Figure 1

Autotrophic uptake of carbon and nitrogen. Correlated TEM and NanoSIMS images of the distribution of ¹³C and ¹⁵N in Symbiodinium and surrounding gastrodermal cell cytoplasm, resulting from simultaneous photosynthetically driven assimilation of [¹³C]bicarbonate (B,E) and [¹⁵N]nitrate (C,F) after a 6 h isotopic pulse in the light. Fixed ¹³C and ¹⁵N were detected throughout the symbiont cell, with carbon mainly concentrated in the primary starch sheet (<) around the dinoflagellate pyrenoid (PY) near thylakoid membranes (TH), in secondary starch granules (ST), symbiont and host lipid bodies (LB), and in extra-algal lipid bodies (LB*). Symbiont nitrogen labelling was lowest in carbon-rich structures and the accumulation body (ACC), and highest in small crystalline-like hotspots. Translocated ¹³C to the host tissue was mainly concentrated in lipid bodies, whereas nuclei (N) and nucleoli (*) in the host cell showed high ¹⁵N labelling. Colours in NanoSIMS maps represent enrichment relative to an unlabelled sample, expressed as atom percent excess (APE). White arrows (A,C,D,F) point to symbiosome membrane. Scale bars are 2 µm.