Effect of polyamine transporter inhibitor on the pH increase in calcifying fluids of juvenile coral polyps possibly involved in calcification processes

Corals made their skeletons in the extracellular fluids called subcalicoblastic calcifying medium (SCM) which are located between calicoblastic cells and carbonate skeletons. There is an alkalization of the calcifying fluids in the calcification processes of corals. Venn et al. (2011) and Ohno et al. (2017) reported that pH of SCM in hard corals was about 0.5 ~1.0 higher than that of ambient seawaters by using pH indicator reagent. Additionally, Allemand et al. (2011) suggested that Ca²⁺-ATPase, which is located in the membranes of calicoblastic epithelial cells surrounding the SCM, contributes to the alkalization of the calcifying fluids. However, the mechanisms involved in the alkalization of the calcifying fluids are largely unknown. We reported that biogenic polyamines, which are grouped to the most common type of intracellular amines, were able to react with carbon dioxide (CO₂) and accelerate calcium carbonate (CaCO₃) formation in seawater (Yasumoto et al., 2014, 2018). Thus, we hypothesized that the biogenic polyamines play important roles in the alkalization of the calcifying fluids. To ascertain such hypothesis, we investigated the effect of polyamine transporter inhibitors (PTI) on the pH increase in the calcifying fluids in a juvenile polyps of Acroporid coral by using pH indicator reagent HTPS. When the primary polyps were treated with 0.5 to 1.0 μM PTI on 1 day post metamorphosis, the calcifying fluids of polyps were about pH 8.4, which were significantly lower than that of no treatment polyps. Our data propose the possibility that the polyamine transporter are involved in the alkalization of the calcifying fluids in primary coral polyps.