Potassium incorporation and isotope fractionation in cultured scleractinian corals

Potassium (K) participates in coral biological activities and accumulates in their skeletons, driving the fractionation of stable K isotopes (⁴¹K/³⁹K). Constraining the influences of biotic and abiotic controls on K isotope fractionation is important for interpreting coral records. However, the processes and mechanisms regulating K incorporation into coral skeletons and K isotope fractionation between seawater and coral skeletons remain unknown. Here, we combined isotopic and synchrotron-based spectroscopic analyses to evaluate the phase distribution and corresponding isotope variation of K in the skeleton of scleractinian (Porites australiensis) corals at a seawater temperature range from 20 to 29 °C in aquaria culture experiments. Potassium in coral skeletons exists mainly as organic-K (K hosted in soluble and insoluble organic matrices) and carbonate-K (K incorporated into K₂CO₃ and aragonite) phases of various proportions. Coral δ⁴¹K values vary substantially in both direction and magnitude from the modern seawater δ⁴¹K composition (∼0.12‰), showing marked deviations (Δ⁴¹K_Coral-Sea) from −2.00 to 0.67‰. As seawater temperature increases, the organic-K fraction increases, whereas δ⁴¹K_Coral decreases. The variation in δ⁴¹K_Coral reflects the relative proportions of organic-bound K to carbonate-associated K. In most cases, coral intracrystalline organic matrices preferentially sequester isotopically lighter K whereas carbonate phases prefer heavier K. Distinguishable inter-colony difference in skeletal δ⁴¹K of corals growing under the same culturing conditions reveals the influence of physiological controls on K partitioning and isotope fractionation. Although calcification rate correlates with temperature to different degrees in the studied corals, likely reflecting control of the difference in zooxanthellae density, we infer that calcification rate is not a major controlling factor on skeletal δ⁴¹K. Rather, skeletal δ⁴¹K correlates closely with K phase partitioning, which is ascribed to temperature-sensitive physiological modulation.