Coupled carbon and nitrogen cycling regulates the cnidarian-algal symbiosis

Efficient nutrient recycling underpins the ecological success of cnidarian-algal symbioses in oligotrophic waters. In these symbioses, nitrogen limitation restricts the growth of algal endosymbionts in hospite and stimulates their release of photosynthates to the cnidarian host. However, the mechanisms controlling nitrogen availability and their role in symbiosis regulation remain poorly understood. Here, we studied the metabolic regulation of symbiotic nitrogen cycling in the sea anemone Aiptasia by experimentally altering labile carbon availability in a series of experiments. Combining 13C and 15N stable isotope labeling experiments with physiological analyses and NanoSIMS imaging, we show that the competition for environmental ammonium between the host and its algal symbionts is regulated by labile carbon availability. Light regimes optimal for algal photosynthesis increase carbon availability in the holobiont and stimulate nitrogen assimilation in the host metabolism. Consequently, algal symbiont densities are lowest under optimal environmental conditions and increase toward the lower and upper light tolerance limits of the symbiosis. This metabolic regulation promotes efficient carbon recycling in a stable symbiosis across a wide range of environmental conditions. Yet, the dependence on resource competition may favor parasitic interactions, explaining the instability of the cnidarian-algal symbiosis as environmental conditions in the Anthropocene shift towards its tolerance limits.

High light quantity suppresses locomotion in symbiotic Aiptasia

Many cnidarians engage in endosymbioses with microalgae of the family Symbiodiniaceae. In this association, the fitness of the cnidarian host is closely linked to the photosynthetic performance of its microalgal symbionts. Phototaxis may enable semi-sessile cnidarians to optimize the light regime for their microalgal symbionts. Indeed, phototaxis and phototropism have been reported in the photosymbiotic sea anemone Aiptasia. However, the influence of light quantity on the locomotive behavior of Aiptasia remains unknown. Here we show that light quantity and the presence of microalgal symbionts modulate the phototactic behavior in Aiptasia. Although photosymbiotic Aiptasia were observed to move in seemingly random directions along an experimental light gradient, their probability of locomotion depended on light quantity. As photosymbiotic animals were highly mobile in low light but almost immobile at high light quantities, photosymbiotic Aiptasia at low light quantities exhibited an effective net movement towards light levels sufficient for positive net photosynthesis. In contrast, aposymbiotic Aiptasia exhibited greater mobility than their photosymbiotic counterparts, regardless of light quantity. Our results suggest that photosynthetic activity of the microalgal symbionts suppresses locomotion in Aiptasia, likely by supporting a positive energy balance in the host. We propose that motile photosymbiotic organisms can develop phototactic behavior as a consequence of starvation linked to symbiotic nutrient cycling.

Implications of bleaching on cnidarian venom ecology

Cnidarian bleaching research often focuses on the effects on a cnidarian's physiological health and fitness, whilst little focus has been towards the impacts of these events on their venom ecology. Given the importance of a cnidarian's venom to their survival and the increasing threat of bleaching events, it is important to understand the effects that this threat may have on this important aspect of their ecology as it may have unforeseen impacts on their ability to catch prey and defend themselves. This review aims to explore evidence that suggests that bleaching may impact on each of the key aspects of a cnidarians' venom ecology: cnidae, venom composition, and venom toxicity. Additionally, the resulting energy deficit, compensatory heterotrophic feeding, and increased defensive measures have been highlighted as possible ecological factors driving these changes. Suggestions are also made to guide the success of research in this field into the future, specifically in regards to selecting a study organism, the importance of accurate symbiont and cnidae identification, use of appropriate bleaching methods, determination of bleaching, and animal handling. Ultimately, this review highlights a significant and important gap in our knowledge into how cnidarians are, and will, continue to be impacted by bleaching stress.

•

Information on the effects of bleaching on cnidarian venom ecology is limited.

•

There is evidence to suggest nematocysts, venom composition and venom toxicity may each be impacted by bleaching.

•

Bleaching may result in depleted energy, increased heterotrophy and/or the need for stronger defensive strategies.

•

To fully understand how cnidarians may be impacted by bleaching stress further research in this field is needed.

•

Future studies should consider the model organism and methodologies, thereby minimising indirect confounding effects.

