Dinoflagellate Symbionts Research Articles

Extensive Differences in Gene Expression Between Symbiotic and Aposymbiotic Cnidarians

Coral reefs provide habitats for a disproportionate number of marine species relative to the small area of the oceans that they occupy. The mutualism between the cnidarian animal hosts and their intracellular dinoflagellate symbionts provides the nutritional foundation for coral growth and formation of reef structures, because algal photosynthesis can provide >90% of the total energy of the host. Disruption of this symbiosis (“coral bleaching”) is occurring on a large scale due primarily to anthropogenic factors and poses a major threat to the future of coral reefs. Despite the importance of this symbiosis, the cellular mechanisms involved in its establishment, maintenance, and breakdown remain largely unknown. We report our continued development of genomic tools to study these mechanisms in Aiptasia, a small sea anemone with great promise as a model system for studies of cnidarian–dinoflagellate symbiosis. Specifically, we have generated de novo assemblies of the transcriptomes of both a clonal line of symbiotic anemones and their endogenous dinoflagellate symbionts. We then compared transcript abundances in animals with and without dinoflagellates. This analysis identified >900 differentially expressed genes and allowed us to generate testable hypotheses about the cellular functions affected by symbiosis establishment. The differentially regulated transcripts include >60 encoding proteins that may play roles in transporting various nutrients between the symbiotic partners; many more encoding proteins functioning in several metabolic pathways, providing clues regarding how the transported nutrients may be used by the partners; and several encoding proteins that may be involved in host recognition and tolerance of the dinoflagellate.

Seasonal mesophotic coral bleaching of Stylophora pistillata in the Northern Red Sea.

Coral bleaching occurs when environmental stress induces breakdown of the coral-algae symbiosis and the host initiates algae expulsion. Two types of coral bleaching had been thoroughly discussed in the scientific literature; the first is primarily associated with mass coral bleaching events; the second is a seasonal loss of algae and/or pigments. Here, we describe a phenomenon that has been witnessed for repeated summers in the mesophotic zone (40–63 m) in the northern Red Sea: seasonal bleaching and recovery of several hermatypic coral species. In this study, we followed the recurring bleaching process of the common coral Stylophora pistillata. Bleaching occurred from April to September with a 66% decline in chlorophyll a concentration, while recovery began in October. Using aquarium and transplantation experiments, we explored environmental factors such as temperature, photon flux density and heterotrophic food availability. Our experiments and observations did not yield one single factor, alone, responsible for the seasonal bleaching. The dinoflagellate symbionts (of the genus Symbiodinium) in shallow (5 m) Stylophora pistillata were found to have a net photosynthetic rate of 56.98–92.19 µmol O2 cm−2 day−1. However, those from mesophotic depth (60 m) during months when they are not bleached are net consumers of oxygen having a net photosynthetic rate between −12.86 - (−10.24) µmol O2 cm−2 day−1. But during months when these mesophotic corals are partially-bleached, they yielded higher net production, between −2.83–0.76 µmol O2 cm−2 day−1. This study opens research questions as to why mesophotic zooxanthellae are more successfully meeting the corals metabolic requirements when Chl a concentration decreases by over 60% during summer and early fall.

Acyl-Homoserine Lactone Quorum Sensing in the Roseobacter Clade

Members of the Roseobacter clade are ecologically important and numerically abundant in coastal environments and can associate with marine invertebrates and nutrient-rich marine snow or organic particles, on which quorum sensing (QS) may play an important role. In this review, we summarize current research progress on roseobacterial acyl-homoserine lactone-based QS, particularly focusing on three relatively well-studied representatives, Phaeobacter inhibens DSM17395, the marine sponge symbiont Ruegeria sp. KLH11 and the dinoflagellate symbiont Dinoroseobacter shibae. Bioinformatic survey of luxI homologues revealed that over 80% of available roseobacterial genomes encode at least one luxI homologue, reflecting the significance of QS controlled regulatory pathways in adapting to the relevant marine environments. We also discuss several areas that warrant further investigation, including studies on the ecological role of these diverse QS pathways in natural environments.

