Photosynthetic carbon acquisition in the red algaGracilaria conferta

Photosynthetic properties of the common red algaGracilaria conferta, collected from the eastern Mediterranean Sea were investigated in 1989, in order to begin evaluating its adaptative strategies with regard to the inorganic carbon composition of seawater, and to test whether the alleged C₄ photosynthesis of anotherGracilaria species is common within the genus. Net photosynthetic rates ofG. conferta were, under ambient conditions of inorganic carbon (ca. 10µM, CO₂ and 2.2 mM HCO ₃ ^- ), not sensitive to O₂ over the range 10 to 300µM, and the CO₂ compensation point was low (ca. 0.005µM). Ribulose-1,5-bisphosphate carboxylase/oxygenase was the major carboxylating enzyme, with a crude extract activity of 175µmol CO₂ g^–1 fresh wt h^–1 while phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase were present at 70 and 20%, respectively, of that activity. No activities of the decarboxylases NAD-and NADP-malic enzyme could be detected. The¹⁴C pulse-chase incorporation pattern showed thatG. conferta fixes inorganic carbon via the photosynthetic carbon reduction cycle only, with no evidence for photosynthetic C₄ acid metabolism. Photosynthesis at the natural seawater pH of 8.2 was, at 25°C and saturating light, saturated at the ambient inorganic carbon concentration of 2.5 mM. It is proposed that, under ambient inorganic carbon conditions, a CO₂ concentrating system other than C₄ metabolism provides an internal CO₂ concentration sufficient to suppress the O₂ effect on ribulose-1,5-bisphosphate carboxylase/oxygenase and, thus, on photorespiration, in a medium where the external free CO₂ concentration is lower than theK _m(CO₂) of the carboxylating enzyme. Since inorganic carbon, under natural saturating light conditions, seems not to be a limiting factor for photosynthesis ofG. conferta, it likely follows that other nutrients limit the growth of this alga in nature.     

Calcification in the articulated coralline alga Corallina pilulifera,with special reference to the effect of elevated CO2 concentration

Calcification in Corallina pilulifera Postels et Ruprecht displayed diurnal variations in aerated (350 ppm CO₂) culture media, with faster rates during the light than during the dark period. Addition of CO₂ (air+1250 ppm) inhibited calcification. This was attributable to the decreased pH resulting from CO₂ addition. Both photosynthesis and calcification were enhanced in seawater, with elevated dissolved inorganic carbon concentrations at a constant pH of 8.2.     

Effects of O2, pH and DIC on photosynthetic net-O2 evolution by marine macroalgae

Net photosynthetic O₂ evolution by five marine macroalgae:Ulva lactuca L.,Enteromorpha sp.,Ceramium strictum Harvey,Fucus serratus L., andF. vesiculosus L., collected from Danish waters in the summer of 1983 was followed at increasing O₂ and with pH either fixed close to pH 7, 8 or 9, or drifting upwards during photosynthesis in a closed chamber to determine the effects of changing O₂, pH and DIC (dissolved inorganic carbon) on photosynthesis. Increasing O₂, increasing pH and decreasing DIC together limited O₂ evolution. Raising the O₂ concentration with pH and DIC held constant resulted in less inhibition of net-O₂ evolution than when all three factors acted together. The O₂ inhibition of photosynthesis was similar to the reported O₂ inhibition of ribulose 1,5-bisphosphate carboxylase isolated from lower and higher plants. Net-O₂ evolution as a function of the molar ratio of O₂ to HCO^– ₃ + CO₂ in solution provided a general, linear relationship (r ² = 0.72 to 0.84), predicting inhibition of photosynthesis based on O₂ pH and DIC changing together. Slopes of this relationship, representing competition between O₂ and carbon based on external concentrations, were similar for the five taxonomically different algae, suggesting that similar processes act to reduce net-O₂ evolution.     

