Cell wall proteomic analysis of the cnidarian photosymbionts Breviolum minutum and Cladocopium goreaui

Unlocking the Complex Cell Biology of Coral-Dinoflagellate Symbiosis: A Model Systems Approach

Symbiotic interactions occur in all domains of life, providing organisms with resources to adapt to new habitats. A prime example is the endosymbiosis between corals and photosynthetic dinoflagellates. Eukaryotic dinoflagellate symbionts reside inside coral cells and transfer essential nutrients to their hosts, driving the productivity of the most biodiverse marine ecosystem. Recent advances in molecular and genomic characterization have revealed symbiosis-specific genes and mechanisms shared among symbiotic cnidarians. In this review, we focus on the cellular and molecular processes that underpin the interaction between symbiont and host. We discuss symbiont acquisition via phagocytosis, modulation of host innate immunity, symbiont integration into host cell metabolism, and nutrient exchange as a fundamental aspect of stable symbiotic associations. We emphasize the importance of using model systems to dissect the cellular complexity of endosymbiosis, which ultimately serves as the basis for understanding its ecology and capacity to adapt in the face of climate change.

The Symbiodinium Proteome Response to Thermal and Nutrient Stresses

Coral bleaching is primarily caused by high sea surface temperatures, and nutrient enrichment of reefs is associated with lower resilience to thermal stress and ecological degradation. Excess inorganic nitrogen relative to phosphate has been proposed to sensitize corals to thermal bleaching. We assessed the physiological and proteomic responses of cultures of the dinoflagellate coral symbiont Symbiodinium microadriaticum to elevated temperature under low-nutrient, high-nutrient and phosphate-limited conditions. Elevated temperature induced reductions of many chloroplast proteins, particularly the light-harvesting complexes, and simultaneously increased the abundance of many chaperone proteins. Proteomes were similar when the N:P ratio was near the Redfield ratio, regardless of absolute N and P concentrations, but were strongly affected by phosphate limitation. Very high N:P inhibited Symbiodinium cell division while increasing the abundance of chloroplast proteins. The proteome response to phosphate limitation was greater than that to elevated temperature, as measured by the number of differentially abundant proteins. Increased physiological sensitivity to high temperatures under high nutrients or imbalanced N:P ratios was not apparent; however, oxidative stress response proteins were enriched among proteins responding to thermal stress under imbalanced N:P ratios. These data provide a detailed catalog of the effects of high temperatures and nutrients on a coral symbiont proteome.

Building consensus around the assessment and interpretation of Symbiodiniaceae diversity

Within microeukaryotes, genetic variation and functional variation sometimes accumulate more quickly than morphological differences. To understand the evolutionary history and ecology of such lineages, it is key to examine diversity at multiple levels of organization. In the dinoflagellate family Symbiodiniaceae, which can form endosymbioses with cnidarians (e.g., corals, octocorals, sea anemones, jellyfish), other marine invertebrates (e.g., sponges, molluscs, flatworms), and protists (e.g., foraminifera), molecular data have been used extensively over the past three decades to describe phenotypes and to make evolutionary and ecological inferences. Despite advances in Symbiodiniaceae genomics, a lack of consensus among researchers with respect to interpreting genetic data has slowed progress in the field and acted as a barrier to reconciling observations. Here, we identify key challenges regarding the assessment and interpretation of Symbiodiniaceae genetic diversity across three levels: species, populations, and communities. We summarize areas of agreement and highlight techniques and approaches that are broadly accepted. In areas where debate remains, we identify unresolved issues and discuss technologies and approaches that can help to fill knowledge gaps related to genetic and phenotypic diversity. We also discuss ways to stimulate progress, in particular by fostering a more inclusive and collaborative research community. We hope that this perspective will inspire and accelerate coral reef science by serving as a resource to those designing experiments, publishing research, and applying for funding related to Symbiodiniaceae and their symbiotic partnerships.

Help Me, Symbionts, You're My Only Hope: Approaches to Accelerate our Understanding of Coral Holobiont Interactions

Tropical corals construct the three-dimensional framework for one of the most diverse ecosystems on the planet, providing habitat to a plethora of species across taxa. However, these ecosystem engineers are facing unprecedented challenges, such as increasing disease prevalence and marine heatwaves associated with anthropogenic global change. As a result, major declines in coral cover and health are being observed across the world's oceans, often due to the breakdown of coral-associated symbioses. Here, we review the interactions between the major symbiotic partners of the coral holobiont-the cnidarian host, algae in the family Symbiodiniaceae, and the microbiome-that influence trait variation, including the molecular mechanisms that underlie symbiosis and the resulting physiological benefits of different microbial partnerships. In doing so, we highlight the current framework for the formation and maintenance of cnidarian-Symbiodiniaceae symbiosis, and the role that immunity pathways play in this relationship. We emphasize that understanding these complex interactions is challenging when you consider the vast genetic variation of the cnidarian host and algal symbiont, as well as their highly diverse microbiome, which is also an important player in coral holobiont health. Given the complex interactions between and among symbiotic partners, we propose several research directions and approaches focused on symbiosis model systems and emerging technologies that will broaden our understanding of how these partner interactions may facilitate the prediction of coral holobiont phenotype, especially under rapid environmental change.

Abundant Species Diversity and Essential Functions of Bacterial Communities Associated with Dinoflagellates as Revealed from Metabarcoding Sequencing for Laboratory-Raised Clonal Cultures

Interactions between algae and bacteria represent an important inter-organism association in aquatic environments, which often have cascading bottom-up influences on ecosystem-scale processes. Despite the increasing recognition of linkages between bacterioplankton and dynamics of dinoflagellate blooms in the field, knowledge about the forms and functions of dinoflagellate-bacteria associations remains elusive, mainly due to the ephemeral and variable conditions in the field. In this study, we characterized the bacterial community associated with laboratory cultures of 144 harmful algal strains, including 130 dinoflagellates (covering all major taxonomic orders of dinoflagellates) and 14 non-dinoflagellates, via high-throughput sequencing for 16S rRNA gene amplicons. A total of 4577 features belonging to bacteria kingdom comprising of 24 phyla, 55 classes, 134 orders, 273 families, 716 genera, and 1104 species were recovered from the algal culture collection, and 3 phyla (Proteobacteria, Bacteroidetes, and Firmicutes) were universally present in all the culture samples. Bacterial communities in dinoflagellates cultures exhibited remarkable conservation across different algal strains, which were dominated by a relatively small number of taxa, most notably the γ-proteobacteria Methylophaga, Marinobacter and Alteromonas. Although the bacterial community composition between dinoflagellates and non-dinoflagellate groups did not show significant difference in general, dinoflagellates harbored a large number of unique features (up to 3811) with relatively low individual abundance and enriched in the potential methylotrophs Methylophaga. While the bacterial assemblages associated with thecate and athecate dinoflagellates displayed no general difference in species composition and functional groups, athecate dinoflagellates appeared to accommodate more aerobic cellulolytic members of Actinobacteria, implying a more possible reliance on cellulose utilization as energy source. The extensive co-occurrence discovered here implied that the relationships between these algal species and the bacterial consortia could be viewed as either bilaterally beneficial (i.e., mutualism) or unilaterally beneficial at least to one party but virtually harmless to the other party (i.e., commensalism), whereas both scenarios support a long-term and stable co-existence rather than an exclusion of one or the other. Our results demonstrated that dinoflagellates-associated bacterial communities were similar in composition, with enrichment of potential uncultured methylotrophs to one-carbon compounds. This work enriches the knowledge about the fundamental functions of bacteria consortia associated with the phycospheres of dinoflagellates and other HABs-forming microalgae.