Shifts in sponge-microbe mutualisms across an experimental irradiance gradient

To investigate how the interactions between the closely related sponge species Aplysina cauliformis and Aplysina fulva and their symbiotic microbial communities vary under changing environmental conditions, we conducted a manipulative shading experiment with treatments spanning a gradient of 6 irradiances. In A. cauliformis, there was a tight coupling of symbiont and host metabolism across treatments, and changes in growth rate were correlated more with shifts in symbiont δ¹³C and δ¹⁵N values than with shade treatment. In contrast, symbiont and host C metabolism were weakly coupled in A. fulva, and the growth of this species was not correlated with shifts in symbiont δ¹³C and δ¹⁵N values. In addition, although photosymbiont metabolism was an important driver of shifts in holobiont C and N metabolism of both host species, host and photosymbiont C metabolism were only correlated in A. cauliformis. Thus, although both species host stable, abundant, and similar photosymbiont communities, each host forms a unique mutualism with its symbionts. These 2 host species may be on different evolutionary trajectories, potentially allowing each to exploit novel niche space in coral reef ecosystems. This study provides data in support of the hypothesis that these symbioses represent a dynamic balance of costs and benefits and provides evidence that, because these costs and benefits are highly species-specific, not all species will respond similarly to environmental fluctuations.