Inorganic carbon uptake by photosynthetically active protoplasts of the red macroalga Chondrus crispus

Photosynthetically active protoplasts were isolated from Chondrus crispus Stackh. by treating thalli with -carrageenase produced from batch culture of Pseudomonas carrageenovora. Using the silicone oil centrifugation technique, it was found that the protoplasts: (1) did not generally accumulate inorganic carbon (C_i) above the concentration in their incubation medium; (2) were saturated at C_i concentrations of 3 to 4 mM; (3) had an intracellular pH of 7.50 when incubated at pH 7.5; and (4) their initial carbon fixation rate was reduced by carbonic anhydrase inhibitors. Although the carbon fixation rate of the protoplasts was about 30% that of thallus fragments, presumably due to the relatively harsh protoplast isolation treatment, the behavior of the protoplasts was similar to that of fragments. This similarity indicates that the protoplasts are photosynthetically active and behave as thallus fragments. Further, the data are consistent with the hypothesis that C. crispus acquires C_i for photosynthesis by the diffusion of CO₂ across the plasma membrane.     

Photosynthetic carbon acquisition in the red alga Gracilaria conferta

In this continuing study on photosynthesis of the marine red alga Gracilaria conferta, it was found that ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in crude extracts had a K _m (CO₂) of 85 M. Since seawater contains only ca. 10 M CO₂, it appears that this alga must possess a CO₂ concetrating system in order to supply sufficient CO₂ to the vicinity of the enzyme. Because this species is a C₃ plant (and thus lacks the C₄ system for concentrating CO₂), but can utilize HCO₃ ^- as an exogenous carbon source, we examined whether HCO₃ ^- uptake could be the initial step of such a CO₂ concetrating system. The surface pH of G. conferta thalli was 9.4 during photosynthesis. At this pH, estimated maximal uncatalyzed HCO₃ ^- dehydration (CO₂ formation) within the unstirred layer was too slow to account for measured phostosynthetic rates, even in the presence of an external carbonic anhydrase inhibitor. This observation, and the marked pH increase in the unstirred layer following the onset of light, suggests that a HCO₃ ^- transport system (probably coupled to transmembrane H⁺/OH^- fluxes) operates at the plasmalemma level. The involvement of surface-bound carbonic anhydrase in such a system remains, however, obscure. The apparent need of marine macroalgae such as G. conferta for CO₂ concentrating mechanisms is discussed with regard to their low affinity of Rubisco to CO₂ and the low rate of CO₂ supply in water. The close similarity between rates of Rubisco carboxylation and measured photosynthesis further suggests that the carboxylase activity, rather than inorganic carbon transport and intercoversion events, could be an internal limiting factor for photosynthetic rates of G. conferta.     

Photosynthetic utilization of CO2(aq) and HCO 3 - in Thalassia testudinum (Hydrocharitaceae)

The effects of total dissolved inorganic carbon (DIC), free carbon dioxide [CO_2(aq)], and bicarbonate (HCO ₃ ^- ) concentrations on net photosynthetic oxygen evolution of the marine angiosperm Thalassia testudinum Banks ex König collected from Biscayne Bay (1988) and from Tampa Bay (1990), Florida, USA, were examined. Rates of photosynthesis declined by 85% from pH 7.25 to 8.75 in buffered seawater media with constant DIC concentration (2.20 mM), suggesting a strong influence of CO_2(aq) concentration. A plateau in the pH-response curve between pH 7.75 and 8.50 indicated possible utilization of HCO ₃ ^- . Responses of photosynthesis measured in buffered seawater media of varying DIC concentrations (0.75 to 13.17 mM) and pH (7.8 to 8.61) demonstrated that photosynthesis is rate-limited at ambient DIC levels. Photosynthesis increased in media with increasing HCO ₃ ^- concentrations but near-constant CO_2(aq) levels, confirming HCO ₃ ^- assimilation. Calculated half-saturation constants (K _s )for CO_2(aq) and HCO ₃ ^- indicated a high affinity for the former [K _s (CO₂)=3 to 18 M] and a much lower affinity for the latter [K _s (HCO ₃ ^- )=1.22 to 8.88 mM]. Calculated V _max values for HCO ₃ ^- were generally higher than those for CO_2(aq), suggesting relatively efficient HCO ₃ ^- utilization, despite the apparent low affinity for this carbon form.     

