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 genusSymbiodinium. Symbiosis is established in most corals through the uptake ofSymbiodiniumfrom the environment. Corals are sessile for most of their life history, whereas free-livingSymbiodiniumare motile; hence, a mechanism to attractSymbiodiniumwould greatly increase the probability of encounter between host and symbiont. Here, we examined whether corals can attract free-living motileSymbiodiniumby their green fluorescence, emitted by the excitation of endogenous GFP by purple-blue light. We found thatSymbiodiniumhave 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),Symbiodiniumwere attracted toward live coral fragments.Symbiodiniumwere also attracted toward an artificial green fluorescence dye with similar excitation and emission spectra to coral-GFP. In the field, moreSymbiodiniumwere 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.