Holobiont biology in light of metaorganism composition: assessing how genotypes of member species assemble and contribute to emergent phenotypes (ASSOCIATE)

• Applicant: Professor Christian Voolstra, Ph.D.
• Subject Area: Ecology and Biodiversity of Animals and Ecosystems, Organismic Interactions ; Oceanography
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 559279431

Stony corals, marine benthic invertebrates in the class Anthozoa, engage in an endosymbiotic relationship with dinoflagellate algae of the family Symbiodiniaceae. This intimate relationship constitutes the foundation of coral reef ecosystems, which provides habitat for an estimated 30% of all described marine species. Corals and the reef ecosystems they build face unprecedented threat from anthropogenic change, e.g. marine heat waves, that trigger bleaching, the expulsion of algal symbionts and consequential whitening of the coral tissue. Without their symbionts, corals eventually starve due to the deprivation of photosynthates, which cover the coral’s energetic needs. Although it was initially assumed that all coral associate with a single algal species, it is now evident that corals associate with a wide array of genetically diverse Symbiodiniaceae that assumingly exhibit high partner fidelity. However, associations can change following extended periods of stress (such as bleaching), but the detailed phenotypic consequences of such altered associations are not well understood, nor whether they are permanent. In fact, for the majority of coral-algal pairings the level of fidelity and specificity of association are unknown, as is knowledge how this affects holobiont phenotypes and member species assemblage at large (microbiome). Here we propose to conduct population-level analyses of coral-algal genotype associations and their resultant phenotypes (bacterial microbiome and thermal tolerance) under original and altered host-symbiont pairings on a large population sample (200 colonies). We will employ a newly developed hybrid-capture based target enrichment approach to even sequencing of the host and algal symbiont portion in conjunction with single cell algal sequencing and standardized acute thermal stress assays and microbiome sequencing as a means to diagnose thermal tolerance phenotypes. Investigations into coral-algal genotype assemblages and associated phenotypes should greatly inform a better understanding of how genotypes/genetic traits of one partner affect selection of certain genotypes of the other partner, and how both affect holobiont phenotypes. This holds important implications for biodiversity and conservation efforts. For instance, the presence of discretely distributed coral-algal genotype assemblages would imply that algal symbiont (genetic) diversity constraints coral (genetic) diversity (and vice versa), and that genetic changes in response to climate change selection in one partner will affect holobiont assemblage at large, thereby affecting the holobiont phenotype and the associated genetic diversity of the holobiont. Understanding how symbiotic species (can) assemble and how this affects holobiont potential to respond to environmental change is critical to forecast species and biodiversity survival.

• DFG Programme: Research Grants