DIMETHYLSULFONIOPROPIONATE PRODUCTION BY PSYCHROPHILIC DIATOM ISOLATES1

Dimethylsulfoniopropionate (DMSP) production by psychrophilic diatom strains, Chaetoceros sp., Navicula sp., and Nitzschia sp., were experimentally analyzed. All strains showed rapid growth (0.3–0.5 d^?1) under cold culture conditions (1.7°C). DMSP concentrations (both as particulate, DMSP_p, and dissolved, DMSP_d) were negligible in cultures of Chaetoceros sp., whereas those for Navicula sp. and Nitzschia sp. increased toward the end of the 56‐day experiments. The ratio of DMSP_p:chl a of the latter two species was approximately 5 in the early exponential growth phase and decreased slightly toward the early stationary phase. During the stationary phase, when chl a and fluorescence remained constant, the ratios in both species increased linearly by up to approximately 6.5 times the value at the start of the stationary phase. This growth‐related DMSP production by diatoms may result in the low concentrations of DMSP_p observed in the early stage of diatom blooms under natural conditions.     

The distribution of dimethylsulfoniopropionate in tropical Pacific coral reef invertebrates

Dimethylsulfoniopropionate (DMSP) is an important component of the global sulfur cycle and may be involved, via its cleavage product dimethylsulfide, in climate regulation. Although it is common in many algae, reports of DMSP in animals, particularly tropical invertebrates, are limited. This study examined the distribution of DMSP in a diverse group of coral reef invertebrates. DMSP was present in all 22 species of cnidarians and ranged from 9 to 723 μmol g⁻¹ of dry mass (DM) with a mean (± 1SD) of 110 ± 180 μmol g⁻¹ DM. It was not detected in a flatworm and an ascidian or in two of five sponges. Concentrations in sponges ranged from undetectable to 16 μmol g⁻¹ DM with a mean of 4 ± 7 μmol g⁻¹ DM. Within the cnidarians, DMSP concentrations did not differ among orders. Among cnidarian species, DMSP concentrations were correlated with symbiotic zooxanthellae densities. Within cnidarian species, DMSP concentrations of individuals were positively correlated with zooxanthellae densities in three of the four species examined. We speculate that DMSP is dietarily derived in sponges and derived from zooxanthellae in the cnidarians. The functions of DMSP in coral reef invertebrates are not known.     

Atmospheric dimethysulphide production from corals in the Great Barrier Reef and links to solar radiation, climate and coral bleaching

Coral zooxanthellae contain high concentrations of dimethylsulphoniopropionate (DMSP), the precursor of dimethylsulphide (DMS), an aerosol substance that could affect cloud cover, solar radiation and ocean temperatures. Acropora intermedia a dominant staghorn coral in the Indo-Pacific region, contain some of the highest concentrations of DMSP reported in the literature but no studies have shown that corals produce atmospheric DMS in situ and thus could potentially participate in sea surface temperature (SST) regulation over reefs; or how production varies during coral bleaching. We show that A. intermedia from the Great Barrier Reef (GBR) produces significant amounts of atmospheric DMS, in chamber experiments, indicating that coral reefs in this region could contribute to an “ocean thermostat” similar to that described for the western Pacific warm pool, where significantly fewer coral reefs have bleached during the last 25?years because of a cloud-SST feedback. However, when Acropora intermedia was stressed with higher light levels and seawater temperatures DMSP production, an indicator of zooxanthellae expulsion, increased markedly in the chamber, whilst atmospheric DMS emissions almost completely shut down. These results suggest that during increased light levels and seawater temperatures in the GBR coral shut-down atmospheric DMS aerosol production, potentially increasing solar radiation levels over reefs and exacerbating coral bleaching.     

