Effect of photosynthetic light dosage on carbon isotope composition in the coral skeleton: Long‐term culture of <i>Porites</i> spp.

Abstract

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 μ mol m −2 s −1 ; 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 ( δ 13 C) became heavier and the oxygen isotope compositions ( δ 18 O) became lighter at higher radiation dose. Skeletal δ 18 O decrease coincided with increasing skeletal growth rate, indicating the influence of so‐called kinetic isotope effects. The observed δ 13 C increase should be subject to both kinetic and metabolic isotope effects, with the latter reflecting skeletal δ 13 C enrichment due to photosynthesis by symbiotic algae. Using a vector approach in the δ 13 C– δ 18 O plane, we discriminated between kinetic and metabolic isotope effects on δ 13 C. The calculated δ 13 C changes from metabolic isotope effects were light dose dependent. The δ 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, δ 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.