Changes in the pathogenic microbiome of Pisaster ochraceus sea stars suffering from sea star wasting disease

Sea star wasting disease (SSWD) affects asteroids across an unprecedented spatial, temporal, and taxonomic range. The symptoms begin with limb twisting and turgor loss; next, their tissues begin to decay, leading to fragmentation and death. Our population surveys of the sea star, Pisaster ochraceus, in Birch Bay, WA indicate that SSWD still affects P. ochraceus stars in the wild and increases in severity from early to late summer. These animals are keystone species; therefore, SSWD could lead to severe ecological changes if populations continue to be diminished by reduced fitness and mortalities. Previous studies suggest that SSWD is likely caused by a microbial agent, but so far, a pathogen has not been conclusively identified. We first sought to characterize potential bacterial pathogens. We isolated bacteria with tissue-degrading potential from diseased and healthy P. ochraceus at two different times from the same population. These bacterial isolates were identified via molecular means. Our results indicated that although the group Gammaproteobacteria dominated our isolates from both healthy and sick animals, the community structure differed at the genus level. While the different bacterial community structures of sick sea stars might in part represent an influx of saprophytic bacteria, the community structure also differed between unaffected tissues of diseased stars and healthy stars. Living, healthy tissues and unaffected tissues should not attract bacteria whose lifestyle is mainly saprophytic. Rather, the microbiome changes are likely selected for at the level of the entire animal rather than just within lesions (decaying tissues). Altogether, these results suggest that the community structure of sea star associated bacteria changes when animals develop SSWD