Green fluorescence from cnidarian hosts attracts symbiotic algae

Abstract

SignificanceReef-building corals cannot survive without symbiotic algae, Symbiodinium, on which they depend for most of their energy. Most coral species gain symbionts from the environment early in life, and possibly after bleaching (i.e., the loss of symbionts in response to stress). However, Symbiodinium density on coral reefs is very low. Although it has long been hypothesized that corals must be able to attract free-living Symbiodinium, such a mechanism has yet to be identified. Here, we use a series of experiments to demonstrate that corals attract Symbiodinium using their endogenous GFP-related green fluorescence, revealing a biological signaling mechanism that underlies the success of this symbioses that is the building block of coral reef ecosystems.

Free‐living and symbiotic lifestyles of a thermotolerant coral endosymbiont display profoundly distinct transcriptomes under both stable and heat stress conditions

“…Blue light photoreceptors have been implicated as a critical regulator of photoprotection in Chlamydomonas reinhardtii , where they control a negative feedback mechanism on photosynthesis under high irradiance via PSII nonphotochemical quenching (Petroutsos et al, ). In Symbiodiniaceae, negative phototaxis in response to blue light was recently demonstrated (Aihara et al, ) and has been previously identified in other eye spot‐containing dinoflagellates (Horiguchi, Kawai, Kubota, Takahashi, & Watanabe, ; Ozasa, Won, Song, Shinomura, & Maeda, ), suggesting avoidance of blue light spectra may be an adaptive behaviour in dinoflagellates. We suggest that host attenuation of blue light represses the transcriptional activation of dinoflagellate blue light photoreceptors in hospite.…”

“…This may be indicative of differences between the free‐living and in hospite environments. Whereas in hospite cells persist within a symbiosome inside of a host, free‐living cells must navigate in space and respond to a diverse array of changing environmental cues such as light (Aihara et al, ; Hollingsworth, Kinzie, Lewis, Krupp, & Leong, ) and chemicals (Fitt, ; Pasternak, Bachr, Abelson, & Achituv, ). On the other hand, the niche occupied by dinoflagellate endosymbionts in coral‐algal symbioses imposes a very particular set of functional requirements on the endosymbiont, which probably requires a reduced signalling repertoire.…”

“…Motility is characteristic of the free‐living state, with early descriptions of Symbiodiniaceae in culture exhibiting highest motility approximately midphotoperiod, from 4–7 hr following cytokinesis, depending upon symbiont type (Fitt, Chang, & Trench, ; Fitt & Trench, ). The function of motility appears to include host‐seeking behaviour, with Symbiodiniaceae attracted to host fluorescence (Aihara et al, ). In hospite symbionts, however, are in an immotile coccoid phase.…”

Physiological Characteristics of Stylophora pistillata Larvae Across a Depth Gradient

“…The ability of individual juveniles to gain and maintain novel symbionts through horizontal acquisition might provide a crucial mechanism to increase the chances of survival in a new environment during dispersal at the larval stage (Byler et al, 2013) and, at the same time, the inheritance of symbionts from the parents would assure the supply of photosynthetic products during the delicate initial larval stages. Previous studies have shown that different clades of Symbiodinium have differential preferences for the light intensity required for photosynthesis and that they are differently affected by the host internal light environment variation, which can be achieved through the modulation of the GFPs fluorescence intensity (Ezzat et al, 2017;Quigley et al, 2018;Aihara et al, 2019). Since the light intensity necessary to attract specific symbionts likely differs with light spectra and depth (Aihara et al, 2019), coral larvae from shallow and deep reefs potentially generate diverse internal light environments, displaying different GFP fluorescence depending on the preferred association with different Symbiodinium clades (Yuyama and Higuchi, 2014).…”

Developmental changes in the intensity and distribution pattern of green fluorescence in coral larvae and juveniles

“…However, the present study suggests that GFP expression in P. damicornis juveniles increased in re sponse to bleaching stress. Recently, Aihara et al (2019) demonstrated that algal symbionts were attracted to green fluorescence emitted by both endogenous GFP of live coral fragments and an artificial green fluorescence dye.…”