Gorgonian disease outbreak in the Gulf of Naples: pathology reveals cyanobacterial infection linked to elevated sea temperatures.

In recent years, mass mortality events of benthic invertebrates in the Mediterranean Sea have been documented to coincide with the increased seawater temperatures associated with global climate change. Following a disease outbreak in gorgonians during the summer seasons of 2008 and 2009 in the Gulf of Naples (Tyrrhenian Sea), we conducted gross and microscopic analyses of healthy and diseased specimens of Eunicella cavolinii and E. singularis using both light and electron microscopy (SEM). Macroscopically, diseased colonies exhibited evident tissue thinning, and dead colonies showed a complete loss of polyps and coenenchyme, exposing their skeletons to settlement by fouling organisms. Histopathology revealed chronic inflammatory lesions at the polyp and axial level, characterized by amoebocyte infiltration of tissue accompanied by new apposition of melanin/gorgonin sheets. We interpreted this response as a defense against different kinds of pathogens-identified as mainly a heterogeneous consortium of filamentous cyanobacteria-and which gradually led to enlargement and hardening of the coral axis, which resembled a wood-like structure at the final stage of the disease. These processes elicited the formation of multiple inflammatory nodules and capsules, some of which were macroscopically visible. A parallel 16S rRNA and ITS analysis of the diseased tissue identified Synechococcus, Arthrospira and other uncultured cyanobacteria grouped within the Oscillatoriales. These results suggest that a cyanobacterial consortium is involved in the pathogenesis of the inflammatory disease leading to the mortality of Gorgoniaceae in the area. Finally, there were anomalously high temperatures (up to 25 degrees C) between 10 and 20 m depth during the sampling period, particularly in June 2009. This supports the hypothesis that the coral skeleton may serve as a reservoir for the pathogens in cooler seasons, with warmer conditions leading to pathogen reactivation and recurring mortality events.