Ammonia transporter 2 as a molecular marker to elucidate the potentials of ammonia transport in phylotypes of Symbiodinium, Cladocopium and Durusdinium in the fluted giant clam, Tridacna squamosa

Abstract

Giant clams harbor coccoid Symbiodiniaceae dinoflagellates that are phototrophic. These dinoflagellates generally include multiple phylotypes (species) of <i>Symbiodinium, Cladocopium,</i> and <i>Durusdinium</i> in disparate proportions depending on the environmental conditions. The coccoid symbionts can share photosynthate with the clam host, which in return supply them with nutrients containing inorganic carbon, nitrogen and phosphorus. Symbionts can recycle nitrogen by absorbing and assimilating the endogenous ammonia produced by the host. This study aimed to use the transcript levels of <i>ammonia transporter</i> 2 (AMT2) in <i>Symbiodinium</i> (Symb-AMT2), <i>Cladocopium</i> (Clad-AMT2) and <i>Durusdinium</i> (Duru-AMT2) as molecular indicators to estimate the potential of ammonia transport in these three genera of Symbiodiniaceae dinoflagellates in different organs of the fluted giant clam, <i>Tridacna squamosa,</i> obtained from Vietnam. We also determined the transcript levels of <i>form</i> II <i>ribulose</i>-1,5-<i>bisphosphate carboxylase/oxygenase</i> (<i>rbc</i>II) and <i>nitrate transporter</i> 2 (NRT2) in <i>Symbiodinium</i> (<i>Symb-rbc</i>II; Symb-NRT2), <i>Cladocopium</i> (<i>Clad-rbc</i>II; <i>Clad</i>-NRT2) and <i>Durusdinium</i> (<i>Duru-rbc</i>II; <i>Duru</i>-NRT2), in order to examine the potential of ammonia transport with reference to the potentials of phototrophy or NO₃− uptake independent of the quantities and proportion of these Symbiodiniaceae phylotypes. Our results indicated for the first time that phylotypes of <i>Symbiodinium</i> and <i>Cladocopium</i> could have different potentials of ammonia transport, and that phylotypes of <i>Symbiodinium</i> might have higher potential of NO₃− transport than ammonia transport. They also suggested that Symbiodiniaceae phylotypes residing in different organs of T. <i>squamosa</i> could have disparate potentials of ammonia transport, alluding to the functional diversity among phylotypes of coccoid <i>Symbiodinium, Cladocopium,</i> and <i>Durusdinium.</i>