Intracellular vesicle-mediated biomineralization of arsenic and barium by a sponge symbiotic bacterium

In their soluble forms, arsenic and barium are ubiquitous toxic elements. Mechanisms for their detoxification include reducing bioavailability by assimilation into organic forms or mineralization. It was previously found that Entotheonella sp., a bacterium common to the Red Sea sponge Theonella swinhoei (Demospongiae, Tetractinellida), accumulates these elements by mineralizing them intracellularly, thus acting as a detoxifying organ to the sponge host. Here, we utilize cryo-TEM and energy-dispersive spectroscopy to investigate the accumulated minerals. Our results show that Entotheonella cells possess an internal membrane-enclosing sphere-like granules that contains barium, arsenic, sulfur, calcium, and phosphorus in high concentrations. Moreover, the bacterial cytoplasm contains many intracellular vesicles (ICVs) enriched with arsenic and sulfur. The coexistence of sulfur and arsenic may suggest the presence of cysteine-containing metal-binding proteins responsible for arsenic uptake and separation within the bacterial cell. To examine that hypothesis, we developed a protocol for vesicle isolation and performed proteomic profiling. Based on the proteins found, ICVs likely originate from the bacteria's outer membrane and contain proteins of known functions, including the transport and detoxification of toxic metals. These findings enhance our understanding of Entotheonella sp. and its host Tamiops swinhoei's unique strategies for hyper-accumulating and neutralizing toxic elements.