The success of coral reefs largely hinges on the symbiosis between coral animal, dinoflagellate algae of the family Symbiodiniaceae and bacteria. While the diversity of these bacteria in the coral holobiont has been extensively studied, little is known about their spatial distribution.
This study is the first to isolate the symbiosome, the compartment within coral cells that houses the Symbiodiniaceae alga, from stony corals and subsequently analyse the bacterial microbiome of this boundary between symbiotic partners.
Cells from the stony coral Acropora loripes were labelled with the fluorescent universal membrane dye FM1-43. Cytometric analysis showed two distinct populations of Symbiodiniaceae cells based on dye intensity. Both were divided into different fractions via fluorescence-assisted cell sorting (FACS).
Microscopy images indicated the presence of a membrane complex enveloping Symbiodiniacaea from the fraction with higher dye intensity, confirming successful isolation of the stony coral symbiosome. Analysis of 16S rRNA gene metabarcoding data showed that bacterial microbiomes differ in diversity and composition between coral tissue cells, symbiosome membrane, and algal surface.
The symbiosome has previously only been isolated in anemones and soft corals. The present study is a major step in understanding the spatial structure of the coral-dinoflagellate-bacterial symbiosis in reef-building stony corals.