Priority Effects Can Shape Strain-Level Community Assembly and Function in a Bioluminescent Symbiosis

Abstract

Symbiotic microbial communities often appear highly variable in their composition and function in ways that environmental factors alone cannot explain. One potential reason for this variability is priority effects, where historical contingency in arrival order affects how symbionts assemble into communities. Focusing on the luminous bacterium (Photobacterium mandapamensis) in the light organ of the sea urchin cardinalfish (Siphamia tubifer), we studied how priority effects might influence bacterial symbiont assembly. In in vitro experiments that used three P. mandapamensis strains isolated from the same individual fish, we found that whichever strain arrived first dominated over the other two strains, indicating strong priority effects. We also found that the strains differed in growth and luminosity, and that the bioluminescence of the three-strain community could not be predicted from individual strain performances. These findings suggest that priority effects can be a major process shaping the composition and function of these symbiotic microbial communities.