Influence of the Quantity and Quality of Light on Photosynthetic Periodicity in Coral Endosymbiotic Algae

Abstract

Symbiotic corals, which are benthic organisms intimately linked with their environment, have evolved many ways to deal with fluctuations in the local marine environment. One possible coping mechanism is the endogenous circadian clock, which is characterized as free running, maintaining a ∼24 h periodicity of circuits under constant stimuli or in the absence of external cues. The quantity and quality of light were found to be the most influential factors governing the endogenous clock for plants and algae. Unicellular dinoflagellate algae are among the best examples of organisms that exhibit circadian clocks using light as the dominant signal. This study is the first to examine the effects of light intensity and quality on the rhythmicity of photosynthesis in the symbiotic dinoflagellate Symbiodinium sp., both as a free-living organism and in symbiosis with the coral Stylophora pistillata. Oxygen production measurements in Symbiodinium cultures exhibited rhythmicity with a periodicity of approximately 24 h under constant high light (LL), whereas under medium and low light, the cycle time increased. Exposing Symbiodinium cultures and corals to spectral light revealed different effects of blue and red light on the photosynthetic rhythm, specifically shortening or increasing the cycle time respectively. These findings suggest that the photosynthetic rhythm is entrained by different light cues, which are wired to an endogenous circadian clock. Furthermore, we provide evidence that mRNA expression was higher under blue light for two potential cryptochrome genes and higher under red light for a phytochrome gene isolated from Symbiodinium. These results offer the first evidence of the impact of the intensity and quality of light on the photosynthetic rhythm in algal cells living freely or as part of a symbiotic association. Our results indicate the presence of a circadian oscillator in Symbiodinium governing the photosynthetic apparatus through a light-induced signaling pathway that has yet to be described.

Photosynthetic circadian rhythmicity patterns of Symbiodium , the coral endosymbiotic algae

“…Thus, the rhythmicity was maintaining daily oscillations with a free-run cycle when conditions switched from LD cycles to LL conditions. The rhythms of photosynthesis in Symbiodinium show circadian pattern and fulfil the three characteristics thereof: first, the rhythm is kept under constant conditions of light; second, light can entrain the rhythm, whereas different light spectrums change the entrainment of the cycle [40]; and third, our former work shows the ability of Symbiodinium in culture as well as inside the coral tissue to compensate for temperature changes and maintain the rhythm under LD and LL [41]. The LL free-run period presented here differs from the LD period mainly in the magnitude of the oxygen evolution.…”

Unfolding the secrets of coral–algal symbiosis

“…The actin gene is highly expressed and conserved in dinoflagellates (Kim et al, 2011). This gene and other HKGs have already been successfully applied as a reference, together with other HKGs, in several recent gene expression studies of different Symbiodinium clades, because of their stable expression during stress exposure (Rosic et al, 2011b;McGinley et al, 2012;Sorek and Levy, 2012;Ogawa et al, 2013). Furthermore, photosynthesis-related transcripts were recognized as shared among the clades and included genes such as Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) and peridinin-chlorophyll a-binding protein that are unique to dinoflagellates (Rowan et al, 1996;Leggat et al, 2011).…”

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

“…In hospite reduction of blue light is a function of host filtering and shifting of spectra via fluorescent proteins (Leutenegger et al, 2007;Quick et al, 2018;Salih et al, 2000;Smith et al, 2013), resulting in an order of magnitude reduction in the amount of blue light transmitted to deeper tissue layers in corals (Lichtenberg, Larkum, & Kühl, 2016). Sorek and Levy (2012) showed that blue light spectra increased mRNA expression of free-living Symiodiniaceae cryptochromes CRY1 and CRY2, but were downregulated when hosted in the coral Stylophora pistillata. Though an E. pallida host GFP has not yet been identified, they exhibit patterns of green autofluorescence when excited by blue light , which could reduce the amount of blue light transmitted to endosymbionts.…”