Prevalence and Photobiology of Photosynthetic Dinoflagellate Endosymbionts in the Nudibranch Berghia stephanieae

Simple Summary

Some sea slugs have evolved highly specialized feeding habits and solely prey upon a reduced number of species. This is the case of Berghia stephanieae, a sea slug that feeds exclusively on glass anemones, such as Exaiptasia diaphana. Glass anemones host photosynthetic microalgae that B. stephanieae ingest when preying upon E. diaphana. The association between these photosynthetic microalgae and sea slugs appears to be limited in time, particularly if B. stephanieae is deprived of prey hosting these microalgae. In the present study, we validate the use of a non-invasive and non-destructive approach that allows monitoring the persistence of this association in live sea slugs by measuring chlorophyll fluorescence. A complete loss of photosynthetic microalgae was observed within 8 days when animals were deprived of food or fed glass anemones with no microalgae (bleached anemones). As such, the association between B. stephanieae and photosynthetic microalgae acquired when preying glass anemones is not a true symbiosis. Future studies may use the technique here described to monitor the prevalence of the association between sea slugs and photosynthetic microalgae, particularly under bleaching events that will impair sea slugs to acquire microalgae by preying upon their invertebrate hosts.

Abstract

Berghia stephanieae is a stenophagous sea slug that preys upon glass anemones, such as Exaiptasia diaphana. Glass anemones host photosynthetic dinoflagellate endosymbionts that sea slugs ingest when consuming E. diaphana. However, the prevalence of these photosynthetic dinoflagellate endosymbionts in sea slugs appears to be short-lived, particularly if B.stephanieae is deprived of prey that host these microalgae (e.g., during bleaching events impacting glass anemones). In the present study, we investigated this scenario, along with food deprivation, and validated the use of a non-invasive and non-destructive approach employing chlorophyll fluorescence as a proxy to monitor the persistence of the association between sea slugs and endosymbiotic photosynthetic dinoflagellates acquired through the consumption of glass anemones. Berghia stephanieae deprived of a trophic source hosting photosynthetic dinoflagellate endosymbionts (e.g., through food deprivation or by feeding on bleached E. diaphana) showed a rapid decrease in minimum fluorescence (F_o) and photosynthetic efficiency (F_v/F_m) when compared to sea slugs fed with symbiotic anemones. A complete loss of endosymbionts was observed within 8 days, confirming that no true symbiotic association was established. The present work opens a new window of opportunity to rapidly monitor in vivo and over time the prevalence of associations between sea slugs and photosynthetic dinoflagellate endosymbionts, particularly during bleaching events that prevent sea slugs from incorporating new microalgae through trophic interactions.

Responding to Threats Both Foreign and Domestic: NOD-Like Receptors in Corals

Historically mechanisms with which basal animals such as reef-building corals use to respond to changing and increasingly stressful environments have remained elusive. However, the increasing availability of genomic and transcriptomic data from these organisms has provided fundamental insights into the biology of these critically important ecosystem engineers. Notably, insights into cnidarians gained in the post-genomics age have revealed a surprisingly complex immune system which bears a surprising level of similarity with the vertebrate innate immune system. This system has been critically linked to how corals respond to the two most prominent threats on a global scale, emerging coral diseases and increasing water temperature, which are recognized cellularly as either foreign or domestic threats, respectively. These threats can arise from pathogenic microbes or internal cellular dysfunction, underscoring the need to further understand mechanisms corals use to sense and respond to threats to their cellular integrity. In this investigation and meta-analysis, we utilize resources only recently available in the post-genomic era to identify and characterize members of an underexplored class of molecules known as NOD-like receptors in the endangered Caribbean coral Orbicella faveolata. We then leverage these data to identify pathways possibly mediated by NLRs in both O. faveolata and the ecologically important branching coral Acropora digitifera. Overall, we find support that this class of proteins may provide a mechanistic link to how reef-building corals respond to threats both foreign and domestic.

Evolution of Photosynthetic Eukaryotes; Current Opinion, Perplexity, and a New Perspective

The evolution of eukaryotic photosynthesis marked a major transition for life on Earth, profoundly impacting the atmosphere of the Earth and evolutionary trajectory of an array of life forms. There are about ten lineages of photosynthetic eukaryotes, including Chloroplastida, Rhodophyta, and Cryptophyta. Mechanistically, eukaryotic photosynthesis arose via a symbiotic merger between a host eukaryote and either a cyanobacterial or eukaryotic photosymbiont. There are, however, many aspects of this major evolutionary transition that remain unsettled. The field, so far, has been dominated by proposals formulated following the principle of parsimony, such as the Archaeplastida hypothesis, in which a taxonomic lineage is often conceptually recognized as an individual cell (or a distinct entity). Such an assumption could lead to confusion or unrealistic interpretation of discordant genomic and phenotypic data. Here, we propose that the free-living ancestors to the plastids may have originated from a diversified lineage of cyanobacteria that were prone to symbioses, akin to some modern-day algae such as the Symbiodiniaceae dinoflagellates and Chlorella-related algae that associate with a number of unrelated host eukaryotes. This scenario, which assumes the plurality of ancestral form, better explains relatively minor but important differences that are observed in the genomes of modern-day eukaryotic algal species. Such a non-typological (or population-aware) way of thinking seems to better-model empirical data, such as discordant phylogenies between plastid and host eukaryote genes.

