Virginia M. Weis

Cellular mechanisms of Cnidarian bleaching: stress causes the collapse of symbiosis

SUMMARY Cnidarian bleaching is a breakdown in the mutualistic symbiosis between host Cnidarians, such as reef building corals, and their unicellular photosynthetic dinoflagellate symbionts. Bleaching is caused by a variety of environmental stressors, most notably elevated temperatures associated with global climate change in conjunction with high solar radiation, and it is a major contributor to coral death and reef degradation. This review examines the underlying cellular events that lead to symbiosis dysfunction and cause bleaching, emphasizing that, to date, we have only some pieces of a complex cellular jigsaw puzzle. Reactive oxygen species (ROS), generated by damage to both photosynthetic and mitochondrial membranes, is shown to play a central role in both injury to the partners and to inter-partner communication of a stress response. Evidence is presented that suggests that bleaching is a host innate immune response to a compromised symbiont, much like innate immune responses in other host–microbe interactions. Finally, the elimination or exit of the symbiont from host tissues is described through a variety of mechanisms including exocytosis, host cell detachment and host cell apoptosis.

Cell Biology of Cnidarian-Dinoflagellate Symbiosis

SUMMARY The symbiosis between cnidarians (e.g., corals or sea anemones) and intracellular dinoflagellate algae of the genus Symbiodinium is of immense ecological importance. In particular, this symbiosis promotes the growth and survival of reef corals in nutrient-poor tropical waters; indeed, coral reefs could not exist without this symbiosis. However, our fundamental understanding of the cnidarian-dinoflagellate symbiosis and of its links to coral calcification remains poor. Here we review what we currently know about the cell biology of cnidarian-dinoflagellate symbiosis. In doing so, we aim to refocus attention on fundamental cellular aspects that have been somewhat neglected since the early to mid-1980s, when a more ecological approach began to dominate. We review the four major processes that we believe underlie the various phases of establishment and persistence in the cnidarian/coral-dinoflagellate symbiosis: (i) recognition and phagocytosis, (ii) regulation of host-symbiont biomass, (iii) metabolic exchange and nutrient trafficking, and (iv) calcification. Where appropriate, we draw upon examples from a range of cnidarian-alga symbioses, including the symbiosis between green Hydra and its intracellular chlorophyte symbiont, which has considerable potential to inform our understanding of the cnidarian-dinoflagellate symbiosis. Ultimately, we provide a comprehensive overview of the history of the field, its current status, and where it should be going in the future.

Cell biology in model systems as the key to understanding corals.

Corals provide the foundation of important tropical reef ecosystems but are in global decline for multiple reasons, including climate change. Coral health depends on a fragile partnership with intracellular dinoflagellate symbionts. We argue here that progress in understanding coral biology requires intensive study of the cellular processes underlying this symbiosis. Such study will inform us on how the coral symbiosis will be affected by climate change, mechanisms driving coral bleaching and disease, and the coevolution of this symbiosis in the context of other host-microbe interactions. Drawing lessons from the broader history of molecular and cell biology and the study of other host-microbe interactions, we argue that a model-systems approach is essential for making effective progress in understanding coral cell biology.

Lectin/glycan interactions play a role in recognition in a coral/dinoflagellate symbiosis

Recognition is an important stage in the establishment of highly specific mutualistic associations. Yet, for the majority of symbioses, very few of the mechanisms involved in recognition and specificity are known. In this study, we provide evidence for a recognition mechanism at the onset of symbiosis between larvae of the coral Fungia scutaria and their endosymbiotic dinoflagellate algae. This recognition step occurs during initial cellular contact between the symbiotic partners through a lectin/glycan interaction. We determined that an intact algal cell surface was required for successful infection of F. scutaria larvae. Modification of the algal cell surface by enzymatic digestion with trypsin or N‐glycosidase significantly reduced infection success, and implicated algal cell surface glycans in recognition. Using flow cytometry, α‐mannose/α‐glucose and α‐galactose residues were identified as potential recognition ligands on the algal cell surface. Finally, inhibition of these cell surface glycans significantly reduced infection of F. scutaria larvae by the algae. These data provide evidence that the algal cell surface contains glycan ligands, such as α‐mannose/α‐glucose and α‐galactose, which play a role in recognition during initial contact at the onset of symbiosis with F. scutaria larvae.