Role of carbonic anhydrase in the supply of inorganic carbon to the giant clam—zooxanthellate symbiosis

The mechanism whereby inorganic carbon (C_i) is acquired by the symbiotic association between the giant clam (Tridacna derasa) and zooxanthellae (Symbiodinium sp.) has been investigated. C_i in the haemolymph of the clam is in equilibrium with the surrounding sea water. The photosynthesis rate exhibited by the intact clam varies as a function of the C_i concentration in the clam haemolymph. The gill tissue contains high carbonic anhydrase activity which may be important in adjusting the C_i equilibrium between haemolymph and sea water. Zooxanthellae (Symbiodinium sp.) isolated from the clam mantle prefer CO₂ to HCO ₃ ^- as a source of inorganic carbon. The zooxanthellae have low levels of carbonic anhydrase on the external surface of the cell; however, mantle extracts display high carbonic anhydrase activity. Carbonic anhydrase is absent from the mantle of aposymbiotic clams (T. gigas), indicating that this enzyme may be essential to the symbiosis. The enzyme is probably associated with the zooxanthellae tubes in the mantle. The results indicate that carbonic anhydrase plays an important role in the supply of carbon dioxide within the clam symbiosis.     

Microsensor studies of photosynthesis and respiration in larger symbiotic foraminifera. I The physico-chemical microenvironment of Marginopora vertebralis, Amphistegina lobifera and Amphisorus hemprichii

 The physico-chemical microenvironment of larger benthic foraminifera was studied with microsensors for O₂, CO₂, pH, Ca²⁺ and scalar irradiance. Under saturating light conditions, the photosynthetic activity of the endosymbiotic algae increased the O₂ up to 183% air saturation and a pH of up to 8.6 was measured at the foraminiferal shell surface. The photosynthetic CO₂ fixation decreased the CO₂ at the shell down to 4.7 μM. In the dark, the respiration of host and symbionts decreased the O₂ level to 91% air saturation and the CO₂ concentration reached up to 12 μM. pH was lowered relative to the ambient seawater pH of 8.2. The endosymbionts responded immediately to changing light conditions, resulting in dynamic changes of O₂, CO₂ and pH at the foraminiferal shell surface during experimentally imposed light–dark cycles. The dynamic concentration changes demonstrated for the first time a fast exchange of metabolic gases through the perforate, hyaline shell of Amphistegina lobifera. A diffusive boundary layer (DBL) limited the solute exchange between the foraminifera and the surrounding water. The DBL reached a thickness of 400–700 μm in stagnant water and was reduced to 100–300 μm under flow conditions. Gross photosynthesis rates were significantly higher under flow conditions (4.7 nmol O₂ cm⁻³ s⁻¹) than in stagnant water (1.6 nmol O₂ cm ⁻³ s⁻¹), whereas net photosynthesis rates were unaffected by flow conditions. The Ca²⁺ microprofiles demonstrated a spatial variation in sites of calcium uptake over the foraminiferal shells. Ca²⁺ gradients at the shell surface showed total Ca²⁺ uptake rates of 0.6 to 4.2 nmol cm⁻² h⁻¹ in A. lobifera and 1.7 to 3.6 nmol cm⁻² h⁻¹ in Marginopora vertebralis. The scattering and reflection of the foraminiferal calcite shell increased the scalar irradiance at the surface up to 205% of the incident irradiance. Transmittance measurements across the calcite shell suggest that the symbionts are shielded from higher light levels, receiving approximately 30% of the incident light for photosynthesis. Received: 6 July 1999 / Accepted: 28 April 2000     

Diffusive boundary layers and photosynthesis of the epilithic algal community of coral reefs

The effects of mass transfer resistance due to the presence of a diffusive boundary layer on the photosynthesis of the epilithic algal community (EAC) of a coral reef were studied. Photosynthesis and respiration of the EAC of dead coral surfaces were investigated for samples from two locations: the Gulf of Aqaba, Eilat (Israel), and One Tree Reef on the Great Barrier Reef (Australia). Microsensors were used to measure O₂ and pH at the EAC surface and above. Oxygen profiles in the light and dark indicated a diffusive boundary layer (DBL) thickness of 180–590 μm under moderate flow (~0.08 m s^?1) and >2,000 μm under quasi-stagnant conditions. Under light saturation the oxygen concentration at the EAC surface rose within a few minutes to 200–550% air saturation levels under moderate flow and to 600–700% under quasi-stagnant conditions. High maximal rates of net photosynthesis of 8–25 mmol O₂ m^?2 h^?1 were calculated from measured O₂ concentration gradients, and dark respiration was 1.3–3.3 mmol O₂ m^?2 h^?1. From light–dark shifts, the maximal rates of gross photosynthesis at the EAC surface were calculated to be 16.5 nmol O₂ cm^?3 s^?1. Irradiance at the onset of saturation of photosynthesis, E _k, was <100 µmol photons m^?2 s^?1, indicating that the EAC is a shade-adapted community. The pH increased from 8.2 in the bulk seawater to 8.9 at the EAC surface, suggesting that very little carbon in the form of CO₂ occurs at the EAC surface. Thus the major source of dissolved inorganic carbon (DIC) must be in the form of HCO₃ ^?. Estimates of DIC fluxes across the DBL indicate that, throughout most of the daytime under in situ conditions, DIC is likely to be a major limiting factor for photosynthesis and therefore also for primary production and growth of the EAC.     

