Ikuko Yuyama

Algal symbiont type affects gene expression in juveniles of the coral Acropora tenuis exposed to thermal stress.

Reef-building corals harbor symbiotic dinoflagellates, Symbiodinium spp., which are currently divided into several clades. The responses of corals associated with different Symbiodinium clades to thermal stress are not well understood, especially at a gene expression level. Juveniles of the coral Acropora tenuis inoculated with different algal types (clade A or D) were exposed to thermal stress and the expression levels of four putative stress-responsive genes, including genes coding green and red fluorescent proteins, an oxidative stress-responsive protein, and an ascorbic acid transporter, were analyzed by quantitative real-time PCR. The expression levels of the four genes decreased at high temperatures if juveniles were associated with clade A symbionts but increased if the symbionts were in clade D. The intensity of green fluorescence increased with temperature in clade D symbionts harboring juveniles, but not in juveniles associated with clade A symbionts. The present results suggest that genotypes of endosymbiotic algae affect the thermal stress responses of the coral juveniles.

Profiling Differential Gene Expression of Symbiotic and Aposymbiotic Corals Using a High Coverage Gene Expression Profiling (HiCEP) Analysis

Coral generally harbors zooxanthellae (genus Symbiodinium) in the body for mutualistic symbiosis, which favors the host through effects on growth, stress response, and nutrient utilization. However, little is known about the molecular mechanisms by which the partners establish and regulate the endosymbiosis. In this study, we conducted a comprehensive transcriptome analysis in the coral Acropora tenuis using a high coverage gene expression profiling (HiCEP) method, to assess the genes that are involved in the coral–zooxanthellae symbiosis. For this purpose, we compared between aposymbiotic juveniles and those inoculated with a cultured monoclonal Symbiodinium species in two different clades (PL-TS-1 or CCMP2467). Among the 765 genes that exhibited different expression profiles between the two groups, 462 were upregulated and 303 downregulated by the symbiosis with somewhat variable responses to the two different symbionts. Among the responsive genes, we could annotate 33 genes by bioinformatic analyses and confirmed that their expression is actually altered in the same direction in the symbiotic individuals using real-time polymerase chain reaction. Functional analyses of the annotated genes indicate that they are involved in carbohydrate and lipid metabolism, intracellular signal transduction, and membrane transport of ions in the host corals as expected from the endosymbiosis of zooxanthellae.

The effects of thermal and high-CO2 stresses on the metabolism and surrounding microenvironment of the coral Galaxea fascicularis.

The effects of elevated temperature and high pCO2 on the metabolism of Galaxea fascicularis were studied with oxygen and pH microsensors. Photosynthesis and respiration rates were evaluated from the oxygen fluxes from and to the coral polyps. High-temperature alone lowered both photosynthetic and respiration rates. High pCO2 alone did not significantly affect either photosynthesis or respiration rates. Under a combination of high-temperature and high-CO2, the photosynthetic rate increased to values close to those of the controls. The same pH in the diffusion boundary layer was observed under light in both (400 and 750 ppm) CO2 treatments, but decreased significantly in the dark as a result of increased CO2. The ATP contents decreased with increasing temperature. The effects of temperature on the metabolism of corals were stronger than the effects of increased CO2. The effects of acidification were minimal without combined temperature stress. However, acidification combined with higher temperature may affect coral metabolism due to the amplification of diel variations in the microenvironment surrounding the coral and the decrease in ATP contents.

Comparing the effects of symbiotic algae (Symbiodinium) clades C1 and D on early growth stages of Acropora tenuis.

Reef-building corals switch endosymbiotic algae of the genus Symbiodinium during their early growth stages and during bleaching events. Clade C Symbiodinium algae are dominant in corals, although other clades — including A and D — have also been commonly detected in juvenile Acroporid corals. Previous studies have been reported that only molecular data of Symbiodinium clade were identified within field corals. In this study, we inoculated aposymbiotic juvenile polyps with cultures of clades C1 and D Symbiodinium algae, and investigated the different effect of these two clades of Symbiodinium on juvenile polyps. Our results showed that clade C1 algae did not grow, while clade D algae grew rapidly during the first 2 months after inoculation. Polyps associated with clade C1 algae exhibited bright green fluorescence across the body and tentacles after inoculation. The growth rate of polyp skeletons was lower in polyps associated with clade C1 algae than those associated with clade D algae. On the other hand, antioxidant activity (catalase) of corals was not significantly different between corals with clade C1 and clade D algae. Our results suggested that clade D Symbiodinium algae easily form symbiotic relationships with corals and that these algae could contribute to coral growth in early symbiosis stages.