Intracellular pH and its response to CO2-driven seawater acidification in symbiotic versus non-symbiotic coral cells

Regulating intracellular pH (pHi) is critical for optimising the metabolic activity of corals, yet the mechanisms involved in pH regulation and the buffering capacity within coral cells are not well understood. Our study investigated how the presence of symbiotic dinoflagellates affects the response of pHi to PCO2-driven seawater acidification in cells isolated from Pocillopora damicornis. Using the fluorescent dye BCECF-AM, in conjunction with confocal microscopy, we simultaneously characterised the pHi response in host coral cells and their dinoflagellate symbionts, in symbiotic and non-symbiotic states under saturating light, with and without the photosynthetic inhibitor DCMU. Each treatment was run under control (pH 7.8) and CO2-acidified seawater conditions (decreasing pH from 7.8 to 6.8). After 105 min of CO2 addition, by which time the external pH (pHe) had declined to 6.8, the dinoflagellate symbionts had increased their pHi by 0.5 pH units above control levels when in the absence of DCMU. In contrast, in both symbiotic and non-symbiotic host coral cells, 15 min of CO2 addition (0.2 pH unit drop in pHe) led to cytoplasmic acidosis equivalent to 0.3-0.4 pH units irrespective of whether DCMU was present. Despite further seawater acidification over the duration of the experiment, the pHi of non-symbiotic coral cells did not change, though in host cells containing a symbiont cell the pHi recovered to control levels when photsynthesis was not inhibited. This recovery was negated when cells were incubated with DCMU. Our results reveal that photosynthetic activity of the endosymbiont is tightly coupled with the ability of the host cell to recover from cellular acidosis after exposure to high CO2/low pH.

Vectored dispersal of Symbiodinium by larvae of a Caribbean gorgonian octocoral

The ability of coral reefs to recover from natural and anthropogenic disturbance is difficult to predict, in part due to uncertainty regarding the dispersal capabilities and connectivity of their reef inhabitants. We developed microsatellite markers for the broadcast spawning gorgonian octocoral Eunicea (Plexaura) flexuosa (four markers) and its dinoflagellate symbiont, Symbiodinium B1 (five markers), and used them to assess genetic connectivity, specificity and directionality of gene flow among sites in Florida, Panama, Saba and the Dominican Republic. Bayesian analyses found that most E.flexuosa from the Florida reef tract, Saba and the Dominican Republic were strongly differentiated from many E.flexuosa in Panama, with the exception of five colonies from Key West that clustered with colonies from Panama. In contrast, Symbiodinium B1 was more highly structured. At least seven populations were detected that showed patterns of isolation by distance. The symbionts in the five unusual Key West colonies also clustered with symbionts from Panama, suggesting these colonies are the result of long-distance dispersal. Migration rate tests indicated a weak signal of northward immigration from the Panama population into the lower Florida Keys. As E.flexuosa clonemates only rarely associated with the same Symbiodinium B1 genotype (and vice versa), these data suggest a dynamic host-symbiont relationship in which E.flexuosa is relatively well dispersed but likely acquires Symbiodinium B1 from highly structured natal areas prior to dispersal. Once vectored by host larvae, these symbionts may then spread through the local population, and/or host colonies may acquire different local symbiont genotypes over time.

Does Coral Disease Affect Symbiodinium? Investigating the Impacts of Growth Anomaly on Symbiont Photophysiology

Growth anomaly (GA) is a commonly observed coral disease that impairs biological functions of the affected tissue. GA is prevalent at Wai ‘ōpae tide pools, southeast Hawai ‘i Island. Here two distinct forms of this disease, Type A and Type B, affect the coral, Montipora capitata . While the effects of GA on biology and ecology of the coral host are beginning to be understood, the impact of this disease on the photophysiology of the dinoflagellate symbiont, Symbiodinium spp., has not been investigated. The GA clearly alters coral tissue structure and skeletal morphology and density. These tissue and skeletal changes are likely to modify not only the light micro-environment of the coral tissue, which has a direct impact on the photosynthetic potential of Symbiodinium spp., but also the physiological interactions within the symbiosis. This study utilized Pulse amplitude modulation fluorometry (PAM) to characterize the photophysiology of healthy and GA-affected M . capitata tissue. Overall, endosymbionts within GA-affected tissue exhibit reduced photochemical efficiency. Values of both Fv/Fm and ΔF/ Fm’ were significantly lower (p<0.01) in GA tissue compared to healthy and unaffected tissues. Tracking the photophysiology of symbionts over a diurnal time period enabled a comparison of symbiont responses to photosynthetically available radiation (PAR) among tissue conditions. Symbionts within GA tissue exhibited the lowest values of ΔF/Fm’ as well as the highest pressure over photosystem II (p<0.01). This study provides evidence that the symbionts within GA-affected tissue are photochemically compromised compared to those residing in healthy tissue.