Uptake of inorganic carbon and internal carbon cycling in symbiont-bearing benthonic foraminifera

Incorporation rates of inorganic carbon and its distribution between the organic matter and the skeleton have been measured using ¹⁴C tracer techniques on two species of symbiont-bearing benthonic foraminifera in the Gulf of Elat: Amphistegina lobifera (a perforate species) and Amphisorus hemprichii (an imperforate species). Under constant experimental conditions, incorporation rates of the radiotracer become linear with time after several hours in A. hemprichii and after one day in A. lobifera. A. lobifera showed a lag time of 24 h for skeletal incorporation, whereas in A. hemprichii uptake into the skeleton started within 2 h. Pulse-chase incubations in radioactive seawater, followed by unlabelled incubations, demonstrate transfer of photosynthetically acquired ¹⁴C into the skeleton of A. lobifera. No such transfer was found in A. hemprichii. The total ¹⁴C uptake by A. lobifera increased during the first 24 h of cold chase incubation. This increase suggests the existence of an internal inorganic carbon pool that was lost (probably evaporated) during the analysis of pulse incubations. However, during the following chase incubations, the ¹⁴C in this pool was incorporated mainly into the skeleton and retained during analysis, causing the increase in the total uptake. No such increase was found in A. hemprichii. Additional ¹⁴C uptake experiments on other species of the genera Operculina, Heterostegina and Borelis suggest that the differences in pathways for incorporation of carbon between A. lobifera and A. hemprichii can be generalized to the perforate and imperforate foraminiferal groups. In perforate species, respired carbon originally taken up through photosynthesis is partly recycled into the skeleton. In imperforate species such a transfer has not been demonstrated. Perforate species seem to have a large internal inorganic carbon pool which serves mainly for calcification and possibly also for photosynthesis, while imperforate species may take up carbon for calcification directly from seawater or have a very small inorganic carbon pool.     

Diurnal changes in seawater carbonate chemistry speciation at increasing atmospheric carbon dioxide

Natural variability in seawater pH and associated carbonate chemistry parameters is in part driven by biological activities such as photosynthesis and respiration. The amplitude of these variations is expected to increase with increasing seawater carbon dioxide (CO₂) concentrations in the future, because of simultaneously decreasing buffer capacity. Here, we address this experimentally during a diurnal cycle in a mesocosm CO₂ perturbation study. We show that for about the same amount of dissolved inorganic carbon (DIC) utilized in net community production diel variability in proton (H⁺) and CO₂ concentrations was almost three times higher at CO₂ levels of about 675 ± 65 in comparison with levels of 310 ± 30 μatm. With a simple model, adequately simulating our measurements, we visualize carbonate chemistry variability expected for different oceanic regions with relatively low or high net community production. Since enhanced diurnal variability in CO₂ and proton concentration may require stronger cellular regulation in phytoplankton to maintain respective gradients, the ability to adjust may differ between communities adapted to low in comparison with high natural variability.     

The size and function of the internal inorganic carbon pool of the foraminifer Amphistegina lobifera

The existence of an internal inorganic carbon pool in the perforate foraminifer Amphistegina lobifera, as suggested recently (ter Kuile and Erez 1987), has been established by direct measurements using a new ¹⁴C tracer method. The imperforate species Amphisorus hemprichii does not contain such a pool. The size of the pool in A. lobifera is proportional to its calcification rate and approximately equals the amount of carbon incorporated into the skeleton during 24 h. Time course experiments show that inorganic carbon (Ci) is photoassimilated at constant rates by the algal symbionts, that the pool is filled to maximum capacity in ca. 24 h, and that Ci incorporation into the skeleton starts only after the pool is filled up. During the chase phase of pulsechase experiments, all ¹⁴C initially residing in the pool is transferred to the skeleton, indicating that the pool serves for calcification and not for photosynthesis. Uptake of Ci into the pool occurs only in the light, indicating that energy may be required for this process. Furthermore, calculations of the Ci concentration inside the pool suggest that it is higher by 2 to 3 orders of magnitude compared to seawater concentration, suggesting that its accumulation is an energy dependent process.     

