Light Induces an Increase in the pH of and a Decrease in the Ammonia Concentration in the Extrapallial Fluid of the Giant ClamTridacna squamosa

Abstract

The objective of this study was to examine whether 12 h of light exposure would lead to an increase in the pH of and a decrease in the concentration of total ammonia in the extrapallial fluid of the giant clam Tridacna squamosa. We also aimed to elucidate indirectly whether movements of ammonia and/or protons (H(+)) occurred between the extrapallial fluid and the outer mantle epithelium. The pH of the extrapallial fluid of T. squamosa exposed to 12 h of light was significantly higher than that of clams exposed to 12 h of darkness. Conversely, the total ammonia concentration in the extrapallial fluid of the former was significantly lower than that of the latter. In addition, the glutamine content in the mantle adjacent to the extrapallial fluid of clams exposed to 12 h of light was significantly greater than that of clams exposed to 12 h of darkness. These results suggest that in the extrapallial fluid of T. squamosa exposed to light, NH(3) combined with H(+) as NH(+)(4) and that NH(+)(4) was transported into the mantle and used as a substrate for glutamine formation. Injection of NH(4)Cl into the extrapallial fluid led to an instantaneous increase in the total ammonia concentration therein, but the total ammonia concentration decreased subsequently and returned to the control value within 1 h. This is in support of the proposition that NH(+)(4) could be transported from the extrapallial fluid to the mantle. Injection of HCl into the extrapallial fluid led to an instantaneous decrease in the pH of the extrapallial fluid. However, there was a significant increase in pH within 1 h in light or darkness, achieving a partial recovery toward the control pH value. The increase in pH within this 1-h period in light or darkness was accompanied by a significant decrease in the total ammonia concentration in the extrapallial fluid, which supports the proposition that H(+) could be transported in combination with NH(3) as NH(+)(4). Therefore, our results prompt a reexamination of the previous proposition that the removal of H(+) by NH(3) can facilitate calcification in molluscs in general and an investigation of the relationship between H(+) removal through NH(+)(4) transport and light-enhanced calcification in T. squamosa.

Effects of temperature andpCO2on the respiration, biomineralization and photophysiology of the giant clamTridacna maxima

“…They also facilitate CaCO 3 precipitation by influencing the Dissolved Inorganic Carbon (DIC) equilibrium by removing the CO 2 via photosynthesis (Goreau, 1959). Such phenomenon has been also described in giant clams (Ip et al, 2006; Ip et al, 2017). In T. squamosa , LEC increases the pH and reduces the ammonia concentration at the interface between the inner mantle and the shell in the extra-pallial fluid, where the biomineralization occurs (Ip et al, 2006).…”

“…Such phenomenon has been also described in giant clams (Ip et al, 2006; Ip et al, 2017). In T. squamosa , LEC increases the pH and reduces the ammonia concentration at the interface between the inner mantle and the shell in the extra-pallial fluid, where the biomineralization occurs (Ip et al, 2006). Recently, Chew et al (2019) reported a light-enhanced expression of a carbonic anhydrase (i.e., CA4-like) in the inner mantle of T. squamosa and suggested that this enzyme is involved in giant clam biomineralization by catalyzing the conversion of HCO - to CO 2 .…”

Impacts of ocean acidification on marine shelled molluscs

“…3) and the organism produces organic compounds that help crystal synthesis (nucleation), selecting the calcium carbonate polymorph (calcite or aragonite), defining the morphology and the shape of the crystal and finally interrupting its growth. It has been hypothesized that shelled molluscs increase the calcium carbonate saturation state in the extrapallial fluid (EPF) by adding calcium and/ or carbonate ions by passive transport or active pumping (Crenshaw and Neff 1969;Ip et al 2006). Moreover, shelled molluscs would make use of the enzyme carbonic anhydrase (CA) that catalyses the conversion of CO 2 to bicarbonate and vice versa.…”

Light-dependent expression of a Na+/H+exchanger 3-like transporter in the ctenidium of the giant clam,Tridacna squamosa, can be related to increased H+excretion during light-enhanced calcification

“…The removal of H + leads to an increase in pH which would raise the supersaturation of aragonite and result in a more rapid precipitation of CaCO 3 . Indeed, exposure to light induces a significant increase in the pH of, and a significant decrease in the total ammonia concentration in, the extrapallial fluid of the fluted giant clam, Tridacna squamosa (Ip et al ). Ip et al.…”