Evidence for a host role in thermotolerance divergence between populations of the mustard hill coral (Porites astreoides) from different reef environments

Studying the mechanisms that enable coral populations to inhabit spatially varying thermal environments can help evaluate how they will respond in time to the effects of global climate change and elucidate the evolutionary forces that enable or constrain adaptation. Inshore reefs in the Florida Keys experience higher temperatures than offshore reefs for prolonged periods during the summer. We conducted a common garden experiment with heat stress as our selective agent to test for local thermal adaptation in corals from inshore and offshore reefs. We show that inshore corals are more tolerant of a 6-week temperature stress than offshore corals. Compared with inshore corals, offshore corals in the 31°C treatment showed significantly elevated bleaching levels concomitant with a tendency towards reduced growth. In addition, dinoflagellate symbionts (Symbiodinium sp.) of offshore corals exhibited reduced photosynthetic efficiency. We did not detect differences in the frequencies of major (>5%) haplotypes comprising Symbiodinium communities hosted by inshore and offshore corals, nor did we observe frequency shifts ('shuffling') in response to thermal stress. Instead, coral host populations showed significant genetic divergence between inshore and offshore reefs, suggesting that in Porites astreoides, the coral host might play a prominent role in holobiont thermotolerance. Our results demonstrate that coral populations inhabiting reefs <10-km apart can exhibit substantial differences in their physiological response to thermal stress, which could impact their population dynamics under climate change.

The Response of the Mediterranean Gorgonian Eunicella singularis to Thermal Stress Is Independent of Its Nutritional Regime

Over the last few decades, sessile benthic organisms from the Mediterranean Sea have suffered from the global warming of the world's oceans, and several mass mortality events were observed during warm summers. It has been hypothesized that mortality could have been due to a nutrient (food) shortage following the stratification of the water column. However, the symbiotic gorgonian Eunicella singularis has also presented a locally exceptional mortality, despite its autotrophic capacities through the photosynthesis of its dinoflagellate symbionts. Thus, this study has experimentally investigated the response of E. singularis to a thermal stress (temperature increase from 18 to 26°C), with colonies maintained more than 2 months under four nutritional diets: autotrophy only (AO), autotrophy and inorganic nitrogen addition (AN), autotrophy and heterotrophy (AH), heterotrophy only (HO). At 18°C, and contrary to many other anthozoans, supplementation of autotrophy with either inorganic nitrogen or food (heterotrophy) had no effect on the rates of respiration, photosynthesis, as well as in the chlorophyll, lipid and protein content. In the dark, heterotrophy maintained the gorgonian's metabolism, except a bleaching (loss of pigments), which did not affect the rates of photosynthesis. At 24°C, rates of respiration, and photosynthesis significantly decreased in all treatments. At 26°C, in addition to a decrease in the lipid content of all treatments, a bleaching was observed after 1 week in the AO treatment, while the AH and AN treatments resisted three weeks before bleaching. These last results suggest that, temperatures above 24°C impair the energetic reserves of this species and might explain the mortality events in the Mediterranean.

Eight polymorphic microsatellite loci for the Indo-Pacific-wide zoanthid, Zoanthus sansibaricus

Next generation sequencing allows rapid development of genetic markers for investigating the ecology and evolution of non-model organisms. In the present study we employed 454 sequencing and high through-put screening to ultimately generate eight highly polymorphic microsatellites for Zoanthus sansibaricus, a common colonial anemone in the order Zoantharia that is widely distributed throughout the Indo-Pacific. Each locus was screened against cultured Symbiodinium as well as on individuals harboring distantly related symbiont species to discount contamination by DNA from their obligate dinoflagellate symbionts. The range of Z. sansibaricus extends across the tropical and subtropical Indian and Pacific oceans. Therefore, these population genetic markers will allow examination of dispersal and gene flow in this species across large tropical and subtropical oceanic ranges.

Isolation of clonal axenic strains of the symbiotic dinoflagellate Symbiodinium and their growth and host specificity(1).