Recovery of the coral Montastrea annularis in the Florida Keys after the 1987 Caribbean “bleaching event”

Many reef-building corals and other cnidarians lost photosynthetic pigments and symbiotic algae (zooxanthellae) during the coral bleaching event in the Caribbean in 1987. The Florida Reef Tract included some of the first documented cases, with widespread bleaching of the massive coral Montastrea annularis beginning in late August. Phototransects at Carysfort Reef showed discoloration of >90% of colonies of this species in March 1988 compared to 0% in July 1986; however no mortality was observed between 1986 and 1988. Samples of corals collected in February and June 1988 had zooxanthellae densities ranging from 0.1 in the most lightly colored corals, to 1.6x10⁶ cells/cm² in the darker corals. Minimum densities increased to 0.5x10⁶ cells/cm² by August 1989. Chlorophyll-a content of zooxanthellae and zooxanthellar mitotic indices were significantly higher in corals with lower densities of zooxanthellae, suggesting that zooxanthellar at low densities may be more nutrientsufficient than those in unbleached corals. Ash-free dry weight of coral tissue was positively correlated with zooxanthellae density at all sample times and was significantly lower in June 1988 compared to August 1989. Proteins and lipids per cm² were significantly higher in August 1989 than in February or June, 1988. Although recovery of zooxanthellae density and coral pigmentation to normal levels may occur in less than one year, regrowth of tissue biomass and energy stores lost during the period of low symbiont densities may take significantly longer.     

Is dimethylsulfoniopropionate (DMSP) produced by the symbionts or the host in an anemone–zooxanthella symbiosis?

Many groups of tropical cnidarians including scleractinian corals, octocorals, corallimorphs, and anemones contain the tertiary sulfonium compound dimethylsulfoniopropionate (DMSP). It is not known if the compound is synthesized by the animals, their microalgal symbionts, or derived through their diet. We determined the source of the DMSP in several species of tropical and temperate anemones using three approaches: (1) conducting comparative measurements of DMSP in aposymbiotic and zooxanthellate anemones of three species that harbor zooxanthellae, and similar measurements in one species that can harbor both zooxanthellae and zoochlorellae, (2) manipulating the presence or absence of zooxanthellae by inoculating juvenile aposymbiotic anemones (Aiptasia pallida) with their symbiont, Symbiodinium bermudense, and (3) manipulating the numbers of S. bermudense by growing aposymbiotic and zooxanthellate A. pallida in the light and the dark. DMSP was present in zooxanthellate anemones in concentrations of 3.4–15 μmol g⁻¹ fresh mass (FM). In aposymbiotic Aiptasia spp. and Anthopleura elegantissima that lacked large numbers of zooxanthellae, concentrations ranged from being undetectable to 0.43 μmol g⁻¹ FM. When aposymbiotic A. pallida were inoculated with zooxanthellae, concentrations of DMSP were an average of 4.24 μmol g⁻¹ FM after 5 weeks; DMSP was undetectable in uninoculated control animals. Aposymbiotic anemones maintained in the light or the dark for 6 weeks contained no DMSP or zooxanthellae. Zooxanthellate anemones in the light contained five times as many zooxanthellae and approximately 7.5 times as much DMSP as zooxanthellate anemones maintained in the dark. Taken together, these data show that the zooxanthellae are the sole source of DMSP in A. pallida. The trends in DMSP concentrations in other species of zooxanthellate anemones suggest that this phenomenon is not limited to A. pallida but may be more generally true for other anemones or even other cnidarians hosting species of Symbiodinium. Communicated by Biology Editor Dr. Michael Lesser     

Differential susceptibility to oxidative stress of two scleractinian corals: antioxidant functioning of mycosporine-glycine

