Thermal stress triggers productive viral infection of a key coral reef symbiont

Abstract

Climate change-driven ocean warming is increasing the frequency and severity of bleaching events, in which corals appear whitened after losing their dinoflagellate endosymbionts (family Symbiodiniaceae). Viral infections of Symbiodiniaceae may contribute to some bleaching signs, but little empirical evidence exists to support this hypothesis. We present the first temporal analysis of a viral lineage—the Symbiodiniaceae-infecting ‘dinoRNAVs’—in coral colonies exposed to a 5-day heat treatment. Throughout the experiment, all colonies were dominated by Symbiodiniaceae in the genus Cladocopium, but 124 dinoRNAV major capsid protein ‘aminotypes’ (unique amino acid sequences) were detected across coral genets and treatments. Seventeen dinoRNAV aminotypes were found only in heat-treated fragments, and 22 aminotypes were detected at higher relative abundances in heat-treated fragments. DinoRNAVs also exhibited higher alpha diversity and dispersion under heat stress. Together, these findings provide the first empirical evidence that exposure to high temperatures triggers some dinoRNAVs to switch from a persistent to a productive infection mode within heat-stressed corals. Over extended time frames, we hypothesize that cumulative dinoRNAV lysis of Symbiodiniaceae cells during productive infections could decrease Symbiodiniaceae densities within corals, observable as bleaching signs. This study sets the stage for reef-scale investigations of dinoRNAV dynamics during bleaching events.