In hospite and ex hospite architecture of photosynthetic thylakoid membranes in Symbiodinium spp. using small-angle neutron scattering.

We demonstrate that small-angle neutron scattering (SANS) can resolve the architecture of photosynthetic thylakoid membranes in live symbiotic algal cells, both extracted from and living inside their respective hosts (ex hospite and in hospite, respectively). This enables a new non-destructive approach to probing thylakoid organization in coral symbioses, relevant to understanding the mechanisms of coral bleaching. A biologically realistic triple-vesicle model, guided by electron microscopy and established biochemical constraints, was fitted to SANS data from live Symbiodinium associated with both the coral analogue Aiptasia and the reef-building coral Acropora. The resulting compartment scattering length densities, together with established biochemical constraints, define a limited compositional range that supports the plausibility of the structural solution. These fits capture key scattering features and yield dimensional parameters, including inter-thylakoid (IT) gap widths, with uncertainties small enough to test models of stress-related membrane rearrangement. A focused covariance analysis shows that this SANS framework can resolve an IT-gap expansion of ∼2.4 nm with >7σ sensitivity, sufficient to distinguish structural changes proposed in thylakoid stress-response models. This provides a robust baseline for future live-cell studies.