This study examined the importance of mycosporine-glycine (Myc-Gly) as a functional antioxidant in the thermal-stress susceptibility of two scleractinian corals, Platygyra ryukyuensis and Stylophora pistillata. Photochemical efficiency of PSII (F_v/F_m), activity of antioxidant enzymes, superoxide dismutase (SOD) and catalase (CAT), and composition and abundance of mycosporine-like amino acids (MAAs) in the coral tissue and in symbiotic zooxanthellae were analyzed during 12-h exposure to high temperature (33 °C). After 6- and 12-h exposures at 33 °C, S. pistillata showed a significantly more pronounced decline in F_v/F_m compared to P. ryukyuensis. A 6-h exposure at 33 °C induced a significant increase in the activities of SOD and CAT in both host and zooxanthellae components of S. pistillata while in P. ryukyuensis a significant increase was observed only in the CAT activity of zooxanthellae. After 12-h exposure, the SOD activity of P. ryukyuensis was unaffected in the coral tissue but slightly increased in zooxanthellae, whereas the CAT activity in the coral tissue showed a 2.5-fold increase. The total activity of antioxidant enzymes was significantly higher in S. pistillata than in P. ryukyuensis, suggesting that P. ryukyuensis is less sensitive to oxidative stress than S. pistillata. This differential susceptibility of the corals is consistent with a 20-fold higher initial concentration of Myc-Gly in P. ryukyuensis compared to S. pistillata. In the coral tissue and zooxanthellae of both species investigated, the first 6 h of exposure to thermal stress induced a pronounced reduction in the abundance of Myc-Gly but not in other MAAs. When exposure was prolonged to 12 h, the Myc-Gly pool continued to decrease in P. ryukyuensis and was completely depleted in S. pistillata. The delay in the onset of oxidative stress in P. ryukyuensis and the dramatic increase in the activities of the antioxidant enzymes in S. pistillata, which contains low concentrations of Myc-Gly suggest that Myc-Gly provides rapid protection against oxidative stress before the antioxidant enzymes are induced. These findings strongly suggest that Myc-Gly is functioning as a biological antioxidant in the coral tissue and zooxanthellae and demonstrate its importance in the survival of reef-building corals under thermal stress.     

Dimethylsulfoniopropionate,superoxide dismutase and glutathione as stress response indicators in three corals under short-term hyposalinity stress

Corals are among the most active producers of dimethylsulfoniopropionate (DMSP), a key molecule in marine sulfur cycling, yet the specific physiological role of DMSP in corals remains elusive. Here, we examine the oxidative stress response of three coral species (Acropora millepora, Stylophora pistillata and Pocillopora damicornis) and explore the antioxidant role of DMSP and its breakdown products under short-term hyposalinity stress. Symbiont photosynthetic activity declined with hyposalinity exposure in all three reef-building corals. This corresponded with the upregulation of superoxide dismutase and glutathione in the animal host of all three species. For the symbiont component, there were differences in antioxidant regulation, demonstrating differential responses to oxidative stress between the Symbiodinium subclades. Of the three coral species investigated, only A. millepora provided any evidence of the role of DMSP in the oxidative stress response. Our study reveals variability in antioxidant regulation in corals and highlights the influence life-history traits, and the subcladal differences can have on coral physiology. Our data expand on the emerging understanding of the role of DMSP in coral stress regulation and emphasizes the importance of exploring both the host and symbiont responses for defining the threshold of the coral holobiont to hyposalinity stress.     

Preliminary evidence for tissue retraction as a factor in photoprotection of corals incapable of xanthophyll cycling

In a 6-h experiment where xanthophyll inter-conversion was blocked by the inhibitor dithiothreitol (DTT), corals in full sunlight showed a significant increase in oxidative damage and cnidarian antioxidant enzyme concentrations compared with controls. By comparison, antioxidant enzyme concentrations did not increase within the symbiotic algae. In addition, the normal reduction in steady state chlorophyll fluorescence (F_t) and maximum fluorescence (F_m′), in response to increased irradiance, was initially suppressed in the DTT-treated corals, but after 6 h, both parameters had reduced to levels similar to controls with a functional xanthophyll cycle. Extreme host retraction was observed in corals treated with DDT in full sunlight compared with untreated corals in full sunlight and DTT-treated corals in dim irradiance. These results suggest that the coral host is capable of using behavioural (tissue retraction) and biochemical defences (antioxidant enzymes) to protect the symbiotic algae under high natural irradiance when the xanthophyll cycle is absent. While these defences could not prevent oxidative damage, nonetheless, algal numbers and algal chlorophyll levels were not affected.     

The effects of elevated temperature on the photosynthetic efficiency of zooxanthellae in hospite from four different species of reef coral: a novel approach

