment with CO2 enhanced growth, while net photosynthesis, gross photosynthesis, dark respiration rates and soluble protein content decreased. The internal C pool remained constant at high CO2, while the assimilated C that was released to the external medium was less than half the values obtained under ambient CO2 levels. This higher retention of C provided the source for extra biomass production under N saturation. In N-sufficient thalli, nitrate-uptake rate and the activity of nitrate reductase (EC 1.6.6.1) increased under high CO2 levels. This did not affect the N content or the internal C:N balance, implying that the extra N-assimilation capacity led to the production of new biomass in proportion to C. Growth enhancement by increased level of CO2 was entirely dependent on the enhancement effect of CO2 on N-assimilation rates. The increase in nitrate reductase activity at high CO2 was not related to soluble carbohydrates or internal C. This indicates that the regulation of N assimilation by CO2 in U. rigida might involve a different pathway from that proposed for higher plants. The role of organic C release as an effective regulatory mechanism maintaining the internal C:N balance in response to different CO2 levels is discussed.

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During the last decade, the Palmones River estualy has undergone severe eutrophication followed by a green tide episode; two species ofUlva, rotundata Blid. and Ulva curvata (Kutz.) De Toni, were the main macroalgae responsible for this bloom. From November 1993 to December 1994, we followed the biomass, the growth dynamics, and tissue elemental composition (C:N:P) of Ulva species, as well as some physicochemical variables in the estualy. Maximum biomass (up to 375 g dly wt.m-2 in some spots, corresponding to a thallus area index of nearly 17 m2 Ulva m-2 sediment) were observed in June and December. However, the biomass vaned among the sampling stations. Water nitrate, ammonia, and phosphate showed high concentrations throughout the year, with extremely high transient pulses, sustaining the high growth rates obseroed. Growth rates were estimated directly i n the field. The rates were generally higher i n Ulva discs maintained in net cages than those estimated by changes in biomass standing stock between two consecutive samplings. The dqerence between both estimates was used to quantijj the importance of the processes causing loss of biomass, which were attrib utable to grazing, exported biomass, and thallus decomposition under anaerobic conditions resulting from extreme selfshading. Maximum chlor@hyll content was found in winter, whereas the minimum was in spring. Atomic N:P ratios were generally higher in the algae than in the water, However, the absolute concentrations of tissue N and P were always higher than the cn'tical h e l s for maximum growth, which suggests that growth was not limited by inorganic N or P availability. The results suggested that the increase in nutrient loading in the river may have triggered the massive development of green algae and that light limitation and temperature stress in summer seem to be the main factors controlling the abundance of Ulva in the estualy. I n addition to light availability and thermal stress, the different loss processes may have a decisive role in the dynamics of Ulva biomass. K q index words: eutrophication; growth; primaly p r e duction; Ulva Local proliferations of masses of green algae are often found in estuaries and coastal waters of areas undergoing eutrophication (Geertz-Hansen et al.

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Thallus discs 9 mm in diameter of Porphyra umbilicalis were cultivated for 4 weeks in either blue or red light at 50 #molm -2 s -1 and 12 h light per day. Growth rate, in terms of weight and area, carbon content and dimensions of intercellular matrix, was higher in thalli grown in red light, while concentrations of soluble protein and photosynthetic pigments (chlorophyll, phycocyanin and phycoerythrin in particular), package effect and cell volume were higher in thalli grown in blue light. The slower growth rate in blue light was ascribed mainly to low efficiency of light absorption (high package effect) and low photosynthetic efficiency (in terms of total photosynthetic pigments) due to little overlap of photosystem I and photosystem II pigments in this red alga in blue light.

Abstract. The short-term effect of a single fire, and the long-term effect of recent fire history and successional stage on total and mineral N concentration, net nitrogen mineralization, and nitrification were evaluated in soils from a steep semi-arid shrubland chronosequence in southeast Spain. A single fire significantly increased soil mineral N availability and net nitrification. Increasing fire frequency in the last few decades was-associated with a sharp decrease in surface soil organic matter and total N concentrations and pools, and with changes in the long-term N dynamic patterns. The surface-soil extractable NH: :NO; ratio increased throughout the chronosequence. All net mineralized N in laboratory incubations from all sites was converted to nitrate, suggesting that allelochemic inhibition of net nitrification is probably not important in this system. Net nitrification in samples during incubation increased through the sere. The maximum rate of net nitrification (k,,,& increased through the first three stages of the sere. A linear relationship was found between total soil N and N mineralization, and both k,, and net nitrification for the first three stages of the sere, suggesting that total N and ammonification are likely to be the control mechanisms of nitrification within the sere. The oldest site exhibited the lowest specific k -and the highest potential soil respiration rate suggesting that a lower N quality and increasing competition for ammonium might also limit nitrification at least in the long-unburned garrigue site.

The seaweed Ulva rigida C. Agardh (Chlorophyta) was cultured under two CO(2) conditions supplied through the air bubbling system: non-manipulated air and 1% CO(2)-enriched aeration. These were also combined with N sufficiency and N limitation, using nitrate as the only N source. High CO(2) in U. rigida led to higher growth rates without increasing the C fixed through photosynthesis under N sufficiency. Quantum yields for charge separation at photosystem II (PSII) reaction centres (phi(PSII)) and for oxygen evolution (phi(O2)) decreased at high CO(2) even in N-sufficient thalli. Cyclic electron flow around PSII as part of a photoprotection strategy accompanied by decreased antennae size was suspected. The new re-arrangement of the photosynthetic energy at high CO(2) included reduced investment in processes other than C fixation, as well as in carbon diverted to respiration. As a result, quantum yield for new biomass-C production (phi(growth)) increased. The calculation of the individual quantum yields for the different processes involved allowed the completion of the energy flow scheme through the cell from incident light to biomass production for each of the CO(2) and N-supply conditions studied.

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