Symbiont carbon and nitrogen assimilation in the Cassiopea–Symbiodinium mutualism

Symbiotic interactions in the marine environment have long been represented by mutualisms between photosymbionts and benthic marine invertebrates like corals and sponges. Although ‘upside-down’ epibenthic jellyfish in the genus Cassiopea also derive a substantial metabolic benefit from abundant communities of the dinoflagellate symbiont Symbiodinium, comparatively little is known about the efficiency of carbon (C) and nitrogen (N) assimilation within the Cassiopea holobiont. Using standardized 6 h incubations with 13Cand 15Nenriched compounds, we assessed symbiont C and N assimilation in both oral arm and bell tissue of C. xamachana under light and dark conditions. Carbon fixation was light dependent and highest in the photosymbiont-rich oral arm tissue. In contrast, NO3 assimilation was light independent in both tissue types and was highest in bell tissue that was sparsely colonized by photosymbionts. This, coupled with higher bell tissue 15N enrichment under dark conditions, implicates nonphotosynthetic microbes in Cassiopea N metabolism. This zonation of microbial activity may allow C. xamachana to simultaneously fix C and assimilate ambient or porewater N released during Cassiopea pumping activity. Although C. xamachana may utilize symbiont-derived N, lower 15N enrichment relative to C fixation suggests that Cassiopea may also rely on exogenous sources of N for growth. This study provides initial evidence that the efficiency of symbiont metabolism within Cassiopea jellyfish is comparable to, or exceeds, that of other common benthic marine invertebrates, sup porting the contention that Cassiopea have an important role in the productivity and nutrient dynamics within their local environment.