Bleaching of reef corals is a phenomenon linked to temperature stress which involves loss of the symbiotic algae of the coral, which are known as zooxanthellae, and/or loss of algal pigments. The photosynthetic efficiency of zooxanthellae within the corals Montastrea annularis, Agaricia lamarki, Agaricia agaricites and Siderastrea radians was examined by pulse-amplitude modulation fluorometry (PAM) during exposure to elevated temperatures (30–36°C). Zooxanthellae within M. annularis and A. lamarki were found to be more sensitive to elevated temperature, virtually complete disruption of photosynthesis being noted during exposure to temperatures of 32 and 34°C. The photosynthetic efficiency of zooxanthellae within S. radians and A. agaricites decreased to a lesser extent. Differences in the loss of algal cells on an aerial basis and in the cellular chlorophyll concentration were also found between these species. By combining the non-invasive PAM technique with whole-cell fluorescence of freshly isolated zooxanthellae, we have identified fundamental differences in the physiology of the symbionts within different species of coral. Zooxanthellae within M. annularis appear to be more susceptible to heat-induced damage at or near the reaction centre of Photosystem II, while zooxanthellae living in S. radians remain capable of dissipating excess excitation energy through non-photochemical pathways, thereby protecting the photosystem from damage during heat exposure.     

Is photoinhibition of zooxanthellae photosynthesis the primary cause of thermal bleaching in corals?

The bleaching of corals in response to increases in temperature has resulted in significant coral reef degradation in many tropical marine ecosystems. This bleaching has frequently been attributed to photoinhibition of photosynthetic electron transport and the consequent photodamage to photosystem II (PSII) and the production of damaging reactive oxygen species (ROS) in the zooxanthellae (Symbiodinium spp.). However, these events may be because of perturbations of other processes occurring within the zooxanthellae or the host cells, and consequently constitute only secondary responses to temperature increase. The processes involved with the onset of photoinhibition of electron transport, photodamage to PSII and pigment bleaching in coral zooxanthellae are reviewed. Consideration is given to how increases in temperature might lead to perturbations of metabolic processes in the zooxanthellae and/or their host cells, which could trigger events leading to bleaching. It is concluded that production of ROS by the thylakoid photosynthetic apparatus in the zooxanthellae plays a major role in the onset of bleaching resulting from photoinhibition of photosynthesis, although it is not clear which particular ROS are involved. It is suggested that hydrogen peroxide generated in the zooxanthellae may have a signalling role in triggering the mechanisms that result in expulsion of zooxanthellae from corals.     

Increasing pCO2 correlates with low concentrations of intracellular dimethylsulfoniopropionate in the sea anemone Anemonia viridis

Marine anthozoans maintain a mutualistic symbiosis with dinoflagellates that are prolific producers of the algal secondary metabolite dimethylsulfoniopropionate (DMSP), the precursor of the climate-cooling trace gas dimethyl sulfide (DMS). Surprisingly, little is known about the physiological role of DMSP in anthozoans and the environmental factors that regulate its production. Here, we assessed the potential functional role of DMSP as an antioxidant and determined how future increases in seawater pCO₂ may affect DMSP concentrations in the anemone Anemonia viridis along a natural pCO₂ gradient at the island of Vulcano, Italy. There was no significant difference in zooxanthellae genotype and characteristics (density of zooxanthellae, and chlorophyll a) as well as protein concentrations between anemones from three stations along the gradient, V1 (3232 μatm CO₂), V2 (682 μatm) and control (463 μatm), which indicated that A. viridis can acclimate to various seawater pCO₂. In contrast, DMSP concentrations in anemones from stations V1 (33.23 ± 8.30 fmol cell⁻¹) and V2 (34.78 ± 8.69 fmol cell⁻¹) were about 35% lower than concentrations in tentacles from the control station (51.85 ± 12.96 fmol cell⁻¹). Furthermore, low tissue concentrations of DMSP coincided with low activities of the antioxidant enzyme superoxide dismutase (SOD). Superoxide dismutase activity for both host (7.84 ± 1.37 U·mg⁻¹ protein) and zooxanthellae (2.84 ± 0.41 U·mg⁻¹ protein) at V1 was 40% lower than at the control station (host: 13.19 ± 1.42; zooxanthellae: 4.72 ± 0.57 U·mg⁻¹ protein). Our results provide insight into coastal DMSP production under predicted environmental change and support the function of DMSP as an antioxidant in symbiotic anthozoans.     