The cnidarian-dinoflagellate mutualism is integral to the survival of the coral-reef ecosystem. Despite the enormous ecological and economic importance of corals, their cellular and molecular biology and the ways in which they respond to environmental change are still poorly understood. We have been developing a proxy system for examining the coral mutualism in which the dinoflagellate symbiont Symbiodinium is introduced into a clonal population of the host Aiptasia, a small sea anemone closely related to corals. To further develop the tools for this system, we generated five clonal, axenic strains of Symbiodinium and verified the lack of contaminants by growth on rich medium, microscopic examination, and PCR analysis. These strains were assigned to clades A (two strains), B, E, and F based on their chloroplast 23S rDNA sequences. Growth studies in liquid cultures showed that the clade B strain and one of the clade A strains were able to grow photoautotrophically (in light with no fixed carbon), mixotrophically (in light with fixed carbon), or heterotrophically (in dark with fixed carbon). The clade E strain, thought to be free-living, was able to grow photoautotrophically but not heterotrophically. Infection of an aposymbiotic Aiptasia host with the axenic strains showed consistent patterns of specificity, with only the clade B and one of the clade A strains able to successfully establish symbiosis. Overall, the Aiptasia-Symbiodinium association represents an important model system for dissecting aspects of the physiology and cellular and molecular biology of cnidarian-dinoflagellate mutualism and exploring issues that bear directly on coral bleaching.

Multiple Symbiont Acquisition Strategies as an Adaptive Mechanism in the Coral Stylophora pistillata

In obligate symbioses, the host’s survival relies on the successful acquisition and maintenance of symbionts. Symbionts can either be transferred from parent to offspring via direct inheritance (vertical transmission) or acquired anew each generation from the environment (horizontal transmission). With vertical symbiont transmission, progeny benefit by not having to search for their obligate symbionts, and, with symbiont inheritance, a mechanism exists for perpetuating advantageous symbionts. But, if the progeny encounter an environment that differs from that of their parent, they may be disadvantaged if the inherited symbionts prove suboptimal. Conversely, while in horizontal symbiont acquisition host survival hinges on an unpredictable symbiont source, an individual host may acquire genetically diverse symbionts well suited to any given environment. In horizontal acquisition, however, a potentially advantageous symbiont will not be transmitted to subsequent generations. Adaptation in obligate symbioses may require mechanisms for both novel symbiont acquisition and symbiont inheritance. Using denaturing-gradient gel electrophoresis and real-time PCR, we identified the dinoflagellate symbionts (genus Symbiodinium) hosted by the Red Sea coral Stylophora pistillata throughout its ontogenesis and over depth. We present evidence that S. pistillata juvenile colonies may utilize both vertical and horizontal symbiont acquisition strategies. By releasing progeny with maternally derived symbionts, that are also capable of subsequent horizontal symbiont acquisition, coral colonies may acquire physiologically advantageous novel symbionts that are then perpetuated via vertical transmission to subsequent generations. With symbiont inheritance, natural selection can act upon the symbiotic variability, providing a mechanism for coral adaptation.

Comparative Lipid Profiling of the Cnidarian Aiptasia pallida and Its Dinoflagellate Symbiont

Corals and other cnidarians house photosynthetic dinoflagellate symbionts within membrane-bound compartments inside gastrodermal cells. Nutritional interchanges between the partners produce carbohydrates and lipids for metabolism, growth, energy stores, and cellular structures. Although lipids play a central role in the both the energetics and the structural/morphological features of the symbiosis, previous research has primarily focused on the fatty acid and neutral lipid composition of the host and symbiont. In this study we conducted a mass spectrometry-based survey of the lipidomic changes associated with symbiosis in the sea anemone Aiptasia pallida, an important model system for coral symbiosis. Lipid extracts from A. pallida in and out of symbiosis with its symbiont Symbiodinium were prepared and analyzed using negative-ion electrospray ionization quadrupole time-of-flight mass spectrometry. Through this analysis we have identified, by exact mass and collision-induced dissociation mass spectrometry (MS/MS), several classes of glycerophospholipids in A. pallida. Several molecular species of di-acyl phosphatidylinositol and phosphatidylserine as well as 1-alkyl, 2-acyl phosphatidylethanolamine (PE) and phosphatidycholine were identified. The 1-alkyl, 2-acyl PEs are acid sensitive suggestive that they are plasmalogen PEs possessing a double bond at the 1-position of the alkyl linked chain. In addition, we identified several molecular species of phosphonosphingolipids called ceramide aminoethylphosphonates in anemone lipid extracts by the release of a characteristic negative product ion at m/z 124.014 during MS/MS analysis. Sulfoquinovosyldiacylglycerol (SQDG), an anionic lipid often found in photosynthetic organisms, was identified as a prominent component of Symbiodinium lipid extracts. A comparison of anemone lipid profiles revealed a subset of lipids that show dramatic differences in abundance when anemones are in the symbiotic state as compared to the non-symbiotic state. The data generated in this analysis will serve as a resource to further investigate the role of lipids in symbiosis between Symbiodinium and A. pallida.

