Symbiont population control by host-symbiont metabolic interaction in Symbiodiniaceae-cnidarian associations

Flexibility of nutritional strategies within a mutualism: food availability affects algal symbiont productivity in two congeneric sea anemone species

Mutualistic symbioses are common, especially in nutrient-poor environments where an association between hosts and symbionts can allow the symbiotic partners to persist and collectively out-compete non-symbiotic species. Usually these mutualisms are built on an intimate transfer of energy and nutrients (e.g. carbon and nitrogen) between host and symbiont. However, resource availability is not consistent, and the benefit of the symbiotic association can depend on the availability of resources to mutualists. We manipulated the diets of two temperate sea anemone species in the genus Anthopleura in the field and recorded the responses of sea anemones and algal symbionts in the family Symbiodiniaceae to our treatments. Algal symbiont density, symbiont volume and photosynthetic efficiency of symbionts responded to changes in sea anemone diet, but the responses depended on the species of sea anemone. We suggest that temperate sea anemones and their symbionts can respond to changes in anemone diet, modifying the balance between heterotrophy and autotrophy in the symbiosis. Our data support the hypothesis that symbionts are upregulated or downregulated based on food availability, allowing for a flexible nutritional strategy based on external resources.

Phylogenetic analysis of cell-cycle regulatory proteins within the Symbiodiniaceae

In oligotrophic waters, cnidarian hosts rely on symbiosis with their photosynthetic dinoflagellate partners (family Symbiodiniaceae) to obtain the nutrients they need to grow, reproduce and survive. For this symbiosis to persist, the host must regulate the growth and proliferation of its symbionts. One of the proposed regulatory mechanisms is arrest of the symbiont cell cycle in the G₁ phase, though the cellular mechanisms involved remain unknown. Cell-cycle progression in eukaryotes is controlled by the conserved family of cyclin-dependent kinases (CDKs) and their partner cyclins. We identified CDKs and cyclins in different Symbiodiniaceae species and examined their relationship to homologs in other eukaryotes. Cyclin proteins related to eumetazoan cell-cycle-related cyclins A, B, D, G/I and Y, and transcriptional cyclin L, were identified in the Symbiodiniaceae, alongside several alveolate-specific cyclin A/B proteins, and proteins related to protist P/U-type cyclins and apicomplexan cyclins. The largest expansion of Symbiodiniaceae cyclins was in the P/U-type cyclin groups. Proteins related to eumetazoan cell-cycle-related CDKs (CDK1) were identified as well as transcription-related CDKs. The largest expansion of CDK groups was, however, in alveolate-specific groups which comprised 11 distinct CDK groups (CDKA-J) with CDKB being the most widely distributed CDK protein. As a result of its phylogenetic position, conservation across Symbiodiniaceae species, and the presence of the canonical CDK motif, CDKB emerged as a likely candidate for a Saccharomyces cerevisiae Cdc28/Pho85-like homolog in Symbiodiniaceae. Similar to cyclins, two CDK-groups found in Symbiodiniaceae species were solely associated with apicomplexan taxa. A comparison of Breviolum minutum CDK and cyclin gene expression between free-living and symbiotic states showed that several alveolate-specific CDKs and two P/U-type cyclins exhibited altered expression in hospite, suggesting that symbiosis influences the cell cycle of symbionts on a molecular level. These results highlight the divergence of Symbiodiniaceae cell-cycle proteins across species. These results have important implications for host control of the symbiont cell cycle in novel cnidarian–dinoflagellate symbioses.

Transcriptome Reprogramming of Symbiodiniaceae Breviolum minutum in Response to Casein Amino Acids Supplementation

Dinoflagellates in the family Symbiodiniaceae can live freely in ocean waters or form a symbiosis with a variety of cnidarians including corals, sea anemones, and jellyfish. Trophic plasticity of Symbiodiniaceae is critical to its ecological success as it moves between environments. However, the molecular mechanisms underlying these trophic shifts in Symbiodiniaceae are still largely unknown. Using Breviolum minutum strain SSB01 (designated SSB01) as a model, we showed that Symbiodiniaceae go through a physiological and transcriptome reprogramming when the alga is grown with the organic nitrogen containing nutrients in hydrolyzed casein, but not with inorganic nutrients. SSB01 grows at a much faster rate and maintains stable photosynthetic efficiency when supplemented with casein amino acids compared to only inorganic nutrients or seawater. These physiological changes are driven by massive transcriptome changes in SSB01 supplemented with casein amino acids. The levels of transcripts encoding proteins involved in altering DNA conformation such as DNA topoisomerases, histones, and chromosome structural components were all significantly changed. Functional enrichment analysis also revealed processes involved in translation, ion transport, generation of second messengers, and phosphorylation. The physiological and molecular changes that underlie in vitro trophic transitions in Symbiodiniaceae can serve as an orthogonal platform to further understand the factors that impact the Symbiodiniaceae lifestyle.