Biotic Control of Skeletal Growth by Scleractinian Corals in Aragonite–Calcite Seas

Modern scleractinian coral skeletons are commonly composed of aragonite, the orthorhombic form of CaCO3. Under certain conditions, modern corals produce calcite as a secondary precipitate to fill pore space. However, coral construction of primary skeletons from calcite has yet to be demonstrated. We report a calcitic primary skeleton produced by the modern scleractinian coral Acropora tenuis. When uncalcified juveniles were incubated from the larval stage in seawater with low mMg/Ca levels, the juveniles constructed calcitic crystals in parts of the primary skeleton such as the septa; the deposits were observable under Raman microscopy. Using scanning electron microscopy, we observed different crystal morphologies of aragonite and calcite in a single juvenile skeleton. Quantitative analysis using X-ray diffraction showed that the majority of the skeleton was composed of aragonite even though we had exposed the juveniles to manipulated seawater before their initial crystal nucleation and growth processes. Our results indicate that the modern scleractinian coral Acropora mainly produces aragonite skeletons in both aragonite and calcite seas, but also has the ability to use calcite for part of its skeletal growth when incubated in calcite seas.

Molecular characterization of coral sulfate transporter homolog that is up‐regulated by the presence of symbiotic algae

Reef-building corals are in symbiosis with the dinoflagellates Symbiodinium spp. In our previous study, the expression of two mRNAs (AtSym-01 and 02) was up-regulated by the presence of Symbiodinium cells (strain PL-TS-1) in juveniles of the reef-building coral Acropora tenuis. In this study, the AtSym-01 mRNA was found to encode a cnidarian ortholog of the vertebrate SLC26A11 sulfate transporter. The AtSym-01 and human SLC26A11 proteins exhibited 46% identity over 542 amino acids. Real-time polymerase chain reaction analysis showed that the expression level of the AtSym-01 mRNA was also increased by the presence of Symbiodinium strain CCMP2467 cells. Immunohistochemical analysis was performed using polyclonal antibodies against the AtSym-01 protein, in order to study the distribution of the protein in A. tenuis tissues. Cell-specific immunoreaction was observed in diverse tissues in juvenile and adult specimens. Notable immunoreaction was observed in mucocytes (mucus cells) in the outer epithelium of juveniles, and gastrodermal cells located between the coelenteron and skeleton of the adult colony. These observations suggest the possibility that the AtSym-01 protein is involved in uptake of sulfate ion for synthesis of sulfated macromolecules that are contained in the mucus and organic matrix of the calcified skeleton.

The northern limit of corals of the genus Acropora in temperate zones is determined by their resilience to cold bleaching.

The distribution of corals in Japan covers a wide range of latitudes, encompassing tropical to temperate zones. However, coral communities in temperate zones contain only a small subset of species. Among the parameters that determine the distribution of corals, temperature plays an important role. We tested the resilience to cold stress of three coral species belonging to the genus Acropora in incubation experiments. Acropora pruinosa, which is the northernmost of the three species, bleached at 13 °C, but recovered once temperatures were increased. The two other species, A. hyacinthus and A. solitaryensis, which has a more southerly range than A. pruinosa, died rapidly after bleaching at 13 °C. The physiological effects of cold bleaching on the corals included decreased rates of photosynthesis, respiration, and calcification, similar to the physiological effects observed with bleaching due to high temperature stress. Contrasting hot bleaching, no increases in antioxidant enzyme activities were observed, suggesting that reactive oxygen species play a less important role in bleaching under cold stress. These results confirmed the importance of resilience to cold stress in determining the distribution and northern limits of coral species, as cold events causing coral bleaching and high mortality occur regularly in temperate zones.

Sulfur utilization of corals is enhanced by endosymbiotic algae.

ABSTRACT Sulfur-containing compounds are important components of all organisms, but few studies have explored sulfate utilization in corals. Our previous study found that the expression of a sulfur transporter (SLC26A11) was upregulated in the presence of Symbiodinium cells in juveniles of the reef-building coral Acropora tenuis. In this study, we performed autoradiography using 35S-labeled sulfate ions (35SO4 2−) to examine the localization and amount of incorporated radioactive sulfate in the coral tissues and symbiotic algae. Incorporated 35SO4 2− was detected in symbiotic algal cells, nematocysts, ectodermal cells and calicoblast cells. The combined results of 35S autoradiography and Alcian Blue staining showed that incorporated 35S accumulated as sulfated glycosaminoglycans (GAGs) in the ectodermal cell layer. We also compared the relative incorporation of 35SO4 2− into coral tissues and endosymbiotic algae, and their chemical fractions in dark versus light (photosynthetic) conditions. The amount of sulfur compounds, such as GAGs and lipids, generated from 35SO4 2− was higher under photosynthetic conditions. Together with the upregulation of sulfate transporters by symbiosis, our results suggest that photosynthesis of algal endosymbionts contributes to the synthesis and utilization of sulfur compounds in corals. Summary: 35S-labeled sulfate incorporated into various cells of coral demonstrates that photosynthesis of endosymbiotic algae contributes to the synthesis and utilization of sulfur compounds.