Subcellular assessment of algal carbon storage and chloroplasts across the microenvironmental landscape of a photosymbiotic coral

Light availability plays a central role in shaping the photophysiology and energy metabolism of photosymbiotic organisms such as reef-building corals. Although light varies greatly within coral colonies, the effects of this spatial heterogeneity on the subcellular organization and energy storage of symbiotic algae (Symbiodiniaceae) remain poorly understood. Here, we combined microscale measurements of light and oxygen across both light-exposed upper regions and shaded basal regions of a Favites abdita colony with three-dimensional subcellular imaging using Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). Our multi-scale approach revealed subcellular heterogeneity among symbiont populations, suggesting different cell cycle stages and physiological states across a spatial stratification in the coral. Subcellular morphometrics revealed that symbiont cells at the top of the colony were twice more voluminous than those at the shaded base with similar plastid volume occupancy. Compared to symbionts at the top of the colony, symbionts in the basal region accumulated nearly three times more starch relative to their cell volume. These findings show that light gradients within coral colonies shape symbiont morphology and energy storage patterns, with important implications for coral stress tolerance and resilience.