Range‐wide population genetic structure of the Caribbean sea fan coral, Gorgonia ventalina

The population structure of benthic marine organisms is of central relevance to the conservation and management of these often threatened species, as well as to the accurate understanding of their ecological and evolutionary dynamics. A growing body of evidence suggests that marine populations can be structured over short distances despite theoretically high dispersal potential. Yet the proposed mechanisms governing this structure vary, and existing empirical population genetic evidence is of insufficient taxonomic and geographic scope to allow for strong general inferences. Here, we describe the range-wide population genetic structure of an ecologically important Caribbean octocoral, Gorgonia ventalina. Genetic differentiation was positively correlated with geographic distance and negatively correlated with oceanographically modelled dispersal probability throughout the range. Although we observed admixture across hundreds of kilometres, estimated dispersal was low, and populations were differentiated across distances <2 km. These results suggest that populations of G. ventalina may be evolutionarily coupled via gene flow but are largely demographically independent. Observed patterns of differentiation corroborate biogeographic breaks found in other taxa (e.g. an east/west divide near Puerto Rico), and also identify population divides not discussed in previous studies (e.g. the Yucatan Channel). High genotypic diversity and absence of clonemates indicate that sex is the primary reproductive mode for G. ventalina. A comparative analysis of the population structure of G. ventalina and its dinoflagellate symbiont, Symbiodinium, indicates that the dispersal of these symbiotic partners is not coupled, and symbiont transmission occurs horizontally.

A Lipidomic Approach to Understanding Free Fatty Acid Lipogenesis Derived from Dissolved Inorganic Carbon within Cnidarian-Dinoflagellate Symbiosis

The cnidarian-dinoflagellate symbiosis is arguably one of the most important within the marine environment in that it is integral to the formation of coral reefs. However, the regulatory processes that perpetuate this symbiosis remain unresolved. It is essential to understand these processes, if we are to elucidate the mechanisms that support growth and resource accumulation by coral host, and conversely, recently observed reduction and/or mortality of corals in response to rapid environmental change. This study specifically focused on one area of metabolic activity within the symbiosis, that of free fatty acid synthesis within both the dinoflagellate symbionts and cnidarian host. The main model system used was Aiptasia pulchella and Symbiodinium sp. in combination with aposymbiotic A. pulchella, the symbiotic coral Acropora millepora system and dinoflagellate culture. Fatty acids (FAs) were selected because of their multiple essential roles inclusive of energy storage (resource accumulation), membrane structure fluidity and cell signaling. The study addressed free FA lipogenesis by using a new method of enriched stable isotopic (13C) incorporation from dissolved inorganic carbon (DI13C) combined with HPLC-MS. FAs derived from DI13C aligned with a mixture of known lipogenesis pathways with the addition of some unusual FAs. After 120 hr, 13C-enriched FA synthesis rates were attributed to only a complex integration of both n–3 and n–6 lipogenesis pathways within the dinoflagellate symbionts. Furthermore, there was no detectible evidence of symbiont derived enriched isotope fatty acids, catabolized 13C derivatives or DI13C being directly utilized, in host late n–6 pathway long-chain FA lipogenesis. These findings do not align with a popular mutualistic translocation model with respect to the use of translocated symbiont photoassimilates in host long-chain FA lipogenesis, which has important connotations for linking nutrient sources with metabolite production and the dynamic regulation of this symbiosis.

Within-Colony Migration of Symbionts During Bleaching of Octocorals

Octocorals compose a major part of cnidarian diversity. As with other symbiont-containing cnidarians, octocorals are susceptible to a stress response and subsequent "bleaching," which typically involves the loss of photosynthetic dinoflagellate symbionts. Studies of bleaching often focus on hexacorals, including sea anemones and scleractinians. The extent to which these results can be generalized to octocorals remains unclear. Bleaching was examined using two representative species of the Holaxonia-Alcyoniina clade of alcyonacean octocorals, Phenganax parrini and Sarcothelia sp. Remarkably, colonies of both species showed the same pattern in response to perturbation: symbionts in the polyps detach or die, leaving the polyps entirely bleached, yet at the same time large numbers of symbionts accumulate in the stolons. These symbionts are contained in host cells, many of which appear to attach to the stolon tissue. A comparison of living and fixed specimens suggests that these cells are loosely bound to, but not actually in, the stolonal tissue. Since gastrovascular fluid in the stolons is driven by cilia, these accumulating cells may lower fluid velocities. The accumulation of symbionts in the stolons during perturbation may have considerable relevance to how octocoral colonies recover from bleaching.

