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

Keywords: GFP; coral; fluorescence; phototaxis; symbiosis.

Fig. 1.

Phototaxis in Symbiodinium. (A) An illustration of the experimental set-up used for evaluating the phototaxis index used in this study. The Symbiodinium OTcH-1 culture in the container was irradiated with light (open arrows) for 10 min, followed by equal partitioning of the container (dashed square) into moieties proximal and distal to the light. The cell densities of the proximal (P) and distal (D) moieties were then used to calculate a phototaxis index, using the equation described in the text. (B) The effect of time of day on phototaxis of Symbiodinium OTcH-1 toward a green LED. The Symbiodinium cells were harvested after the onset of light exposure, from 2 to 12 h, and were placed in a plastic container for monitoring phototaxis (n = 3 biological samples). (C) Contour map of the phototaxis action spectrum of Symbiodinium OTcH-1. The phototaxis index was determined from measurements taken under 18 different monochromatic light spectra at eight different light intensities (n = 3 biological samples).

Fig. 2.

Attraction of Symbiodinium cells to a green fluorescent coral. (A) Top surface of the plate-like coral, E. aspera, used in this study. Photograph was taken under natural light conditions. (Scale bar, 1 cm.) (B) Fluorescence spectrum (blue, excitation; green, emission) of the coral body. (C) Schematic illustration of the fluorescence attraction assay. (D) Representative pictures of the Symbiodinium accumulation around the green fluorescent coral fragment (square 8 mm on a side; Lower), but not around the coral skeleton control (Upper) during the exposure to blue light (20 µmol photons · m⁻²⋅s⁻¹) for 10 min. (E) Attraction of Symbiodinium cells around the coral fragments on exposure to different colors of light (20 µmol photons · m⁻²⋅s⁻¹) for 10 min. The values are relative to cell density of the culture before irradiation (n = 3 biological samples; bars, ±SE).

Fig. 3.

Attraction of Symbiodinium cells to an artificial green fluorescent object. (A) The fluorescence spectrum (blue, excitation; green, emission) of the GFD used. (B) Symbiodinium accumulation around the GFD-painted disk (8 mm in diameter; Lower), but not around the control disk (Upper), during the exposure to blue light (20 µmol photons m⁻²⋅s⁻¹) for 10 min. (C) Attraction of Symbiodinium cells to the GFD-painted disk on exposure to different colors of light for 10 min. The values are relative to cell density of the culture before irradiation. *P < 0.05; **P < 0.01, Student’s t test. (D) Effect of the intensity of green fluorescence of the coral and GFD on attraction of Symbiodinium cells on the exposure to blue-light (20 µmol photons m⁻²⋅s⁻¹) for 10 min. The arrow shows the GFD intensity used in experiments for Symbiodinium attraction (A–C). The value for the coral was the same as in Fig. 2E. (C and D) n = 5 biological samples for GFD-painted disk; n = 3 biological samples for the coral. Bars, ±SE.

Fig. 4.

Attraction of Symbiodinium cells to green fluorescent traps in coral reefs. (A) Seawater traps with and without green fluorescent dye. (B) Box-and-whisker plot for the density of Symbiodinium in the green fluorescent trap relative to that in the non-GDF painted (white) trap. The box, line, and square represent the quartiles, median, and average, respectively. Data were taken at nine sampling sites.