The use of algal symbiont cultures (Family Symbiodiniaceae) as model systems to study stony coral tissue loss disease: Use of fractionated disease isolates to help with pathogen identification

Since its initial appearance in late summer 2014, Florida’s Coral Reef has experienced unprecedented losses of brain and boulder corals due to stony coral tissue loss disease (SCTLD). These coral species appear to be unusually susceptible to this disease, but the factors underlying differential susceptibility (at both the species and colony level) are not well understood. Studies have shown that algal symbionts (Family Symbiodiniaceae) are implicated in disease etiology (Landsberg et al., 2020) and that different algal symbionts can modulate susceptibility, both within and between species (Dennison et al., 2021). Subsequent work on algal symbiont cultures showed that some algal symbionts, particularly certain taxa in the genus Breviolum and Durusdinium, but also members of Effrenium (but not Symbiodinium or Fugacium) may be the direct targets of a SCTLD pathogen(s) when exposed to seawater from disease colonies filtered to 10 μm, suggesting that cultured symbionts may be informative model systems for understanding SCTLD, due to the relative ease of maintaining clonal lineages in the laboratory under standard conditions that can be easily replicated for comparative experiments across space and time (Karp et al., 2022). Here, we expanded on this approach by fractionating the disease-exposed seawater to identify the lowest size fraction that elicits this disease response in algal symbiont cultures. We systematically exposed five strains (Symbiodinium microadriaticum, Breviolum minutum, Breviolum psygmophilium, Cladocopium goreaui, and Durusdinium trenchii) to six treatments (control 0.2 μm f/2, “healthy” coral water filtered to 10 μm, and seawater sourced from around diseased corals that was filtered to 10 μm, 3 μm, 0.8 μm, and 0.2 μm). We monitored cell culture response for 10 days using light microscopy and chlorophyll fluorescence, and preserved samples for downstream analysis using TEM, fluorescence microscopy, and transcriptomic analyses. Furthermore, we preserved samples for other collaborators to study metabolomics (Dr. Neha Garg, Georgia Tech) and screen for the SCTLD pathogen through metagenomics (Dr. Tony Goldberg, University of Wisconsin-Madison). Our findings show that B. minutum, C. goreaui, and D. trenchii are directly impacted by seawater surrounding coral colonies presenting with active SCTLD lesions. Furthermore, the responses of these cultures are similar even when the source water is filtered to 0.8 μm (but generally not when filtered to 0.2 μm), which is consistent with the pathogen being a very small bacterium or a giant virus. Algal symbiont cultures thus may be useful laboratory models to further identify the pathogen through targeted studies, removing the complications of using corals with varied environmental histories, holobiome compositions, and (often unknown) SCTLD exposure histories, while also limited the extraction or use of valuable coral resources for routine SCTLD experiments.