[1] Whereas the oxygen isotope ratio of the coral skeleton is used for reconstruction of past information on seawater, the carbon isotope ratio is considered a proxy for physiological processes, principally photosynthesis and respiration. However, the fractionation of carbon isotopes in biogenic carbonate such as coral skeleton is still unclear. We conducted a long-term culture experiment of Porites spp. corals at different light dosages (light intensity, 100, 300, or 500 mmol m À2 s À1 ; daily light period, 10 or 12 h) at 25 ± 0.6°C to examine the contribution of photosynthetic activity to skeletal carbon isotope composition. Corals were grown in sand-filtered seawater and not fed; thus, they subsisted from photosynthesis of symbiotic algae. As the daily dose of photosynthetically active radiation increased, the rate of annual extension also increased. Mean isotope compositions shifted; the carbon isotope compositions (d 13 C) became heavier and the oxygen isotope compositions (d 18 O) became lighter at higher radiation dose. Skeletal d 18 O decrease coincided with increasing skeletal growth rate, indicating the influence of so-called kinetic isotope effects. The observed d 13 C increase should be subject to both kinetic and metabolic isotope effects, with the latter reflecting skeletal d 13 C enrichment due to photosynthesis by symbiotic algae. Using a vector approach in the d 13 C-d 18 O plane, we discriminated between kinetic and metabolic isotope effects on d 13 C. The calculated d 13 C changes from metabolic isotope effects were light dose dependent. The d 13 C fractionation curve related to metabolic isotope effects is very similar to the photosynthesis-irradiance curve, indicating the direct contribution of photosynthetic activity to metabolic isotope effects. In contrast, d 13 C fractionation related to kinetic isotope effects gradually increased as the growth rate increased. Our experiment demonstrated that the kinetic and metabolic isotope effects in coral skeleton were successfully differentiated.Citation: Omata, T., A. Suzuki, T. Sato, K. Minoshima, E. Nomaru, A. Murakami, S. Murayama, H. Kawahata, and T. Maruyama (2008), Effect of photosynthetic light dosage on carbon isotope composition in the coral skeleton: Long-term culture of Porites spp.,

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Seawater around deep-sea hydrothermal vents and cold seeps contain high levels of hydrogen sulfide, which is toxic to most animals. Invertebrates inhabiting these environments have been reported to accumulate high levels of thiotaurine, a sulfur-containing amino acid. Thiotaurine is likely to play an important role in sulfide detoxification, but its functions in the detoxification process are still unknown. We cloned methane-seep mussel Bathymodiolus platifrons cDNA encoding the taurine transporter (TAUT), which transports thiotaurine and its precursors across the cell membrane. Phylogenetic analyses revealed that the predicted peptide formed a clade with the TAUTs of shallow-water mussel Mytilus galloprovincialis and the hydrothermal vent mussel Bathymodiolus septemdierum that harbors thioautotrophic bacteria. We then reared B. platifrons in the presence or absence of Na 2 S and quantified TAUT mRNA using a real-time PCR system. The amount of TAUT mRNA in the gills of B. platifrons increased with rearing in the presence of Na 2 S for 69 days, but no change was observed in the absence of sulfide. These results suggest that TAUT plays an important role in sulfide detoxification, even in species that do not harbor thioautotrophic bacteria. The TAUT mRNA level was variable in the mantle and low in the foot throughout the entire rearing period, regardless of the presence/absence of sulfide, suggesting that TAUT gene expression is regulated differentially in each tissue.

To study the mechanism of element incorporation into bivalve shells living in hydrothermal environments, the chemical composition of deep-sea mussel (Bathymodiolus platifrons) shell was analyzed using three microanalytical techniques. Mg/Ca, Sr/Ca, Mn/Ca, and Ba/Ca ratios were measured in ~8-µm-diameter spots by secondary ion mass spectrometry (NanoSIMS). Mg, Sr, Ca, and S distributions were analyzed by electron probe microanalyzer (EPMA; spatial resolution, 2 µm). Micro-scale shell structures were observed by SEM and optical microscope. The inner aragonitic layer comprised three sublayer types. The thick organic sublayers and homogeneous sublayers were interlaminated sporadically among usually deposited nacreous layers. Elemental compositions were significantly associated with shell sublayer types. The thick organic sublayers contained higher Mg/Ca, Sr/Ca and Ba/Ca ratios by factors of up to 3.0, 2.6 and 3.2, respectively, compared to the adjacent nacreous layers. On the other hand, Mn/Ca ratios in homogeneous sublayers were depleted down to one third of those in adjacent nacreous layers. Our data suggest that biological effects are significant for micro scale elemental distributions.