Acquisition of obligate mutualist symbionts during the larval stage is not beneficial for a coral host

Theory suggests that the direct transmission of beneficial endosymbionts (mutualists) from parents to offspring (vertical transmission) in animal hosts is advantageous and evolutionarily stable, yet many host species instead acquire their symbionts from the environment (horizontal acquisition). An outstanding question in marine biology is why some scleractinian corals do not provision their eggs and larvae with the endosymbiotic dinoflagellates that are necessary for a juvenile's ultimate survival. We tested whether the acquisition of photosynthetic endosymbionts (family Symbiodiniaceae) during the planktonic larval stage was advantageous, as is widely assumed, in the ecologically important and threatened Caribbean reef-building coral Orbicella faveolata. Following larval acquisition, similar changes occurred in host energetic lipid use and gene expression regardless of whether their symbionts were photosynthesizing, suggesting the symbionts did not provide the energetic benefit characteristic of the mutualism in adults. Larvae that acquired photosymbionts isolated from conspecific adults on their natal reef exhibited a reduction in swimming, which may interfere with their ability to find suitable settlement substrate, and also a decrease in survival. Larvae exposed to two cultured algal species did not exhibit differences in survival, but decreased their swimming activity in response to one species. We conclude that acquiring photosymbionts during the larval stage confers no advantages and can in fact be disadvantageous to this coral host. The timing of symbiont acquisition appears to be a critical component of a host's life history strategy and overall reproductive fitness, and this timing itself appears to be under selective pressure.

Feeding increases the number of offspring but decreases parental investment of Red Sea coral Stylophora pistillata

Successful reproductive output and recruitment is crucial to coral persistence and recovery following anthropogenic stress. Feeding is known to alter coral physiology and increase resilience to bleaching.The goal of the study was to address the knowledge gap of the influence of feeding on reproductive output and offspring phenotype.Colonies of Stylophora pistillata from the Northern Gulf of Aqaba (Red Sea) were fed an Artemia diet or unfed for 5 months during gametogenesis, fertilization, and brooding. In addition, time to settlement and mortality of planulae were assessed at water temperatures ranging from winter temperature (22°C) to three degrees above average peak summer temperature (31°C). A range of physiological parameters was measured in parents and offspring.In brooding parents, feeding significantly increased protein concentration and more than tripled the number of released planulae. Planulae from unfed colonies had higher chlorophyll per symbiont concentration and concomitantly higher photosynthetic efficiency compared to planulae from fed parents. In settlement assays, planulae showed a similar thermal resistance as known for this Red Sea adult population. Mortality was greater in planulae from unfed parents at ambient and 3°C above ambient temperature despite higher per offspring investment in terms of total fatty acid content. Fatty acid profiles and relative abundances were generally conserved between different fed and unfed colonies but planulae were enriched in monounsaturated fatty acids relative to adults, that is, 16:1, 18:1, 20:1, 22:1, and 24:1 isomers.Ultimately the availability of zooplankton could influence population physiology and recruitment in corals.

Global Shifts in Gene Expression Profiles Accompanied with Environmental Changes in Cnidarian-Dinoflagellate Endosymbiosis

