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(The Gist of Science Reporter) Tripartite Symbiosis

[NOVEMBER-2019]

Tripartite Symbiosis

Tripartite association involving three different organisms is common symbiosis, an in plants but rare in animals. However, researchers at Princeton and the University of Maryland Center for Environmental sciences studied a marine mollusc Elysia rufescens, commonly known as sea-slug which feeds on marine algae of the genus Bryopsis.
The slug gets not only the food but also a defensive chemical found in the algae, which is a product of the bacteria named Candidatus endobryopsis kahalalidifaciens that produces about fifteen or so different toxins known as Kahalalides. These chemicals are known to act as a deterrent to surrounding fish and other marine animals. One of the toxins, kahalalide has been evaluated as a potential cancer drug due to its potent toxicity.
The bacterial genus Candidatus does not possess the genes required for survival outside the algal species Bryopsis. Their genetic capacity is such that they can pump out toxic molecules that stop predators from eating the algae. The only one predator that can eat the toxin is the slug Elysia rufescens that stores the toxin in a more concentrated form than found in algae.
The scientists compared the bacteria to a factory as they consume raw materials in the form of amino acids from the algae and release a finished product in the form of toxic chemicals. The bacterial symbionts make defensive molecules for the host in exchange for a protected living space.
None of the amino acid substrates that make up the kahalalides can be produced by Candidatus itself; therefore these substances are mostly provided by the autotrophic Bryopsis species, highlighting an unusual strategy of collaborative biosynthesis between a symbiotic bacterium and its host.