LATE LARVAL DEVELOPMENT AND ONSET OF SYMBIOSIS IN THE SCLERACTINIAN CORAL FUNGIA SCUTARIA

Many corals that harbor symbiotic algae (zooxanthellae) produce offspring that initially lack zooxanthellae. This study examined late larval development and the acquisition of zooxanthellae in the scleractinian coral Fungia scutaria, which produces planula larvae that lack zooxanthellae. Larvae reared under laboratory conditions developed the ability to feed 3 days after fertilization; feeding behavior was stimulated by homogenized Artemia. Larvae began to settle and metamorphose 5 days after fertilization. In laboratory experiments, larvae acquired experimentally added zooxanthellae by ingesting them while feeding. Zooxanthellae entered the gastric cavity and were phagocytosed by endodermal cells. As early as 1 h after feeding, zooxanthellae were observed in both endodermal and ectodermal cells. Larvae were able to form an association with three genetically distinct strains of zooxanthellae. Both zooxanthellate and azooxanthellate larvae underwent metamorphosis, and azooxanthellate polyps were able to acquire zooxanthellae from the environment. Preliminary evidence suggests that the onset of symbiosis may influence larval development; in one study symbiotic larvae settled earlier than aposymbiotic larvae. Protein profiles of eggs and larvae throughout development revealed a putative yolk protein doublet that was abundant in eggs and 1-day-old larvae and was absent by day 6. This study is the first to examine the onset of symbiosis between a motile cnidarian host and its algal symbiont.

Transcriptome analysis of a cnidarian-dinoflagellate mutualism reveals complex modulation of host gene expression

BackgroundCnidarian – dinoflagellate intracellular symbioses are one of the most important mutualisms in the marine environment. They form the trophic and structural foundation of coral reef ecosystems, and have played a key role in the evolutionary radiation and biodiversity of cnidarian species. Despite the prevalence of these symbioses, we still know very little about the molecular modulators that initiate, regulate, and maintain the interaction between these two different biological entities. In this study, we conducted a comparative host anemone transcriptome analysis using a cDNA microarray platform to identify genes involved in cnidarian – algal symbiosis.ResultsWe detected statistically significant differences in host gene expression profiles between sea anemones (Anthopleura elegantissima) in a symbiotic and non-symbiotic state. The group of genes, whose expression is altered, is diverse, suggesting that the molecular regulation of the symbiosis is governed by changes in multiple cellular processes. In the context of cnidarian – dinoflagellate symbioses, we discuss pivotal host gene expression changes involved in lipid metabolism, cell adhesion, cell proliferation, apoptosis, and oxidative stress.ConclusionOur data do not support the existence of symbiosis-specific genes involved in controlling and regulating the symbiosis. Instead, it appears that the symbiosis is maintained by altering expression of existing genes involved in vital cellular processes. Specifically, the finding of key genes involved in cell cycle progression and apoptosis have led us to hypothesize that a suppression of apoptosis, together with a deregulation of the host cell cycle, create a platform that might be necessary for symbiont and/or symbiont-containing host cell survival. This first comprehensive molecular examination of the cnidarian – dinoflagellate associations provides critical insights into the maintenance and regulation of the symbiosis.

Nitric oxide and cnidarian bleaching: an eviction notice mediates breakdown of a symbiosis

SUMMARY Nitric oxide (NO) is a free radical implicated in numerous cell signaling, physiological and pathophysiological processes of eukaryotic cells. Here, we describe the production of NO as part of the cellular stress response of the symbiotic sea anemone Aiptasia pallida, which hosts dinoflagellates from the genus Symbiodinium. We show that exposure to elevated temperatures induces symbiotic anemones to produce high levels of NO, leading to the collapse of the symbiosis. These results shed light on the poorly understood cellular mechanism through which elevated seawater temperature causes the release of symbiotic algae from symbiotic cnidarians, a detrimental process known as coral (cnidarian) bleaching. The results presented here show that the host cell is a major source of NO during exposure to elevated temperatures and that this constitutes a cytotoxic response leading to bleaching. These results have important evolutionary implications as the observed NO production in these basal metazoans displays many parallels to the cytotoxic inflammatory response to pathogens, a well-understood process in mammalian model systems.

Apoptosis and autophagy as mechanisms of dinoflagellate symbiont release during cnidarian bleaching: every which way you lose.