Regulation and control of intracellular algae (= zooxanthellae) in hard corals

To examine algal (= zooxanthellae) regulation and control, and the factors determining algal densities in hard corals, the zooxanthellae mitotic index and release rates were regularly determined in branch tips from a colony of a staghorn coral, Acropora formosa, recovering from a coral ''bleaching'' event (the stress-related dissociation of the coral–algal symbiosis). Mathematical models based upon density-dependent decreases in the algal division frequency and increases in algal release rates during the post-bleaching recovery period accurately predict the observed recovery period (ca. 20 weeks). The models suggest that (i) the colony recovered its algal population from the division of the remaining zooxanthellae, and (ii) the continual loss of zooxanthellae significantly slowed the recovery of the coral. Possible reasons for the ''paradoxical'' loss of healthy zooxanthellae from the bleached coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthellae to aposymbiotic host cells. At a steady-state algal density of 2.1 x 10⁶ zooxanthellae cm^-2 at the end of the recovery period, the zooxanthellae would have to form a double layer of cells in the coral tissues, consistent with microscopic observations. Neighbouring colonies of A. formosa with inherently higher algal densities possess proportionately smaller zooxanthellae. Results suggest that space availability and the size of the algal symbionts determines the algal densities in the coral colonies. The large increases in the algal densities reported in corals exposed to elevated nutrient concentrations (i.e between a two- and five-fold increase in the algal standing stock) are not consistent with this theory. We suggest that increases of this magnitude are a product of the experimental conditions: reasons for this statement are discussed. We propose that the stability of the coral–algal symbiosis under non-stress conditions, and the constancy of zooxanthellae densities in corals reported across growth form, depth and geographic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.     

Symbiont diversity in scleractinian corals from tropical reefs and subtropical non-reef communities in Taiwan

We examined zooxanthellae diversity in scleractinian corals from southern Taiwan and the Penghu Archipelago, a tropical coral reef and a subtropical non-reefal community, respectively. Zooxanthellae diversity was investigated in 52 species of scleractinian corals from 26 genera and 13 families, using restriction fragment length polymorphism (RFLP), and phylogenetic analyses of the nuclear small-subunit ribosomal DNA (nssrDNA) and large-subunit ribosomal DNA (nlsrDNA). RFLP and phylogenetic analyses of nuclear-encoded ribosomal RNA genes showed that Symbiodinium clade C was the dominant zooxanthellae in scleractinian corals in the seas around Taiwan; Symbiodinium clade D was also found in some species. Both Symbiodinium clade C and D were found in colonies of seven species of scleractinian corals. Symbiodinium clade D was associated with corals that inhabit either shallow water or the reef edge in deep water, supporting the hypothesis that Symbiodinium clade D is a relatively stress-tolerant zooxanthellae found in marginal habitats.Communicated by Biological Editor H.R. Lasker     

Symbiosis-specific changes in dimethylsulphoniopropionate concentrations in Stylophora pistillata along a depth gradient

Scleractinian corals are prolific producers of dimethylsulphoniopropionate (DMSP), but ecophysiological mechanisms influencing cellular concentrations are uncertain. While DMSP is often proposed to function as an antioxidant, interactions between specific host–symbiont genotype associations, plasticity in DMSP concentrations and environmental conditions that can either exert or alleviate oxidative stress are unclear. We used long-term (6 months) reciprocal transplantation of Stylophora pistillata hosting two distinct symbiont phylotypes along a depth gradient, clades A (<20 m) and C (>20 m), to assess the effect of change in depth (light intensity) on DMSP concentrations in relation to symbiont genotype and photoacclimation in corals between 3 and 50 m in the Gulf of Aqaba. Bathymetric distribution of total DMSP (DMSPt) per cell varied significantly while particulate DMSP (DMSPp) appeared to be unaffected by depth. Highest DMSPt concentrations in control corals occurred at 20 m. While 3-m transplants showed a significant increase in DMSPt concentration at 20 m and became affiliated with an additional genotype (C72), 50-m transplants largely persisted with their original genotype and exhibited no significant changes in DMSPt concentrations. DMSPt concentrations in transplants at both 3 and 50 m, on the other hand, increased significantly while all corals maintained their original symbiont genotypes. Photoacclimation differed significantly with transplantation direction relative to the controls. Symbionts in 3-m transplants at 20 m exhibited no changes in chlorophyll a (chl a) concentration, cell density or cell diameter while symbiont densities decreased and chl a concentrations increased significantly at 50 m. In contrast, symbiont densities in 50-m transplants remained unaffected across depths while symbiont diameters decreased. Chl a concentrations decreased at 20 m and increased at 3 m. Our results indicate that DMSPt concentrations following changes in depth are not only a function of symbiont genotype but result from different acclimation abilities of both symbiotic partners.

