“What we found was that in body-wall samples from the three sea pig species, one bacterium accounted for more than 70% of bacterial 16S rRNA gene sequences,” Yu Yoshida said.
Researchers at the University of Tokyo uncovered this unusual bacterium beneath the skin of translucent sea pigs as the creatures trundle across the deep ocean floor. Using fluorescent imaging, the team located the organism and named it Candidatus Abyssoplasma scotoplanesicola.
DNA analysis revealed a stripped-down genome of just 649,000 bases inside the newly identified microbe. More than a quarter of this genetic material is dedicated to building surface-associated protein groups.
“The complete genome was very small, only 649,000 bases,” Yoshida said. “By comparison, the E. coli genome is approximately seven times larger.”
The research team also discovered giant proteins exceeding 5,000 amino acids in length within the bacterial genome. For perspective, bacterial proteins average about 300 amino acids in length.
Sea pigs are plump, soft-bodied sea cucumbers that reach around 15 centimeters in length. They walk along the ocean floor on enlarged, leglike tube feet, sometimes gathering in large groups to feed on organic-rich seafloor sediments, including those around whale falls.
Beyond recycling organic matter and nutrients, their squidgy bodies provide shelter from predators to young king crabs. Subcuticular bacteria grow in the layers which encase the body of these marine invertebrates.
“Taxonomic and genomic information about these body-wall inhabiting bacteria is scarce, and very few studies of deep-sea species have linked this information to the bacteria’s location within host tissues,” Yoshida said.
The study provides clues as to which functions remain in symbiotic bacteria living closely with their host. Researchers said that the bacterium is thought to rely heavily on host tissues to obtain amino acids, nucleic acid precursors, lipids, and mineral nutrients.
“We would really like to understand how this bacterium could become so dominant in the body wall, the functions of the giant surface proteins, what competition it faces from other bacteria, and of course the effects on the host,” Yoshida said.