Regulation of cnidarian–dinoflagellate mutualisms: Evidence that activation of a host TGFβ innate immune pathway promotes tolerance of the symbiont

Animals must manage interactions with beneficial as well as detrimental microbes. Immunity therefore includes strategies for both resistance to and tolerance of microbial invaders. Transforming growth factor beta (TGFβ) cytokines have many functions in animals including a tolerance-promoting (tolerogenic) role in immunity in vertebrates. TGFβ pathways are present in basal metazoans such as cnidarians but their potential role in immunity has never been explored. This study takes a two-part approach to examining an immune function for TGFβ in cnidarians. First bioinformatic analyses of the model anemone Aiptasia pallida were used to identify TGFβ pathway components and explore the hypothesis that an immune function for TGFβs existed prior to the evolution of vertebrates. A TGFβ ligand from A. pallida was identified as one that groups closely with vertebrate TGFβs that have an immune function. Second, cellular analyses of A. pallida were used to examine a role for a TGFβ pathway in the regulation of cnidarian–dinoflagellate mutualisms. These interactions are stable under ambient conditions but collapse under elevated temperature, a phenomenon called cnidarian bleaching. Addition of exogenous human TGFβ suppressed an immune response measured as LPS-induced nitric oxide (NO) production by the host. Addition of anti-TGFβ to block a putative TGFβ pathway resulted in immune stimulation and a failure of the symbionts to successfully colonize the host. Finally, addition of exogenous TGFβ suppressed immune stimulation in heat-stressed animals and partially abolished a bleaching response. These findings suggest that the dinoflagellate symbionts somehow promote host tolerance through activation of tolerogenic host immune pathways, a strategy employed by some intracellular protozoan parasites during their invasion of vertebrates. Insight into the ancient, conserved nature of host–microbe interactions gained from this cnidarian–dinoflagellate model is valuable to understanding the evolution of immunity and its role in the regulation of both beneficial and detrimental associations.

Thermal Stress Promotes Host Mitochondrial Degradation in Symbiotic Cnidarians: Are the Batteries of the Reef Going to Run Out?

The symbiotic relationship between cnidarians and their dinoflagellate symbionts, Symbiodinium spp, which underpins the formation of tropical coral reefs, can be destabilized by rapid changes to environmental conditions. Although some studies have concluded that a breakdown in the symbiosis begins with increased reactive oxygen species (ROS) generation within the symbiont due to a decoupling of photosynthesis, others have reported the release of viable symbionts via a variety of host cell derived mechanisms. We explored an alternative model focused upon changes in host cnidarian mitochondrial integrity in response to thermal stress. Mitochondria are often likened to being batteries of the cell, providing energy in the form of ATP, and controlling cellular pathway activation and ROS generation. The overall morphology of host mitochondria was compared to that of associated symbionts under an experimental thermal stress using confocal and electron microscopy. The results demonstrate that hyperthermic stress induces the degradation of cnidarian host mitochondria that is independent of symbiont cellular deterioration. The potential sites of host mitochondrial disruption were also assessed by measuring changes in the expression of genes associated with electron transport and ATP synthesis using quantitative RT-PCR. The primary site of degradation appeared to be downstream of complex III of the electron transport chain with a significant reduction in host cytochrome c and ATP synthase expression. The consequences of reduced expression could limit the capacity of the host to mitigate ROS generation and maintain both organelle integrity and cellular energy supplies. The disruption of host mitochondria, cellular homeostasis, and subsequent cell death irrespective of symbiont integrity highlights the importance of the host response to thermal stress and in symbiosis dysfunction that has substantial implications for understanding how coral reefs will survive in the face of climate change.

