Using Transcript Levels of Nitrate Transporter 2 as Molecular Indicators to Estimate the Potentials of Nitrate Transport in Symbiodinium, Cladocopium, and Durusdinium of the Fluted Giant Clam, Tridacna squamosa

Giant clams are important ecosystem engineers of coral reefs because they harbor large quantities of phototrophic Symbiodiniaceae dinoflagellates of mainly genera Symbiodinium, Cladocopium, and Durusdinium. The coccoid dinoflagellates donate photosynthate and amino acids to the clam host, which in return needs to supply inorganic carbon and nitrogen to them. The host can conduct light-enhanced absorption of nitrate (NO3–), which can only be metabolized by the symbionts. This study aimed to clone nitrate transporter 2 (NRT2) from the symbionts of the fluted giant clam, Tridacna squamosa. Here, we report three major sequences of NRT2 derived from Symbiodinium (Symb-NRT2), Cladocopium (Clad-NRT2) and Durusdinium (Duru-NRT2). Phenogramic analysis and molecular characterization confirmed that these three sequences were NRT2s derived from dinoflagellates. Immunofluorescence microscopy localized NRT2 at the plasma membrane and cytoplasmic vesicles of the symbiotic dinoflagellates, indicating that it could partake in the uptake and transport of NO3–. Therefore, the transcript levels of Symb-NRT2, Clad-NRT2, and Duru-NRT2 could be used as molecular indicators to estimate the potential of NO3– transport in five organs of 13 T. squamosa individuals. The transcript levels of form II ribulose-1, 5-bisphosphate carboxylase/oxygenase (rbcII) of Symbiodinium (Symb-rbcII), Cladocopium (Clad-rbcII) and Durusdinium (Duru-rbcII) were also determined in order to calculate the transcript ratios of Symb-NRT2/Symb-rbcII, Clad-NRT2/Clad-rbcII, and Duru-NRT2/Duru-rbcII. These ratios expressed the potentials of NO3– transport with reference to the phototrophic potentials in a certain genus of coccoid dinoflagellate independent of its quantity. Results obtained indicate that Symbiodinium generally had a higher potential of NO3– transport than Cladocopium and Durusdinium at the genus level. Furthermore, some phylotypes (species) of Symbiodinium, particularly those in the colorful outer mantle, had very high Symb-NRT2/Symb-rbcII ratio (7–13), indicating that they specialized in NO3– uptake and nitrogen metabolism. Overall, our results indicate for the first time that different phylotypes of Symbiodiniaceae dinoflagellates could have dissimilar abilities to absorb and assimilate NO3–, alluding to their functional diversity at the genus and species levels.

Diversity of Three Small Type’s Giant Clams and Their Associated Endosymbiotic Symbiodiniaceae at Hainan and Xisha Islands, South China Sea

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates, however, the diversity of the giant clams, as well as the diversity and distribution of Symbiodiniaceae in different Tridacnine species remain relatively poorly studied in the South China Sea. In this study, a total of 100 giant clams belonging to small type’s giant clams, Tridacna maxima, T. crocea, and T. noae, were collected from Hainan and Xisha Islands. Based on mtDNA cytochrome c oxidase subunit 1 gene (COI) and 16S rRNA fragments, T. maxima and T. crocea showed a closer phylogenetic relationship than T. noae. All the three species of giant clams hosted Symbiodiniaceae including genera Symbiodinium (formerly Clade A) and Cladocopium (formerly Clade C). Geographically, symbionts in Cladocopium are restricted to Xisha Islands, probably because Cladocopium prefers to inhabit in waters with higher mean temperatures. The endosymbiont specificity among the three giant clam species was also detected. T. noae and T. crocea are found to harbor Symbiodinium preferentially, compared with Cladocopium. These results could provide important information to understand various endosymbionts occurring in giant clams in the South China Sea.

Biodiversity of Associated Megabenthic Invertebrate of Corall Reef Ecosystem of Petong Island Batam Indonesia

Abstracts Global warming and climate changes were of the main environmental concern of marine scientist in the last three decade. As a biodiversity hotspot, marine coastal ecosystem faced an environmental threat because of increasing sea surface temperature (SST) and land base effluent, which in turn had an impact on the biodiversity of megabenthic faunal on coral reef community. The present paper studied the current condition of reef ecosystem as well as biodiversity of megabenthic community in Petong Island, a core zone of Batam Marine Management Area (BMMA), Riau Archipelago Province. Gradual changes of physical and chemical properties of seawater originated from coastal activities presumably has caused an effect on benthic faunal community structure. It revealed that benthic lifeform coral cover account for 30,53%, categorized into fairly good, whereas abiotic sand cover of 16,53%. The lifeform consists of Acropora (AC)1.73% and non-Acropora 28,80%. Death coral with algae (DCA) account for 40,40% and death corall (DC) 2,80%, Soft Corall (SC) 1,20%, fleshy seaweed (FS) 0,07%, other fauna (OT) 1,47%, and rubble (R) 7,00%. Apparently, only 4 of target species megabenthic fauna from eight, were eccounterred including; seaurchin (Diadema sp.), topshell (Drupella sp.), giant clam (Tridacna sp.), trochus (Trochus sp.), with density of each megabenthic fauna calculated 5929, 1857, 71 and 71 ind/ha, respectively. It appeared that sea urchin were the highest density and followed by topshell (Drupella sp). Sea urchin were noted as a bioindicator of reef ecosystem health. High density of sea urchin may indicate that reef in unhealthy state. The presence of Diadema Sitosum indicate that dead coral were present and it feed on algae growing in dead coral. High density of sea urchin may indicate that reef in unhealthy state

Giant clam growth in the Gulf of Aqaba is accelerated compared to fossil populations

The health of reef-building corals has declined due to climate change and pollution. However, less is known about whether giant clams, reef-dwelling bivalves with a photosymbiotic partnership similar to that found in reef-building corals, are also threatened by environmental degradation. To compare giant clam health against a prehistoric baseline, we collected fossil and modern Tridacna shells from the Gulf of Aqaba, Northern Red Sea. After calibrating daily/twice-daily growth lines from the outer shell layer, we determined that modern individuals of all three species ( Tridacna maxima , T. squamosa and T. squamosina ) grew faster than Holocene and Pleistocene specimens. Modern specimens also show median shell organic δ 15 N values 4.2‰ lower than fossil specimens, which we propose is most likely due to increased deposition of isotopically light nitrate aerosols in the modern era. Nitrate fertilization accelerates growth in cultured Tridacna , so nitrate aerosol deposition may contribute to faster growth in modern wild populations. Furthermore, colder winter temperatures and past summer monsoons may have depressed fossil giant clam growth. Giant clams can serve as sentinels of reef environmental change, both to determine their individual health and the health of the reefs they inhabit.