Evidence for a role of protein phosphorylation in the maintenance of the cnidarian-algal symbiosis

A genome-centric view of the role of the Acropora kenti microbiome in coral health and resilience

Microbial diversity has been extensively explored in reef-building corals. However, the functional roles of coral-associated microorganisms remain poorly elucidated. Here, we recover 191 bacterial and 10 archaeal metagenome-assembled genomes (MAGs) from the coral Acropora kenti (formerly A. tenuis) and adjacent seawater, to identify microbial functions and metabolic interactions within the holobiont. We show that 82 MAGs were specific to the A. kenti holobiont, including members of the Pseudomonadota, Bacteroidota, and Desulfobacterota. A. kenti-specific MAGs displayed significant differences in their genomic features and functional potential relative to seawater-specific MAGs, with a higher prevalence of genes involved in host immune system evasion, nitrogen and carbon fixation, and synthesis of five essential B-vitamins. We find a diversity of A. kenti-specific MAGs encode the biosynthesis of essential amino acids, such as tryptophan, histidine, and lysine, which cannot be de novo synthesised by the host or Symbiodiniaceae. Across a water quality gradient spanning 2° of latitude, A. kenti microbial community composition is correlated to increased temperature and dissolved inorganic nitrogen, with corresponding enrichment in molecular chaperones, nitrate reductases, and a heat-shock protein. We reveal mechanisms of A. kenti-microbiome-symbiosis on the Great Barrier Reef, highlighting the interactions underpinning the health of this keystone holobiont.

Symbiosis maintenance in the facultative coral, Oculina arbuscula, relies on nitrogen cycling, cell cycle modulation, and immunity

Symbiosis with unicellular algae in the family Symbiodiniaceae is common across tropical marine invertebrates. Reef-building corals offer a clear example of cellular dysfunction leading to a dysbiosis that disrupts entire ecosystems in a process termed coral bleaching. Due to their obligate symbiotic relationship, understanding the molecular underpinnings that sustain this symbiosis in tropical reef-building corals is challenging, as any aposymbiotic state is inherently coupled with severe physiological stress. Here, we leverage the subtropical, facultatively symbiotic and calcifying coral Oculina arbuscula to investigate gene expression differences between aposymbiotic and symbiotic branches within the same colonies under baseline conditions. We further compare gene ontology (GO) and KOG enrichment in gene expression patterns from O. arbuscula with prior work in the sea anemone Exaiptasia pallida (Aiptasia) and the salamander Ambystoma maculatum-both of which exhibit endophotosymbiosis with unicellular algae. We identify nitrogen cycling, cell cycle control, and immune responses as key pathways involved in the maintenance of symbiosis under baseline conditions. Understanding the mechanisms that sustain a healthy symbiosis between corals and Symbiodiniaceae algae is of urgent importance given the vulnerability of these partnerships to changing environmental conditions and their role in the continued functioning of critical and highly diverse marine ecosystems.

Broad scale proteomic analysis of heat-destabilised symbiosis in the hard coral Acropora millepora

Coral reefs across the globe are threatened by warming oceans. The last few years have seen the worst mass coral bleaching events recorded, with more than one quarter of all reefs irreversibly impacted. Considering the widespread devastation, we need to increase our efforts to understanding the physiological and metabolic shifts underlying the breakdown of this important symbiotic ecosystem. Here, we investigated the proteome (PRIDE accession # PXD011668) of both host and symbionts of the reef-building coral Acropora millepora exposed to ambient (~ 28 °C) and elevated temperature (~ 32 °C for 2 days, following a five-day incremental increase) and explored associated biomolecular changes in the symbiont, with the aim of gaining new insights into the mechanisms underpinning the collapse of the coral symbiosis. We identified 1,230 unique proteins (774 host and 456 symbiont) in the control and thermally stressed corals, of which 107 significantly increased and 125 decreased in abundance under elevated temperature relative to the control. Proteins involved in oxidative stress and proteolysis constituted 29% of the host proteins that increased in abundance, with evidence of impairment to endoplasmic reticulum and cytoskeletal regulation proteins. In the symbiont, we detected a decrease in proteins responsible for photosynthesis and energy production (33% of proteins decreased in abundance), yet minimal signs of oxidative stress or proteolysis. Lipid stores increased > twofold despite reduction in photosynthesis, suggesting reduced translocation of carbon to the host. There were significant changes in proteins related to symbiotic state, including proteins linked to nitrogen metabolism in the host and the V-ATPase (-0.6 fold change) known to control symbiosome acidity. These results highlight key differences in host and symbiont proteomic adjustments under elevated temperature and identify two key proteins directly involved in bilateral nutrient exchange as potential indicators of symbiosis breakdown.

