Final report on electron microscopy of Florida corals affected with stony coral tissue loss disease (SCTLD).

Based on microscopy, hypotheses as to possible causes of SCTLD include infectious agents not visible at the light microscopy level or some sort of toxicosis, perhaps originating from Zooxanthellae. To clarify this, we examined corals affected with SCTLD using electron microscopy which permits visualization of details not visible on light microscopy. Here we show that Zooxanthellae in SCTLD-affected corals consistently manifest varying degrees of pathology associated with elongated particles compatible in morphology with filamentous single stranded RNA viruses of plants (anisometric viral like particles- AVLP). Our findings showed what appeared to be a progression from early to late replication of AVLP in chloroplasts with little to no coral host cell pathology other than cell lysis. In addition to AVLP, Zooxanthellae showed pathology in chloroplasts consistent with that seen in plants infected by viruses. Based on these findings, we hypothesize that SCTLD is a viral disease of Zooxanthellae leading to coral host death.