Opensheart, We really are in uncharted waters here as the changes in ocean chemistry are happening at a pace that exceeds all precedent in evolutionary history on this planet ( last 500 million years ).
The starfish wasting disease is apparently caused by a parvovirus that has been documented in invertebrates for 70 years so it's current virulence is a bit of a mystery still. Changes in ocean chemistry ( acidification ),changes in composition of viral communities, or potentially increased virulence due to newly mutated forms are all potential players. Methanotrophs in my opinion would be not so likely a contributor, at least directly, but they do contribute to acidification as they convert methane to Co2 and so contribute to acidification.
There has been work done at Tatoosh Island ,Washington by Cathy Pfister and her husband Tim Wooten that documents pH changes happening there that have exceeded expectations for most of the drivers of acidification we currently understand. Along with those pH changes the shells of mussels currently collected show a marked Carbon 12 excursion from decades old collections as well as shells found in Indian Shell Middens of centuries past. I have been wondering if large methane venting and associated pH shifts might also play some role in the Carbon excursion? Time will tell I suppose but if the reception the Wooten/Pfister pair received when they published is any indication of the very hard push back other researches receive on this subject I would expect few takers or funding to pursue this issue. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0025766
Also on the starfish wasting , please read commentshttp://www.virology.ws/2014/11/17/a-virus-that-melts-sea-stars/