One of nature’s strangest symbioses reveals its secrets

UNOT LIKE THEIR Terrestrial cousins, sea slugs are widely considered to be some of the most beautiful animals on the planet. In some cases, some of this beauty comes from subcellular structures called chloroplasts, which they extract intact from the algae they eat, then sequester in the intestinal diverticula for their own purposes.

Chloroplasts are the distant descendants of photosynthetic bacteria that became symbiotic a little less than 2 billion years ago with an organism ancestral to algae and plants today. From the point of view of their hosts, their goal is to use the energy of sunlight to split water into hydrogen and oxygen, then react the hydrogen with carbon dioxide to form molecular precursors. glucose. Other reactions also transform these basic materials into other biochemicals. By rearranging the atoms of carbon, hydrogen and oxygen and adding nitrogen, for example, we get amino acids, the building blocks of proteins.

The best hypothesis was therefore that the slugs sequester the chloroplasts in their diverticula so that they can continue to photosynthesize there. But the details were obscure. So, to shed more light, sort of, Sónia Cruz from the University of Aveiro, Portugal, and her colleagues collected sea slugs and carried out some experiments.

As they write in a journal of the Acts of the Royal Society, they supplied the slugs with simple chemicals (sodium bicarbonate and ammonia) containing unusual carbon and nitrogen atoms. Instead of normal carbon, which has six neutrons, bicarbonate contained carbon with seven neutrons. Likewise, ammonia contained nitrogen with eight neutrons instead of the normal seven. Molecules containing these rare isotopes can be detected using a technique called mass spectrometry, so the fate of the atoms involved is easy to follow.

Having treated their slugs this way, the team exposed half of them to light for 36 consecutive hours and kept the other half in the dark. As expected, they saw the unusual isotopes of carbon and nitrogen penetrate the chloroplasts of slugs exposed to light. In contrast, little heavy nitrogen and no heavy carbon entered the chloroplasts of slugs kept in the dark. What intrigued them, however, was the subsequent fate of these isotopes.

A naive hypothesis would be that the products of the captured chloroplasts would be dispersed around the body of a slug, and thus made available to all of its organs. This is because most of them end up in a structure called the albumin gland, which produces nutrients which are then incorporated into the eggs of the animal (the species concerned, Shy elysia, is a hermaphrodite, therefore all individuals carry eggs). The rest migrated directly into the gonads of the slugs.

This result suggests that, rather than aiding the general welfare of an animal, chloroplasts were co-opted by Elysia for a specific role in reproduction. To verify this idea, the team conducted another experiment. They kept pairs of slugs for 28 days in either normal light or very dim light conditions and counted the number of eggs laid.

Pairs exposed to normal light averaged 238 eggs per week during the experiment. Those kept almost in darkness averaged 129. This roughly confirmed the idea that the ingested chloroplasts somehow became extensions of. Elysia, and that chloroplasts, those superbly adapted symbionts of algae and plants, have also been able to adapt to life as part of a very different host. ■

A first version of this article was published online on October 6, 2021

This article appeared in the Science & Technology section of the print edition under the title “Sea slugs and chloroplasts”