Diel rhythmicity of lipid-body formation in coral – Symbiodinium endosymbiosis

The biogenesis of intracellular lipid bodies (LBs) is dependent upon the symbiotic status between host corals and their intracellular dinoflagellates (genus Symbiodinium ), though aside from this observation, little is known about LB behavior and function in this globally important endosymbiosis. The present research aimed to understand how LB formation and density are regulated in the gastrodermal tissue layer of the reef-building coral Euphyllia glabrescens . After tissue fixation and labeling with osmium tetroxide, LB distribution and density were quantified by imaging analysis of serial cryo-sections, and a diel rhythmicity was observed; the onset of solar irradiation at sunrise initiated an increase in LB density and size, which peaked at sunset. Both LB density and size then decreased to basal levels at night. On a seasonal timescale, LB density was found to be significantly positively correlated with seasonal irradiation, with highest densities found in the summer and lowest in the fall. In terms of LB lipid composition, only the concentration of wax esters, and not triglycerides or sterols, exhibited diel variability. This suggests that the metabolism and accumulation of lipids in LBs is at least partially light dependent. Ultrastructural examinations revealed that the LB wax ester concentration correlated with the number of electron-transparent inclusion bodies. Finally, there was a directional redistribution of the LB population across the gastroderm over the diel cycle. Collectively, these data reveal that coral gastrodermal LBs vary in composition and intracellular location over diel cycles, features which may shed light on their function within this coral–dinoflagellate mutualism.

Introduction

Mutualistic associations between dinoflagellates (Symbiodinium sp.) and cnidarians (e.g., corals and sea anemones) are ecologically important and are responsible for the construction of coral reefs. As such, they have been the subject of intensive investigation during the past decades at the ecological scale (LaJeunesse et al. 2010). However,
microscale investigations of the symbiosis at the cell and molecular levels have been slower to arrive, due in part to the challenges inherent in studying intimate endosymbioses (Weis et al. 2008; Weis and Allemand 2009). Given the looming threats of global climate change on coral reef ecosystems, there is an urgent need to better understand
the basic biology of these fundamentally important endosymbioses…