Insights into coral bleaching under heat stress from analysis of gene expression in a sea anemone model system

Loss of endosymbiotic algae ("bleaching") under heat stress has become a major problem for reef-building corals worldwide. To identify genes that might be involved in triggering or executing bleaching, or in protecting corals from it, we used RNAseq to analyze gene-expression changes during heat stress in a coral relative, the sea anemone Aiptasia. We identified >500 genes that showed rapid and extensive up-regulation upon temperature increase. These genes fell into two clusters. In both clusters, most genes showed similar expression patterns in symbiotic and aposymbiotic anemones, suggesting that this early stress response is largely independent of the symbiosis. Cluster I was highly enriched for genes involved in innate immunity and apoptosis, and most transcript levels returned to baseline many hours before bleaching was first detected, raising doubts about their possible roles in this process. Cluster II was highly enriched for genes involved in protein folding, and most transcript levels returned more slowly to baseline, so that roles in either promoting or preventing bleaching seem plausible. Many of the genes in clusters I and II appear to be targets of the transcription factors NFκB and HSF1, respectively. We also examined the behavior of 337 genes whose much higher levels of expression in symbiotic than aposymbiotic anemones in the absence of stress suggest that they are important for the symbiosis. Unexpectedly, in many cases, these expression levels declined precipitously long before bleaching itself was evident, suggesting that loss of expression of symbiosis-supporting genes may be involved in triggering bleaching.

Different levels of energetic coupling between photosynthesis and respiration do not determine the occurrence of adaptive responses of Symbiodiniaceae to global warming

Disentangling the metabolic functioning of corals' endosymbionts (Symbiodiniaceae) is relevant to understanding the response of coral reefs to warming oceans. In this work, we first question whether there is an energetic coupling between photosynthesis and respiration in Symbiodiniaceae (Symbiodinium, Durusdinium and Effrenium), and second, how different levels of energetic coupling will affect their adaptive responses to global warming. Coupling between photosynthesis and respiration was established by determining the variation of metabolic rates during thermal response curves, and how inhibition of respiration affects photosynthesis. Adaptive (irreversible) responses were studied by exposing two Symbiodinium species with different levels of photosynthesis-respiration interaction to high temperature conditions (32°C) for 1 yr. We found that some Symbiodiniaceae have a high level of energetic coupling; that is, photosynthesis and respiration have the same temperature dependency, and photosynthesis is negatively affected when respiration is inhibited. Conversely, photosynthesis and respiration are not coupled in other species. In any case, prolonged exposure to high temperature caused adjustments in both photosynthesis and respiration, but these changes were fully reversible. We conclude that energetic coupling between photosynthesis and respiration exhibits wide variation amongst Symbiodiniaceae and does not determine the occurrence of adaptive responses in Symbiodiniaceae to temperature increase.

Sub-cellular imaging shows reduced photosynthetic carbon and increased nitrogen assimilation by the non-native endosymbiont Durusdinium trenchii in the model cnidarian Aiptasia

Hosting different symbiont species can affect inter-partner nutritional fluxes within the cnidarian-dinoflagellate symbiosis. Using nanoscale secondary ion mass spectrometry (NanoSIMS), we measured the spatial incorporation of photosynthetically fixed ¹³ C and heterotrophically derived ¹⁵ N into host and symbiont cells of the model symbiotic cnidarian Aiptasia (Exaiptasia pallida) when colonized with its native symbiont Breviolum minutum or the non-native Durusdinium trenchii. Breviolum minutum exhibited high photosynthetic carbon assimilation per cell and translocation to host tissue throughout symbiosis establishment, whereas D. trenchii assimilated significantly less carbon, but obtained more host nitrogen. These findings suggest that D. trenchii has less potential to provide photosynthetically fixed carbon to the host despite obtaining considerable amounts of heterotrophically derived nitrogen. These sub-cellular events help explain previous observations that demonstrate differential effects of D. trenchii compared to B. minutum on the host transcriptome, proteome, metabolome and host growth and asexual reproduction. Together, these differential effects suggest that the non-native host-symbiont pairing is sub-optimal with respect to the host's nutritional benefits under normal environmental conditions. This contributes to our understanding of the ways in which metabolic integration impacts the benefits of a symbiotic association, and the potential evolution of novel host-symbiont pairings.