Surface Topography, Bacterial Carrying Capacity, and the Prospect of Microbiome Manipulation in the Sea Anemone Coral Model Aiptasia

Aiptasia is an emerging model organism to study cnidarian symbioses due to its taxonomic relatedness to other anthozoans such as stony corals and similarities of its microalgal and bacterial partners, complementing the existing Hydra (Hydrozoa) and Nematostella (Anthozoa) model systems. Despite the availability of studies characterizing the microbiomes of several natural Aiptasia populations and laboratory strains, knowledge on basic information, such as surface topography, bacterial carrying capacity, or the prospect of microbiome manipulation is lacking. Here we address these knowledge gaps. Our results show that the surface topographies of the model hydrozoan Hydra and anthozoans differ substantially, whereas the ultrastructural surface architecture of Aiptasia and stony corals is highly similar. Further, we determined a bacterial carrying capacity of ∼10⁴ and ∼10⁵ bacteria (i.e., colony forming units, CFUs) per polyp for aposymbiotic and symbiotic Aiptasia anemones, respectively, suggesting that the symbiotic status changes bacterial association/density. Microbiome transplants from Acropora humilis and Porites sp. to gnotobiotic Aiptasia showed that only a few foreign bacterial taxa were effective colonizers. Our results shed light on the putative difficulties of transplanting microbiomes between cnidarians in a manner that consistently changes microbial host association at large. At the same time, our study provides an avenue to identify bacterial taxa that exhibit broad ability to colonize different hosts as a starting point for cross-species microbiome manipulation. Our work is relevant in the context of microbial therapy (probiotics) and microbiome manipulation in corals and answers to the need of having cnidarian model systems to test the function of bacteria and their effect on holobiont biology. Taken together, we provide important foundation data to extend Aiptasia as a coral model for bacterial functional studies.

The Molecular Language of the Cnidarian-Dinoflagellate Symbiosis

The cnidarian-dinoflagellate symbiosis is of huge importance as it underpins the success of coral reefs, yet we know very little about how the host cnidarian and its dinoflagellate endosymbionts communicate with each other to form a functionally integrated unit. Here, we review the current knowledge of interpartner molecular signaling in this symbiosis, with an emphasis on lipids, glycans, reactive species, biogenic volatiles, and noncoding RNA. We draw upon evidence of these compounds from recent omics-based studies of cnidarian-dinoflagellate symbiosis and discuss the signaling roles that they play in other, better-studied symbioses. We then consider how improved knowledge of interpartner signaling might be used to develop solutions to the coral reef crisis by, for example, engineering more thermally resistant corals.

Evidence for a role of protein phosphorylation in the maintenance of the cnidarian-algal symbiosis

The endosymbiotic relationship between cnidarians and photosynthetic dinoflagellate algae provides the foundation of coral reef ecosystems. This essential interaction is globally threatened by anthropogenic disturbance. As such, it is important to understand the molecular mechanisms underpinning the cnidarian–algal association. Here we investigated phosphorylation‐mediated protein signalling as a mechanism of regulation of the cnidarian–algal interaction, and we report on the generation of the first phosphoproteome for the coral model system Aiptasia. Mass spectrometry‐based phosphoproteomics using data‐independent acquisition allowed consistent quantification of over 3,000 phosphopeptides totalling more than 1,600 phosphoproteins across aposymbiotic (symbiont‐free) and symbiotic anemones. Comparison of the symbiotic states showed distinct phosphoproteomic profiles attributable to the differential phosphorylation of 539 proteins that cover a broad range of functions, from receptors to structural and signal transduction proteins. A subsequent pathway enrichment analysis identified the processes of “protein digestion and absorption,” “carbohydrate metabolism,” and “protein folding, sorting and degradation,” and highlighted differential phosphorylation of the “phospholipase D signalling pathway” and “protein processing in the endoplasmic reticulum.” Targeted phosphorylation of the phospholipase D signalling pathway suggests control of glutamate vesicle trafficking across symbiotic compartments, and phosphorylation of the endoplasmic reticulum machinery suggests recycling of symbiosome‐associated proteins. Our study shows for the first time that changes in the phosphorylation status of proteins between aposymbiotic and symbiotic Aiptasia anemones may play a role in the regulation of the cnidarian–algal symbiosis. This is the first phosphoproteomic study of a cnidarian–algal symbiotic association as well as the first application of quantification by data‐independent acquisition in the coral field.