Isabelle Bourge

An improved 45Ca protocol for investigating physiological mechanisms in coral calcification

A sensitive experimental protocol using cloned corals (hereafter “microcolonies”) of the branching scleractinian coral Stylophora pistillata and 45Ca has been developed to enable reproducible measurements of physiological and biochemical mechanisms involved in calcium transport and compartmentalization during coral calcification. Cloned S. pistillata microcolonies were propagated in the laboratory from small fragments of parent colonies collected in 1990 in the Gulf of Aqaba, Jordan. Cloned microcolonies have several intrinsic properties that help to reduce unwanted biological variability: (1) same genotype; (2) similar sizes and shapes; and (3) absence of macroscopic boring organisms. Errors specifically associated with long-standing problems to do with isotopic exchange were further reduced by producing microcolonies with no skeletal surfaces exposed to the radioisotope-labelled incubation medium. The value of the technique resides principally in its superior ability to elucidate transportation pathways and processes and not in its ability to quantitatively estimate calcium deposition by corals in nature. We describe here a rapidly exchangeable calcium pool in which up to 90% of the radioactive label taken up during incubations is located. This pool (72.9±1.4 nmol Ca mg-1 protein) is presumably located within the coelenteric cavity as suggested by the following: (1) it has 4-min half-time saturation kinetics; (2) the accumulation of calcium is linearly correlated with the calcium concentration of sea-water; and (3) its insensitivity to metabolic and ion transport inhibitors indicate that membranes do not isolate this compartment. Washout of this large extracellular pool greatly improved estimates of calcium deposition as evidenced by 10 to 40% reduction in coefficients of variation when compared with previous 45Ca2+ methods described in the literature. Comparisons of calcification measurements simultaneously carried out using the alkalinity anomaly technique and the 45Ca protocol described here show that the correlation coefficient of both techniques is close to 1. Unlike previous reports, our 45Ca2+-derived measurements are slightly lower than those computed from the alkalinity depletion technique.

Distribution of surface seawater partial C02 pressure in the English Channel and in the Southern Bight of the North Sea

Abstract The concentrations of dissolved CO 2 dissolved oxygen and chlorophyll have been determined in the surface seawater of the English Channel and of the Southern Bight of the North Sea in June 1992, April–May 1993 and September–October 1993. The partial pressure of CO 2 (PCO 2 ) displays large spatial and temporal variations ranging from 100 ppm up to 670 ppm. The relationship between seawater pCO 2 and chlorophyll distribution is discussed and compared with data recently obtained in the North Atlantic by A. J. Watson, C. Robinson, J. E. Robertson, P. J. le B. Williams and J. R. Fasham (1991) Nature , 350, 50–53. Air-sea CO 2 fluxes have been calculated from wind speed and pCO 2 data and, due to the combined effect of both of those parameters, the flux can be as high as about 150 mmol m −2 d −1 . This implies that shelves could play an appreciable role in the global C budget. Further extended C0 2 level measurements should be made to improve flux estimations for the coastal zone on a global basis.

Distribution of inorganic carbon and related parameters in surface seawater of the English Channel during spring 1994

Abstract The complete CO 2 system, dissolved oxygen, nutrients and chlorophyll concentrations, fluorescence, salinity and temperature of surface seawater have been determined in the English Channel and the Southern Bight of the North Sea during Spring 1994. As already mentioned in a previous study, this area is characterized by important spatial and temporal variations of the considered set of parameters. This paper focuses on the distribution of the homogeneous buffer factor [ β = dln(pCO 2 ) dln(TCO 2 ) ]. It is shown that this parameter is a useful tool for identifying processes that mainly affect the CO 2 distribution, such as primary production or water mass mixing, even in such a complex area.