Adverse effects of total phosphate load from the environment on the skeletal formation of coral juveniles

Nitrogen's impact on corals has been widely studied, but the role of phosphate is often overlooked due to its low concentrations in seawater. Previous studies have suggested that phosphate can penetrate intercellular spaces to reach the extracellular calcifying medium (ECM), where it adsorbs onto skeletal surfaces and disrupts calcium carbonate crystallization, thereby inhibiting skeletal growth. Based on this mechanism, we hypothesized that skeletal growth inhibition depends not only on phosphate concentration but also on total phosphate load (flow volume × concentration). To test this hypothesis, we conducted experiments in which coral juveniles were cultured under conditions where phosphate concentrations as low as 0.5 μM and daily seawater exchanges of 0.9 L significantly inhibited skeletal formation. Furthermore, increasing the flow volume of phosphate-enriched seawater exacerbated calcification impairment. These findings underscore the importance of evaluating both phosphate concentration and total phosphate load when assessing phosphate pollution in natural environments.