Photophysiological performance of zooxanthellate endosymbionts (Symbiodiniaceae) in the golden jellyfish Mastigias papua across a natural environmental gradient in marine lakes of Palau (Micronesia)

Introduction Understanding how organisms adapt to complex and variable environments requires in situ analysis of ecological performance in different environments. Methods In this multi-year study, we investigate the photophysiological performance of dinoflagellate endosymbionts (Symbiodiniaceae) of the golden jellyfish Mastigias papua across a natural environmental gradient of marine lakes and semienclosed coves in the Palau Archipelago (Western Caroline Islands, Pacific Ocean). Results These sites vary in terms of age, distance from the nearby ocean (lagoon) and underwater light conditions, all of which influence the ecology of the host and that of its symbionts. Jellyfish endosymbionts from isolated lakes ‐ remote from the ocean and characterized by more turbid conditions – were found to exhibit significantly lower photosynthetic performance (quantum yield and PSII performance index) than endosymbionts from sites more connected to the nearby ocean and with clearer waters. The pigment composition of endosymbionts varied little between those collected from inland lakes and ocean-connected coves, and showed little sign of adaptation to light spectra dominated by reddish wavelengths. Discussion This suggests a mismatch between the optical characteristics of waters in more turbid lake environments and those of pigments that appear to remain optimized for blue light use in marine dinoflagellates. While published studies show evidence of morphological and behavioral adaptations in Mastigias jellyfish living in Palau’s marine lakes, this study suggests that their symbionts are limited by reduced pigment plasticity and lower photosynthetic performance. These results highlight the importance of symbiont characteristics and physiological compatibility in driving host adaptation, and the potential for evolutionary mismatch in tightly integrated symbioses, under novel environmental conditions.