Photosynthetic utilisation of inorganic carbon by seagrasses from Zanzibar, East Africa

Photosynthetic rates of eight seagrass species from Zanzibar were limited by the inorganic carbon composition of natural seawater (2.1 mM, mostly in the form of HCO₃ ⁻), and they exhibited more than three time higher rates at inorganic carbon saturation (>6 mM). The intertidal species that grew most shallowly, Halophila ovalis, Halodule wrightii and Cymodocea rotundata, showed the highest affinity for inorganic carbon (K _1/2 = ca. 2.5 mM), followed by the subtidal species (K _1/2 > 5 mM). Photosynthesis of H. wrightii, C. rotundata, Cymodocea serrulata and Enhalus acoroides was >50% inhibited by acetazolamide, a membrane-impermeable inhibitor of carbonic anhydrase, indicating that extracellular HCO₃ ⁻ dehydration is an important part of their inorganic carbon uptake. Photosynthetic rates of H. wrightii, Thalassia hemprichii, Thalassodendron ciliatum, C. serrulata and E. acoroides were strongly reduced by changing the seawater pH from 8.2 to 8.6 in a closed system. In H. ovalis, C. rotundata and Syringodiumisoetifolium, photosynthesis at pH 8.6 was maintained at a higher level than could be caused by the ca. 30% CO₂ concentration which remained in the closed experimental systems at that pH, pointing toward HCO₃ ⁻ uptake in those species. It is suggested that the ability of H. ovalis and C. rotundata to grow in the high, frequently air-exposed, intertidal zone may be related to a capability to take up HCO₃ ⁻ directly, since this is a more efficient way of HCO₃ ⁻ utilisation than extracellular HCO₃ ⁻ dehydration under such conditions. The inability of all species to attain maximal photosynthetic rates under natural conditions of inorganic carbon supports the notion that seagrasses may respond favourably to any future increases in marine CO₂ levels. Received: 19 March 1997 / Accepted: 31 March 1997     

Photosynthesis and respiration in the alga Ahnfeltia plicata in a flow-through system

Photosynthesis and respiration in Ahnfeltia plicata (Huds.) Fries (Gigartinales) was measured in a seawater flowthrough system at different temperatures, salinities and photon flux densities (PFD). The exchanges of dissolved oxygen and inorganic carbon were continuously recorded with an oxygen probe and a pH electrode measuring variation in CO₂–HCO ₃ ^- equilibrium as pH changes. Highest apparent photosynthesis at moderate photon flux density (PFD 50 E m^-2 s^-1) was found at 15°C and 33 S. Photosynthesis was measured up to PFD 500 E m^-2 s^-1 and no light saturation was documented. In the present experimental set-up, with continuous supply of fresh seawater, the number of limiting factors during photosynthesis measurements is reduced.     

Photosynthesis and amelioration of desiccation in the intertidal saccate alga Colpomenia peregrina

Rates of gross photosynthesis for the intertidal saccate alga Colpomenia peregrina (Sauv.) Hamel were determined under submersed and emersed conditions. Maximal photosynthetic rates were lower than for most seaweeds but comparable with other saccate members of the genus. By fitting the data to a hyperbolic tangent function, maximal photosynthetic rates were estimated to be 5.29 mmol CO₂ m^-2 h^-1 under submersed conditions and 2.06 mmol CO₂ m^-2 h^-1 under emersed conditions. I_k for submersed thalli was 69.1 E m^-2 s^-1, wherea for emersed thalli it was 149.0 E m^-2 s^-1, or 2.2 times higher. At low tide in the field and under saturating irradiance, carbon from seawater retained within the thallus cavity was assimilated at 0.9 mmol CO₂ m^-2 h^-1. In the laboratory under emersed conditions, carbon from this source was taken up at 0.6 mmol CO₂ m^-2 h^-1 at 20°C and at 0.34 mmol CO₂ m^-2 h^-1. Retained seawater also greatly reduced drying under desiccating conditions. Experimental thalli from which seawater had been removed lost thallus water continuously throughout the drying period (120 min). On the other hand, control, thalli lost water for the first 15 min, after which no further water loss occurred. At the termination of the experiment, control thalli had lost 7.2% of their water, whereas experimental thalli had lost 39.2%. Desiccation affected the emersed photosynthetic rate of experimental and control thalli. Emersed photosynthetic rates for thalli dried for 15 min were higher than for fullyhydrated thalli. However, emersed photosynthesis of thalli dried for longer than 15 min was lower than fully-hydrated rates and was directly related to percent water loss. Utilizing data from this study, a model was constructed to determine total photosynthetic production of C. peregrina over a single daylight period. From these calculations it was determined that emersed photosynthesis can increase daily photosynthetic production of C. peregrina by 50%.     

