Green fluorescence from cnidarian hosts attracts symbiotic algae.

Abstract

Reef-building corals thrive in nutrient-poor marine environments because of an obligate symbiosis with photosynthetic dinoflagellates of the genus <i>Symbiodinium</i> Symbiosis is established in most corals through the uptake of <i>Symbiodinium</i> from the environment. Corals are sessile for most of their life history, whereas free-living <i>Symbiodinium</i> are motile; hence, a mechanism to attract <i>Symbiodinium</i> would greatly increase the probability of encounter between host and symbiont. Here, we examined whether corals can attract free-living motile <i>Symbiodinium</i> by their green fluorescence, emitted by the excitation of endogenous GFP by purple-blue light. We found that <i>Symbiodinium</i> have positive and negative phototaxis toward weak green and strong purple-blue light, respectively. Under light conditions that cause corals to emit green fluorescence, (e.g., strong blue light), <i>Symbiodinium</i> were attracted toward live coral fragments. <i>Symbiodinium</i> were also attracted toward an artificial green fluorescence dye with similar excitation and emission spectra to coral-GFP. In the field, more <i>Symbiodinium</i> were found in traps painted with a green fluorescence dye than in controls. Our results revealed a biological signaling mechanism between the coral host and its potential symbionts.