Stable endosymbiotic relationships between cnidarian animals and dinoflagellate algae are vital for sustaining coral reef ecosystems. Recent studies have shown that elevated seawater temperatures can cause the collapse of their endosymbiosis, known as 'bleaching', and result in mass mortality. However, the molecular interplay between temperature responses and symbiotic states still remains unclear. To identify candidate genes relevant to the symbiotic stability, we performed transcriptomic analyses under multiple conditions using the symbiotic and apo-symbiotic (symbiont free) Exaiptasia diaphana, an emerging model sea anemone. Gene expression patterns showed that large parts of differentially expressed genes in response to heat stress were specific to the symbiotic state, suggesting that the host sea anemone could react to environmental changes in a symbiotic state-dependent manner. Comparative analysis of expression profiles under multiple conditions highlighted candidate genes potentially important in the symbiotic state transition under heat-induced bleaching. Many of these genes were functionally associated with carbohydrate and protein metabolisms in lysosomes. Symbiont algal genes differentially expressed in hospite encode proteins related to heat shock response, calcium signaling, organellar protein transport, and sugar metabolism. Our data suggest that heat stress alters gene expression in both the hosts and symbionts. In particular, heat stress may affect the lysosome-mediated degradation and transportation of substrates such as carbohydrates through the symbiosome (phagosome-derived organelle harboring symbiont) membrane, which potentially might attenuate the stability of symbiosis and lead to bleaching-associated symbiotic state transition.

Binning enables efficient host genome reconstruction in cnidarian holobionts

Background

Many cnidarians, including stony corals, engage in complex symbiotic associations, comprising the eukaryotic host, photosynthetic algae, and highly diverse microbial communities—together referred to as holobiont. This taxonomic complexity makes sequencing and assembling coral host genomes extremely challenging. Therefore, previous cnidarian genomic projects were based on symbiont-free tissue samples. However, this approach may not be applicable to the majority of cnidarian species for ecological reasons. We therefore evaluated the performance of an alternative method based on sequence binning for reconstructing the genome of the stony coral Porites rus from a hologenomic sample and compared it to traditional approaches.

Results

Our results demonstrate that binning performs well for hologenomic data, producing sufficient reads for assembling the draft genome of P. rus. An assembly evaluation based on operational criteria showed results that were comparable to symbiont-free approaches in terms of completeness and usefulness, despite a high degree of fragmentation in our assembly. In addition, we found that binning provides sufficient data for exploratory k-mer estimation of genomic features, such as genome size and heterozygosity.

Conclusions

Binning constitutes a powerful approach for disentangling taxonomically complex coral hologenomes. Considering the recent decline of coral reefs on the one hand and previous limitations to coral genome sequencing on the other hand, binning may facilitate rapid and reliable genome assembly. This study also provides an important milestone in advancing binning from the metagenomic to the hologenomic and from the prokaryotic to the eukaryotic level.

Polymorphic adaptations in metazoans to establish and maintain photosymbioses

Mutualistic symbioses are common throughout the animal kingdom. Rather unusual is a form of symbiosis, photosymbiosis, where animals are symbiotic with photoautotrophic organisms. Photosymbiosis is found among sponges, cnidarians, flatworms, molluscs, ascidians and even some amphibians. Generally the animal host harbours a phototrophic partner, usually a cyanobacteria or a unicellular alga. An exception to this rule is found in some sea slugs, which only retain the chloroplasts of the algal food source and maintain them photosynthetically active in their own cytosol - a phenomenon called 'functional kleptoplasty'. Research has focused largely on the biodiversity of photosymbiotic species across a range of taxa. However, many questions with regard to the evolution of the ability to establish and maintain a photosymbiosis are still unanswered. To date, attempts to understand genome adaptations which could potentially lead to the evolution of photosymbioses have only been performed in cnidarians. This knowledge gap for other systems is mainly due to a lack of genetic information, both for non-symbiotic and symbiotic species. Considering non-photosymbiotic species is, however, important to understand the factors that make symbiotic species so unique. Herein we provide an overview of the diversity of photosymbioses across the animal kingdom and discuss potential scenarios for the evolution of this association in different lineages. We stress that the evolution of photosymbiosis is probably based on genome adaptations, which (i) lead to recognition of the symbiont to establish the symbiosis, and (ii) are needed to maintain the symbiosis. We hope to stimulate research involving sequencing the genomes of various key taxa to increase the genomic resources needed to understand the most fundamental question: how have animals evolved the ability to establish and maintain a photosymbiosis?