RQ of benthic marine invertebrates

This study investigated an incubation method which employed simultaneous measurement of CO₂ production and O₂ consumption rates to calculate the RQ (respiratory quotient; CO₂ production rate: O₂ consumption rate) of individual benthic marine invertebrates. Carbon dioxide production rates were calculated from changes in CO₂ concentration determined using seawater pH. O₂ consumption rates were calculated from changes in O₂ concentration with a correction applied for O₂ flux across the air/water interface due to gaseous exchange. Species examined were Triphyllozoon sp. cf. moniliferum (MacGillivray 1860), a bryozoan; Herdmania momus (Savigny), a solitary ascidian; Poneroplax albida (Blainville 1825), a chiton; and Haliotis roei (Gray 1826), an abalone. Six individuals of each were collected on 14 November 1985 from the limestone walls of a cave in a nearshore reef off Marmion, Western Australia. After acclimation for 6 h in experimental conditions, rates of CO₂ production and O₂ consumption were measured. A minimum period of 4 h was required to obtain consistent RQ values for each species. The standard error (SE) of the (calculated) RQ ratio was 14 to 33% of the mean in incubations of 4 h, and less than 14% in incubations of 4 to 12 h. The RQ is commonly used as an indicator of unknown catabolic substrates by comparing it with biochemically determined limits for known substrates. This study provides a strong argument against using the RQ of individual animals to draw any conclusions about catabolic substrates. Unexplained variation in the components of the RQ of an individual, measured over short time periods, and the potential involvement of stored reserves in catabolism, over longer time periods, obscure the relationship between the RQ of individual animals and the ratio's biochemically determined limits.     

Mechanisms for the uptake of inorganic carbon by two species of symbiont-bearing foraminifera

The mechanisms for uptake of inorganic carbon (Ci) for photosynthesis and calcification of a perforate foraminifer, Amphistegina lobifera Larsen, and an imperforate species, Amphisorus hemprichii Ehrenberg, from the Gulf of Eilat, Red Sea were studied in 1986–1987 using ¹⁴C tracer techniques. Total Ci uptake of A. lobifera and photosynthetic carbon uptake of A. hemprichii fit the Hill-Whittingham equation that describes the overall rate of enzymatic reactions that are provided with their substrate through a diffusion barrier. This suggests that diffusion is the rate limiting step for total Ci uptake in A. lobifera. Photosynthesis by the isolated symbionts and uptake of CO₃ ^2- for calcification obey Michaelis-Menten kinetics indicating that enzymatic reactions determine the rate of the separate processes. Both photosynthesis and calcification can be inhibited without affecting each other. Calcification rates in A. lobifera were optimal at Ca levels around normal seawater concentration and were sensitive to inhibitors of respiratory adenosine triphosphate (ATP) generation and Ca-ATP-ase. This indicates that Ca uptake is also active. Calcification rates of A. hemprichii increased linearly as a function of external Ci concentration over the entire experimental range (0 to 4 mM Ci). In contrast, photosynthetic rates showed Hill-Whittingham type kinetics. The dependence of calcification on the CO₃ ^2- concentration was also linear, suggesting that its diffusion is the rate limiting step for calcification in A. hemprichii. Increasing Ca concentrations yielded higher calcification rates over the entire range measured (0 to 40 mM Ca). Calcification in A. hemprichii was less sensitive to inhibitors of ATP generation than in A. lobifera, suggesting that in A. hemprichii energy supply is less important for this process.     

