Filamentous virus-like particles are present in coral dinoflagellates across genera and ocean basins

Abstract

Filamentous viruses are hypothesized to play a role in stony coral tissue loss disease (SCTLD) through infection of the endosymbiotic dinoflagellates (Family Symbiodiniaceae) of corals. To evaluate this hypothesis, it is critical to understand the global distribution of filamentous virus infections across the genetic diversity of Symbiodiniaceae hosts. We present transmission electron microscopy images demonstrating filamentous virus-like particles (VLPs) are present in 61 and 70% of Symbiodiniaceae cells (genus Cladocopium) within Pacific corals (Acropora hyacinthus, Porites c.f. lobata, respectively); these VLPs are more prevalent in Symbiodiniaceae of in situ colonies experiencing heat stress. Symbiodiniaceae expelled from A. hyacinthus also contain filamentous VLPs, and these cells are more degraded than their in hospite counterparts. Similar to the VLPs reported from SCTLD-affected Caribbean reefs, VLPs range from ~ 150–1500 nm in length and 15–37 nm in diameter and appear to constitute various stages in a replication cycle. Finally, we demonstrate that SCTLD-affected corals containing filamentous VLPs are dominated by diverse Symbiodiniaceae lineages from the genera Breviolum, Cladocopium, and Durusdinium. Although this study cannot definitively confirm or refute the role of filamentous VLPs in SCTLD, it demonstrates that filamentous VLPs are not solely observed in SCTLD-affected corals or reef regions, nor are they solely associated with corals dominated by members of a particular Symbiodiniaceae genus. We hypothesize that filamentous viruses are a widespread, common group that infects Symbiodiniaceae. Genomic characterization of these viruses and empirical tests of the impacts of filamentous virus infection on Symbiodiniaceae and coral colonies should be prioritized.

Novel metagenomics analysis of stony coral tissue loss disease

“…The stony coral (order Scleractinia) microbiome is a complex system of interactions between the host, bacteria, viruses, fungi, archaea, and algal symbionts (Bourne et al 2009); thus a disturbance in any number of these symbiotic relationships could be involved in SCTLD progression. Multiple studies have explored viruses that may infect stony coral symbionts, notably Symbiodiniaceae, but no causative relationships have been detected (Work et al 2021;Veglia et al 2022;Beavers et al 2023;Howe-Kerr et al 2023). Bacterial species are particularly under scrutiny for their potential involvement in SCTLD, due to the effectiveness of antibiotics in halting lesion progression in multiple affected coral species (Aeby et al 2019, Neely et al 2020Shilling et al 2021;Studivan et al 2023).…”