     

Forever in the dark: the cave-dwelling?azooxanthellate reef coral Leptoseris troglodyta sp. n. (Scleractinia,?Agariciidae)

The coral species Leptoseris troglodyta sp. n. (Scleractinia, Agariciidae) is described as new to science. It is the first known azooxanthellate shallow-water agariciid and is recorded from the ceilings of caves at 5-35 m depth in West Pacific coral reefs. The corals have monocentric cup-shaped calices. They may become colonial through extramural budding from the basal coenosteum, which may cause adjacent calices to fuse. The size, shape and habitat of Leptoseris troglodyta are unique compared to other Leptoseris species, many of which have been recorded from mesophotic depths. The absence of zooxanthellae indicates that it may survive well in darkness, but endolithic algae in some corals indicate that they may be able to get some light. The presence of menianes on the septal sides, which may help to absorb light at greater depths in zooxanthellate corals, have no obvious adaptive relevance in the new species and could have been inherited from ancestral species that perhaps were zooxanthellate. The new species may be azooxanthellate as derived through the loss of zooxanthellae, which would be a reversal in Leptoseris phylogeny.     

Heterotrophy in Tropical Scleractinian Corals

The dual character of corals, that they are both auto- and heterotrophs, was recognized early in the twentieth Century. It is generally accepted that the symbiotic association between corals and their endosymbiotic algae (called zooxanthellae) is fundamental to the development of coral reefs in oligotrophic tropical oceans because zooxanthellae transfer the major part of their photosynthates to the coral host (autotrophic nutrition). However, numerous studies have confirmed that many species of corals are also active heterotrophs, ingesting organisms ranging from bacteria to mesozooplankton. Heterotrophy accounts for between 0 and 66% of the fixed carbon incorporated into coral skeletons and can meet from 15 to 35% of daily metabolic requirements in healthy corals and up to 100% in bleached corals. Apart from this carbon input, feeding is likely to be important to most scleractinian corals, since nitrogen, phosphorus, and other nutrients that cannot be supplied from photosynthesis by the coral's symbiotic algae must come from zooplankton capture, particulate matter or dissolved compounds. A recent study showed that during bleaching events some coral species, by increasing their feeding rates, are able to maintain and restore energy reserves.
This review assesses the importance and effects of heterotrophy in tropical scleractinian corals. We first provide background information on the different food sources (from dissolved organic matter to meso- and macrozooplankton). We then consider the nutritional inputs of feeding. Finally, we review feeding effects on the different physiological parameters of corals (tissue composition, photosynthesis and skeletal growth).     

The genetic identity of dinoflagellate symbionts in Caribbean octocorals

Many cnidarians (e.g., corals, octocorals, sea anemones) maintain a symbiosis with dinoflagellates (zooxanthellae). Zooxanthellae are grouped into clades, with studies focusing on scleractinian corals. We characterized zooxanthellae in 35 species of Caribbean octocorals. Most Caribbean octocoral species (88.6%) hosted clade B zooxanthellae, 8.6% hosted clade C, and one species (2.9%) hosted clades B and C. Erythropodium caribaeorum harbored clade C and a unique RFLP pattern, which, when sequenced, fell within clade C. Five octocoral species displayed no zooxanthella cladal variation with depth. Nine of the ten octocoral species sampled throughout the Caribbean exhibited no regional zooxanthella cladal differences. The exception, Briareum asbestinum, had some colonies from the Dry Tortugas exhibiting the E. caribaeorum RFLP pattern while elsewhere hosting clade B. In the Caribbean, octocorals show more symbiont specificity at the cladal level than scleractinian corals. Both octocorals and scleractinian corals, however, exhibited taxonomic affinity between zooxanthella clade and host suborder.Communicated by R.C. Carpenter     

Comparison of stress susceptibility of in hospite and isolated zooxanthellae among five coral species

