The colorful mantle of the giant clam Tridacna squamosa expresses a homolog of electrogenic sodium: Bicarbonate cotransporter 2 that mediates the supply of inorganic carbon to photosynthesizing symbionts

Abstract

Giant clams live in symbiosis with phototrophic dinoflagellates, which reside extracellularly inside zooxanthellal tubules located mainly in the colourful and extensible outer mantle. As symbiotic dinoflagellates have no access to the ambient seawater, they need to obtain inorganic carbon (Ci) from the host for photosynthesis during illumination. The outer mantle has a host-mediated and light-dependent carbon-concentrating mechanism to augment the supply of Ci to the symbionts during illumination. Iridocytes can increase the secretion of H+ through vacuolar H+-ATPase to dehydrate HCO3− present in the hemolymph to CO2. CO2 can permeate the basolateral membrane of the epithelial cells of the zooxanthellal tubules, and rehydrated back to HCO3− in the cytoplasm catalysed by carbonic anhydrase 2. This study aimed to elucidate the molecular mechanism involved in the transport of HCO3− across the apical membrane of these epithelial cells into the luminal fluid surrounding the symbionts. We had obtained the complete cDNA coding sequence of a homolog of electrogenic Na+-HCO3− cotransporter 2 (NBCe2-like gene) from the outer mantle of the fluted giant clam, Tridacna squamosa. NBCe2-like gene comprised 3,399 bp, encoding a protein of 1,132 amino acids of 127.3 kDa. NBCe2-like protein had an apical localization in the epithelial cells of zooxanthellal tubules, denoting that it could transport HCO3− between the epithelial cells and the luminal fluid. Furthermore, illumination augmented the transcript level and protein abundance of NBCe2-like gene/NBCe2-like protein in the outer mantle, indicating that it could mediate the increased transport of HCO3− into the luminal fluid to support photosynthesis in the symbionts.

Ultradian rhythms in shell composition of photosymbiotic and non-photosymbiotic mollusks

“…This daily periodicity may be caused by the ctenidium in tridacnids working on a daily rhythm to keep the acid-base balance in the hemolymph of the clams to offset the CO2 depletion by photosymbionts (which is paced to the day-night cycle of light availability). In the process, Ca 2+ -channels and Na + /H + -exchangers work to keep the charge balance in the internal fluid and provide nutrients and ions for shell mineralization, letting in compatible trace elements such as Sr 2+ (Ip and Chew, 2021). This mechanism could explain the indirect link between trace element uptake in the shell in tridacnids and the day-night cycle without a direct causal relationship between trace element concentration and light availability (as demonstrated by the strong daily cycle in trace elements in the shaded TSM1 specimen).…”

“…This suggests that the 24h cycle has a much larger relative influence on trace element composition (especially Sr/Ca and Ba/Ca) in tridacnids than in pectinids. This seems to point towards a role of the photosymbionts in calcification by tridacnids, such as through symbiontmediated diurnal variation in the pH of the extrapallial fluid (Ip et al, 2006), as well as active transport of HCO3for calcification (Chew et al, 2019) and as a C supply to the symbionts from the host (Boo et al, 2021). Given the differences in absolute ratios between these two groups of bivalves, comparing variance yields a more robust assessment of the relative importance of tidal or diurnal variability on shell composition than looking at the absolute size (amplitude) of the chemical cycle.…”

“…CC BY 4.0 License. section 4.2; Ip and Chew, 2021). Symbionts have been shown to directly aid in calcification in terms of proton pumping (Armstrong et al, 2018), influencing internal acid-base chemistry (Ip et al, 2006), and valvometric studies show the clams bask in sunlight in daylight hours and close partially at night when symbiosis is likely reduced (Schwartzmann et al, 2011).…”

“…Alternatively, the presence of daily cyclicity in shell chemistry may yield information about the paleobiology of extinct mollusks, such as the presence of photosymbiosis (e.g., Sano et al, 2012;Warter et al, 2018;de Winter et al, 2020). The latter seems plausible given the effect of photosymbiosis on shell mineralization in modern tridacnids (Ip and Chew, 2021) and on the trace element composition of aragonite in modern photosymbiotic scleractinian corals (Cohen et al, 2002;Meibom et al, 2003;Inoue et al, 2018). If proven true, daily variability in bivalve shells may serve as a proxy for photosymbiosis in the fossil record (e.g., de Winter et al, 2020).…”