Effects of Symbiodinium Colonization on Growth and Cell Proliferation in the Giant Clam Hippopus hippopus

Giant clams (subfamily Tridacnidae) house their obligate symbionts, Symbiodinium sp., in a specialized tubular system. Rapid uptake of Symbiodinium has been shown to increase early clam survival, suggesting that symbionts play an essential role in host growth and development. To determine whether symbionts influence development in the giant clam Hippopus hippopus, we compared growth patterns and cell proliferation in two groups of clams inoculated or not inoculated (control) with Symbiodinium sp. Symbiont uptake occurred continuously from days 8 to 26 post-fertilization, with, on average, ∼5% per day colonized. The control treatment grew even without symbionts (1.03 ± 0.41 µm per day, standard error). Inoculated individuals grew significantly faster (2.91 ± 0.37 µm per day) than control individuals (P < 0.001). However, daily shell length measurements did not significantly differ between treatments until day 22, and ∼97% of control individuals metamorphosed by day 24, suggesting a delay in growth effects. Consistent with this, at day 13, clam cell proliferation was not correlated with symbiont abundance in inoculated individuals (P = 0.13), while at day 26, it was (P < 0.01). The proliferating cell pattern also changed from being randomly distributed (P = 0.99) at day 13 to non-randomly distributed (P = 0.002), with increased likelihood of proliferation within ∼25 µm of a symbiont, at day 26. Our results indicate that H. hippopus has a longer Symbiodinium acquisition period than previously recorded, after which proliferation and development are enhanced but during which growth is unaffected by Symbiodinium.

Using Aiptasia as a Model to Study Metabolic Interactions in Cnidarian-Symbiodinium Symbioses

The symbiosis between cnidarian hosts and microalgae of the genus Symbiodinium provides the foundation of coral reefs in oligotrophic waters. Understanding the nutrient-exchange between these partners is key to identifying the fundamental mechanisms behind this symbiosis, yet has proven difficult given the endosymbiotic nature of this relationship. In this study, we investigated the respective contribution of host and symbiont to carbon and nitrogen assimilation in the coral model anemone Aiptaisa. For this, we combined traditional measurements with nanoscale secondary ion mass spectrometry (NanoSIMS) and stable isotope labeling to investigate patterns of nutrient uptake and translocation both at the organismal scale and at the cellular scale. Our results show that the rate of carbon and nitrogen assimilation in Aiptasia depends on the identity of the host and the symbiont. NanoSIMS analysis confirmed that both host and symbiont incorporated carbon and nitrogen into their cells, implying a rapid uptake and cycling of nutrients in this symbiotic relationship. Gross carbon fixation was highest in Aiptasia associated with their native Symbiodinium communities. However, differences in fixation rates were only reflected in the δ¹³C enrichment of the cnidarian host, whereas the algal symbiont showed stable enrichment levels regardless of host identity. Thereby, our results point toward a "selfish" character of the cnidarian-Symbiodinium association in which both partners directly compete for available resources. Consequently, this symbiosis may be inherently instable and highly susceptible to environmental change. While questions remain regarding the underlying cellular controls of nutrient exchange and the nature of metabolites involved, the approach outlined in this study constitutes a powerful toolset to address these questions.

First insight into the viral community of the cnidarian model metaorganism Aiptasia using RNA-Seq data

Current research posits that all multicellular organisms live in symbioses with associated microorganisms and form so-called metaorganisms or holobionts. Cnidarian metaorganisms are of specific interest given that stony corals provide the foundation of the globally threatened coral reef ecosystems. To gain first insight into viruses associated with the coral model system Aiptasia (sensu Exaiptasia pallida), we analyzed an existing RNA-Seq dataset of aposymbiotic, partially populated, and fully symbiotic Aiptasia CC7 anemones with Symbiodinium. Our approach included the selective removal of anemone host and algal endosymbiont sequences and subsequent microbial sequence annotation. Of a total of 297 million raw sequence reads, 8.6 million (∼3%) remained after host and endosymbiont sequence removal. Of these, 3,293 sequences could be assigned as of viral origin. Taxonomic annotation of these sequences suggests that Aiptasia is associated with a diverse viral community, comprising 116 viral taxa covering 40 families. The viral assemblage was dominated by viruses from the families Herpesviridae (12.00%), Partitiviridae (9.93%), and Picornaviridae (9.87%). Despite an overall stable viral assemblage, we found that some viral taxa exhibited significant changes in their relative abundance when Aiptasia engaged in a symbiotic relationship with Symbiodinium. Elucidation of viral taxa consistently present across all conditions revealed a core virome of 15 viral taxa from 11 viral families, encompassing many viruses previously reported as members of coral viromes. Despite the non-random selection of viral genetic material due to the nature of the sequencing data analyzed, our study provides a first insight into the viral community associated with Aiptasia. Similarities of the Aiptasia viral community with those of corals corroborate the application of Aiptasia as a model system to study coral holobionts. Further, the change in abundance of certain viral taxa across different symbiotic states suggests a role of viruses in the algal endosymbiosis, but the functional significance of this remains to be determined.