Cnidarian bleaching results from the breakdown in the symbiosis between the host cnidarian and its dinoflagellate symbiont. Coral bleaching in recent years has increasingly caused degradation and mortality of coral reefs on a global scale. Although much is understood about the environmental causes of bleaching, the underlying cellular mechanisms of symbiont release that drive the process are just beginning to be described. In this study, we investigated the roles of two cellular pathways, host cell apoptosis and autophagy, in the bleaching process of the symbiotic anemone Aiptasia pallida. Host cell apoptosis was experimentally manipulated using gene knockdown of an anemone caspase by RNA interference, chemical inhibition of caspase using ZVAD-fmk and an apoptosis-inducer wortmannin. Autophagy was manipulated by chemical inhibition using wortmannin or induction using rapamycin. The applications of multiple single treatments resulted in some increased bleaching in anemones under control conditions but no significant drop in bleaching in individuals subjected to a hyperthermic stress. These results indicated that no single pathway is responsible for symbiont release during bleaching. However, when multiple inhibitors were applied simultaneously to block both apoptosis and autophagy, there was a significant reduction in bleaching in heat-stressed anemones. Our results allow us to formulate a model for cellular processes involved in the control of cnidarian bleaching where apoptosis and autophagy act together in a see-saw mechanism such that if one is inhibited the other is induced. Similar interconnectivity between apoptosis and autophagy has previously been shown in vertebrates including involvement in an innate immune response to pathogens and parasites. This suggests that the bleaching response could be a modified immune response that recognizes and removes dysfunctional symbionts.

Host-symbiont specificity during onset of symbiosis between the dinoflagellates Symbiodinium spp. and planula larvae of the scleractinian coral Fungia scutaria

Abstract. Many corals which engage in symbioses with dinoflagellates from the genus Symbiodinium (zooxanthellae) produce offspring which initially lack zooxanthellae. These species must choose their symbionts from numerous genetically distinct strains of zooxanthellae co-occurring in the environment. In most cases, symbiosis onset results in an association between a specific host coral and a specific strain of algal symbiont. This is the first study to examine host-symbiont specificity during symbiosis onset in a larval cnidarian, and the first to examine such events in a scleractinian of any life stage. We infected planula larvae of the solitary Hawaiian scleractinian Fungia scutaria with both homologous zooxanthellae, freshly isolated from F. scutaria adults, and heterologous zooxanthellae, isolated from Montipora verrucosa, Porites compressa, and Pocillopora damicornis, three species of scleractinians which co-occur with F. scutaria. We found that homologous zooxanthellae were better able to establish symbioses with larval hosts than were heterologous isolates, by two separate measures: percent of a larval population infected, and densities of zooxanthellae per larva. We also measured algal densities in larvae over a 4-day period until the onset of settlement and metamorphosis. We found no changes in zooxanthella population densities, regardless of zooxanthella type or the light environment in which they were incubated. Strong infection of host larvae with homologous algae compared to heterologous algae suggests that there is a specificity process which occurs sometime during the early stages of infection between the partners, and which results in the establishment of a specific symbiosis.

The genome of Aiptasia, a sea anemone model for coral symbiosis

Significance Coral reefs form marine-biodiversity hotspots of enormous ecological, economic, and aesthetic importance that rely energetically on a functional symbiosis between the coral animal and a photosynthetic alga. The ongoing decline of corals worldwide due to anthropogenic influences, including global warming, ocean acidification, and pollution, heightens the need for an experimentally tractable model system to elucidate the molecular and cellular biology underlying the symbiosis and its susceptibility or resilience to stress. The small sea anemone Aiptasia is such a system, and our analysis of its genome provides a foundation for research in this area and has revealed numerous features of interest in relation to the evolution and function of the symbiotic relationship. The most diverse marine ecosystems, coral reefs, depend upon a functional symbiosis between a cnidarian animal host (the coral) and intracellular photosynthetic dinoflagellate algae. The molecular and cellular mechanisms underlying this endosymbiosis are not well understood, in part because of the difficulties of experimental work with corals. The small sea anemone Aiptasia provides a tractable laboratory model for investigating these mechanisms. Here we report on the assembly and analysis of the Aiptasia genome, which will provide a foundation for future studies and has revealed several features that may be key to understanding the evolution and function of the endosymbiosis. These features include genomic rearrangements and taxonomically restricted genes that may be functionally related to the symbiosis, aspects of host dependence on alga-derived nutrients, a novel and expanded cnidarian-specific family of putative pattern-recognition receptors that might be involved in the animal–algal interactions, and extensive lineage-specific horizontal gene transfer. Extensive integration of genes of prokaryotic origin, including genes for antimicrobial peptides, presumably reflects an intimate association of the animal–algal pair also with its prokaryotic microbiome.