Rapid Identification of Long-Chain Polyunsaturated Fatty Acids in a Marine Extract by HPLC-MS Using Data-Dependent Acquisition

The collision-induced dissociation (CID) of a range of deprotonated fatty acid standards was studied using linear ion trap mass spectrometry. Neutral losses of 78, 98, and 136 Da were consistently observed for fatty acids with five or more double bonds. Comparison of the MS/MS spectra of docosahexaenoic acid (DHA) and universally (13)C-labeled DHA allowed the molecular formulas for these neutral losses to be determined as C(6)H(6), C(5)H(6)O(2), and C(8)H(8)O(2). Knowledge of fatty acid fragmentation processes was then applied to identify fatty acids from a sea anemone, Aiptasia pulchella, and dinoflagellate symbiont, Symbiodinium sp. extract. Using HPLC-MS, fatty acids were separated and analyzed by tandem mass spectrometry in data-dependent acquisition mode. Neutral loss chromatograms for 78, 98, and 136 Da allowed the identification of long-chain fatty acids with five or more double bonds. On the basis of precursor ion m/z ratios, chain length and degree of unsaturation for these fatty acids were determined. The application of this technique to an Aiptasia sp.-Symbiodinium sp. lipid extract enabled the identification of the unusual, long-chain fatty acids 24:6, 26:6, 26:7, 28:7, and 28:8 during a single 40 min HPLC-MS analysis.

Nitrogen isotopic composition of planktonic foraminifera from the modern ocean and recent sediments

We investigated the controls on the δ15N values of shell‐bound organic matter of planktonic foraminifera (foraminifera‐bound δ15N, or FB‐δ15N). The bulk biomass δ15N of live foraminifera collected from plankton tows at Sta. S in the Sargasso Sea is within ∼1‰ of the FB‐δ15N of the same species picked from surface sediments from the low‐latitude North Atlantic. The FB‐δ15N value in the surface sediments is strongly correlated with the δ15N of thermocline nitrate, the dominant source of new N to the euphotic zone. The three euphotic‐zone‐dwelling, symbiotic, spinose species, Globigerinoides ruber, Globigerinoides sacculifer, and Orbulina universa, have a FB‐δ15N similar to or slightly higher than that of the nitrate supply to the euphotic zone, whereas the deeper‐dwelling, non‐spinose, and/or asymbiotic forms have higher δ15N. In the Cariaco Basin sediment trap samples, the FB‐δ15N of O. universa varies substantially (1.2‰ between the lowest and highest value), in some cases in step with δ15N changes in the bulk sinking N, while the subeuphotic‐zone‐dwelling, asymbiotic, and/or non‐spinose species are generally higher in FB‐δ15N and less variable through the time series. The higher and less temporally variable FB‐δ15N values of the deeper dwellers are consistent with their partial reliance on subsurface suspended particulate nitrogen, the δ15N of which is elevated and relatively stable over time. As an alternative, possibly additional explanation for the lower FB‐δ15N of the euphotic‐zone dwellers (despite their likely tendency to feed on high‐δ15N zooplankton), the dinoflagellate symbionts may reduce the δ15N elevation of a foraminifera's biomass relative to its diet by reducing the efflux of low‐δ15N ammonium.

A hypothesis linking sub-optimal seawater &amp;lt;I&amp;gt;p&amp;lt;/I&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; conditions for cnidarian-&amp;lt;I&amp;gt;Symbiodinium&amp;lt;/I&amp;gt; symbioses with the exceedence of the interglacial threshold (&amp;gt;260 ppmv)

Abstract. Most scleractinian corals and many other cnidarians host intracellular photosynthetic dinoflagellate symbionts ("zooxanthellae"). The zooxanthellae contribute to host metabolism and skeletogenesis to such an extent that this symbiosis is well recognised for its contribution in creating the coral reef ecosystem. The stable functioning of cnidarian symbioses is however dependent upon the host's ability to maintain demographic control of its algal partner. In this review, I explain how the modern envelope of seawater conditions found within many coral reef ecosystems (characterised by elevated temperatures, rising pCO2, and enriched nutrient levels) are antagonistic toward the dominant host processes that restrict excessive symbiont proliferation. Moreover, I outline a new hypothesis and initial evidence base, which support the suggestion that the additional "excess" zooxanthellae fraction permitted by seawater pCO2 levels beyond 260 ppmv significantly increases the propensity for symbiosis breakdown ("bleaching") in response to temperature and irradiance extremes. The relevance of this biological threshold is discussed in terms of historical reef extinction events, glacial-interglacial climate cycles and the modern decline of coral reef ecosystems.