Coral Productivity Is Co-Limited by Bicarbonate and Ammonium Availability

The nitrogen environment and nitrogen status of reef-building coral endosymbionts is one of the important factors determining the optimal assimilation of phototrophic carbon and hence the growth of the holobiont. However, the impact of inorganic nutrient availability on the photosynthesis and physiological state of the coral holobiont is partly understood. This study aimed to determine if photosynthesis of the endosymbionts associated with the coral Stylophora pistillata and the overall growth of the holobiont were limited by the availability of dissolved inorganic carbon and nitrogen in seawater. For this purpose, colonies were incubated in absence or presence of 4 µM ammonium and/or 6 mM bicarbonate. Photosynthetic performances, pigments content, endosymbionts density and growth rate of the coral colonies were monitored for 3 weeks. Positive effects were observed on coral physiology with the supplementation of one or the other nutrient, but the most important changes were observed when both nutrients were provided. The increased availability of DIC and NH₄⁺ significantly improved the photosynthetic efficiency and capacity of endosymbionts, in turn enhancing the host calcification rate. Overall, these results suggest that in hospite symbionts are co-limited by nitrogen and carbon availability for an optimal photosynthesis.

Host and Symbiont Cell Cycle Coordination Is Mediated by Symbiotic State, Nutrition, and Partner Identity in a Model Cnidarian-Dinoflagellate Symbiosis

Biomass regulation is critical to the overall health of cnidarian-dinoflagellate symbioses. Despite the central role of the cell cycle in the growth and proliferation of cnidarian host cells and dinoflagellate symbionts, there are few studies that have examined the potential for host-symbiont coregulation. This study provides evidence for the acceleration of host cell proliferation when in local proximity to clusters of symbionts within cnidarian tentacles. The findings suggest that symbionts augment the cell cycle of not only their enveloping host cells but also neighboring cells in the epidermis and gastrodermis. This provides a possible mechanism for rapid colonization of cnidarian tissues. In addition, the cell cycles of symbionts differed depending on nutritional regime, symbiotic state, and species identity. The responses of cell cycle profiles to these different factors implicate a role for species-specific regulation of symbiont cell cycles within host cnidarian tissues.

The cell cycle is a critical component of cellular proliferation, differentiation, and response to stress, yet its role in the regulation of intracellular symbioses is not well understood. To explore host-symbiont cell cycle coordination in a marine symbiosis, we employed a model for coral-dinoflagellate associations: the tropical sea anemone Aiptasia (Exaiptasia pallida) and its native microalgal photosymbionts (Breviolum minutum and Breviolum psygmophilum). Using fluorescent labeling and spatial point-pattern image analyses to characterize cell population distributions in both partners, we developed protocols that are tailored to the three-dimensional cellular landscape of a symbiotic sea anemone tentacle. Introducing cultured symbiont cells to symbiont-free adult hosts increased overall host cell proliferation rates. The acceleration occurred predominantly in the symbiont-containing gastrodermis near clusters of symbionts but was also observed in symbiont-free epidermal tissue layers, indicating that the presence of symbionts contributes to elevated proliferation rates in the entire host during colonization. Symbiont cell cycle progression differed between cultured algae and those residing within hosts; the endosymbiotic state resulted in increased S-phase but decreased G₂/M-phase symbiont populations. These phenotypes and the deceleration of cell cycle progression varied with symbiont identity and host nutritional status. These results demonstrate that host and symbiont cells have substantial and species-specific effects on the proliferation rates of their mutualistic partners. This is the first empirical evidence to support species-specific regulation of the symbiont cell cycle within a single cnidarian-dinoflagellate association; similar regulatory mechanisms likely govern interpartner coordination in other coral-algal symbioses and shape their ecophysiological responses to a changing climate.