Coral species in a similar habitat often show different bleaching susceptibilities. It is not understood which partner of coral-zooxanthellae complexes is responsible for differential stress susceptibility. Stress susceptibilities of in hospite and isolated zooxanthellae from five species of corals collected from shallow water in Okinawa were compared. To estimate stress susceptibility, we measured the maximum quantum yields (F_v/F_m) of in hospite and isolated zooxanthellae after 3-h exposure to either 28 or 34 °C at various light intensities and their recovery after 12 h under dim light at 26 °C. Significant reduction in photochemical efficiency (F_v/F_m) of photosystem II (PSII) was observed in in hospite zooxanthellae exposed to high light intensity (1000 μmol quanta m⁻² s⁻¹), while PSII activity of isolated zooxanthellae decreased significantly even at a lower light intensity (70 μmol quanta m⁻² s⁻¹). The recovery of the PSII activity after 12 h was incomplete in both in hospite and isolated zooxanthellae, indicating the presence of chronic photoinhibition. The stress susceptibility of isolated zooxanthellae was more variable among species than in hospite zooxanthellae. The order of stress susceptibility among the five coral species was different between in hospite and isolated zooxanthellae. The present results suggest that the host plays a significant role in determining bleaching susceptibility of corals, though zooxanthellae from different host have different stress susceptibilities.     

Identity and diversity of coral endosymbionts (zooxanthellae) from three Palauan reefs with contrasting bleaching, temperature and shading histories

The potential of corals to associate with more temperature-tolerant strains of algae (zooxanthellae, Symbiodinium) can have important implications for the future of coral reefs in an era of global climate change. In this study, the genetic identity and diversity of zooxanthellae was investigated at three reefs with contrasting histories of bleaching mortality, water temperature and shading, in the Republic of Palau (Micronesia). Single-stranded conformation polymorphism and sequence analysis of the ribosomal DNA internal transcribed spacer (ITS)1 region was used for genotyping. A chronically warm but partly shaded coral reef in a marine lake that is hydrographically well connected to the surrounding waters harboured only two single-stranded conformation polymorphism profiles (i.e. zooxanthella communities). It consisted only of Symbiodinium D in all 13 nonporitid species and two Porites species investigated, with the remaining five Porites harbouring C*. Despite the high temperature in this lake (> 0.5 degrees above ambient), this reef did not suffer coral mortality during the (1998) bleaching event, however, no bleaching-sensitive coral families and genera occur in the coral community. This setting contrasts strongly with two other reefs with generally lower temperatures, in which 10 and 12 zooxanthella communities with moderate to low proportions of clade D zooxanthellae were found. The data indicate that whole coral assemblages, when growing in elevated seawater temperatures and at reduced irradiance, can be composed of colonies associated with the more thermo-tolerant clade D zooxanthellae. Future increases in seawater temperature might, therefore, result in an increasing prevalence of Symbiodinium phylotype D in scleractinian corals, possibly associated with a loss of diversity in both zooxanthellae and corals.     

Discordant coral–symbiont structuring: factors shaping geographical variation of Symbiodinium communities in a facultative zooxanthellate coral genus,Oculina

Understanding the factors that help shape the association between corals and their algal symbionts, zooxanthellae (Symbiodinium), is necessary to better understand the functional diversity and acclimatization potential of the coral host. However, most studies focus on tropical zooxanthellate corals and their obligate algal symbionts, thus limiting our full comprehension of coral–algal symbiont associations. Here, we examine algal associations in a facultative zooxanthellate coral. We survey the Symbiodinium communities associated with Oculina corals in the western North Atlantic and the Mediterranean using one clade-level marker (psbA coding region) and three fine-scale markers (cp23S–rDNA, b7sym15 flanking region, and b2sym17). We ask whether Oculina spp. harbor geographically different Symbiodinium communities across their geographic range and, if so, whether the host’s genetics or habitat differences are correlated with this geographical variation. We found that Oculina corals harbor different Symbiodinium communities across their geographical range. Of the habitat differences (including chlorophyll a concentration and depth), sea surface temperature is better correlated with this geographical variation than the host’s genetics, a pattern most evident in the Mediterranean. Our results suggest that although facultative zooxanthellate corals may be less dependent on their algal partners compared to obligate zooxanthellate corals, the Symbiodinium communities that they harbor may nevertheless reflect acclimatization to environmental variation among habitats.