The synergistic effects of hydrogen peroxide and elevated seawater temperature on the metabolic activity of the coral <Emphasis Type="Italic">Galaxea fascicularis</Emphasis>

We examined quantitative changes in the metabolism of the coral Galaxea fascicularis caused by increases in both hydrogen peroxide (H₂O₂) concentration and seawater temperature. Seawater temperatures were maintained at 27 or 31°C in a well-controlled incubation chamber, and three levels of H₂O₂ concentration (0, 0.3, 3.0 μM) were used in experimental treatments. Gross primary production, calcification rates and respiration rates were all affected by increased H₂O₂ concentrations and high seawater temperatures. Individual treatments of high H₂O₂ or elevated seawater temperature alone caused significant declines in coral photosynthesis and calcification rates within the 3-day incubation period. The synergistic effect of high H₂O₂ combined with high seawater temperature resulted in a 134% increase in respiration rates, which surpassed the effect of either H₂O₂ or high seawater temperature alone. Our results suggest that both high H₂O₂ concentrations and elevated temperatures in seawater can strongly affect coral metabolism; however, these effects cannot be estimated by simply summing the effects of individual stress parameters.     

Oxygen and carbon stable isotopic profiles of the fan mussel, Pinna nobilis, and reconstruction of sea surface temperatures in the Mediterranean

Stable oxygen and carbon isotope profiles ('¹⁸O_skeletal and '¹³C_skeletal), taken along the direction of growth from the umbo to the shell margin in shells of the pinnid Pinna nobilis, were used to reconstruct sea surface temperatures (SST) in the south-east Mediterranean and ontogenetic records of metabolic CO₂ incorporation. Comparison of the seasonal cycle of SST, predicted from the '¹⁸O_skeletal record of a small (young) rapidly growing pinnid and temperature measured with a continuous in situ recorder showed that P. nobilis calcifies under isotopic equilibrium with surrounding seawater, thus indicating that P. nobilis shells can be used as a reliable predictor of SST. A 10-year SST record for the south-east Mediterranean was reconstructed from the shell profiles of four pinnid shells of different sizes and ages collected in 1995 and 1996. Reliable resolution of the seasonal SST could only be achieved during the first 4 years of shell growth. As the pinnids grew older, the temperature record was poorly resolved because the shell growth had diminished with age, resulting in time-averaging of the record. The amplitude of the generated seasonal temperature cycle compared favourably (DŽ°C) with a long-term temperature record from northern Mediterranean waters. Clear seasonal cycles in '¹³C_skeletal were observed with an amplitude of ~1.0‰, similar to the calculated seasonal changes in '¹³C of seawater (0.6‰) overlying seagrass meadows. An ontogenetic trend towards less positive '¹³C_skeletal values was too large to be attributed to any decrease in '¹³C in seawater resulting from the invasion of anthropogenic CO₂. It is suggested that the temporal changes of '¹³C_skeletal are due to incorporation of respiratory CO₂ into the extrapallial fluid and reflect changes in the metabolic activity of the pinnid rather than changes in the isotopic composition of dissolved inorganic carbon within the surrounding seawater.     

Carbon acquisition strategies for marine macroalgae

A model system was developed to analyse differences in carbon acquisition strategies among macroalgae. During photosynthesis in a limited volume of seawater the capability of the algae to assimilate inorganic carbon as well as to change the alkalinity of the seawater was analysed. These properties were then related to the status of the carbonate equilibrium system of the seawater. The experimental system was assumed to simulate the conditions in the boundary layer during periods of low water exchange or high intensity irradiations. Fundamental differences were found between different algal classes, suggesting that capabilities to adapt to specific environmental conditions may be connected with dissimilarities in carbon acquisition strategies. In general, green algae were able to reach the highest pH (10.8 at 5°C), and thus to achieve the highest reduction in the level of inorganic carbon via a simple HCO₃ ^–/OH^– ion exchange process. For brown algae, pH increases due to carbon uptake never exceeded pH 9.7 (9.5 in a saltwater scale). In spite of this, members of the Fucaceae (littoral brown algae) were able to extract almost all of the dissolved inorganic carbon (DIC). This was achieved through a gradual decrease in the alkalinity of the enclosed water, so that the carbon assimilation could continue without any concomitant increase in pH. For red algae, the specific response was an increase in the level of inorganic carbon. Thus, for this algal class, no specific strategy for handling a shortage of inorganic carbon was documented. Within each algal class, differences in pH and DIC compensation points could be related to differences in the depths at which the algal species occurred. This paper also introduces a low cost and convenient method of analysing DIC in seawater.