Usurping a ‘superpower’

A recent investigation conducted by Harvard biologists explains how certain sea slugs consume algae and integrate their photosynthetic organelles into their own cells. These organelles keep functioning, supplying nutrients and energy to their hosts, acting as emergency reserves during starvation.

“This is a creature that can appropriate elements from other organisms and utilize them within its own cells,” stated Corey Allard, the principal author of the study and a former postdoctoral researcher in the Department of Molecular and Cellular Biology. “I considered that to be some of the most astonishing biology I had ever encountered.”

The study, published in the journal Cell, illustrates how these so-called “solar-powered” sea slugs maintain the organelles within “kleptosomes” — specialized membranes acting as biological storage sacks. This research could provide insights into the development of eukaryotic cells and pave the way for potential biomedical applications.

“The astonishing aspect is that sea slugs can effectively seize ‘superpowers’ — specifically the capacity to generate energy from light via algae,” expressed Amy Si-Ying Lee, an assistant professor of cell biology at Harvard Medical School, a researcher at the Dana-Farber Cancer Institute, and a co-author of the study. “Others harness abilities like stinging or luminescence. What’s incredibly fascinating is we discovered how they sustain these acquired superpowers for their own survival.”

The investigation commenced several years prior when Allard, now an assistant professor at the Medical School, was affiliated with the Bellono Lab, focused on endosymbiosis, wherein one species resides within another. Unlike corals, which integrate entire algal cells, sea slugs utilize only fragments — minuscule organelles from their prey’s cells.

In the new publication, the team details how the sea slug Elysia crispata, a species found in the tropical regions of the western Atlantic and Caribbean, consumes algae but does not completely digest the chloroplasts.

Instead, the slugs redirect these organelles into intestinal sacs and encase them within a unique membrane that researchers designated a “kleptosome.” Inside this distinctive slug structure, the appropriated organelles are preserved to continue photosynthesis.

It appears that the slugs have adapted to downregulate the lysosomes, the cellular “waste disposal” organelles typically responsible for breaking down such material.

Chemical examinations indicated that the appropriated chloroplasts contained slug proteins, suggesting that the hosts were sustaining the stolen organelles. Meanwhile, the organelles continued to produce their own algae proteins, demonstrating they were still operational inside the slugs.

The slugs preserved the appropriated organelles in leaf-like formations atop their backs, (“Essentially, it’s a solar panel,” Allard comments) and well-nourished slugs exhibited a greenish hue.

Researchers then observed another anomaly: When slugs experienced starvation, their bodies turned orange like autumn leaves. Evidently, the chlorophyll (the green pigment in chloroplasts) was decomposed when the appropriated organelles were consumed as a “last resort” for energy.

Some existing scientific research suggested that slugs solely thrive on solar energy, but Allard contends that photosynthesis alone isn’t adequate for their survival.

“The actual role of these structures may be much more intricate than mere solar panels,” he stated. “They could serve as food reserves, camouflage, or render them unpalatable to predators. Likely, they encompass all these functions.”

The unassuming slugs may offer insights into significant events in life’s history.

Endosymbiosis has been a crucial force for evolutionary innovation. For instance, both chloroplasts (which facilitate photosynthesis in plants and algae) and mitochondria (the energy-producing organelles in cells) were initially autonomous cells incorporated as organelles within host cells.

“In numerous endosymbiotic systems, such as our mitochondria or plant chloroplasts, this is how it commenced: An ancient prokaryotic cell was engulfed and incorporated into the host,” explained Nick Bellono, a professor of molecular and cellular biology and the senior author of the publication. “In the case of the slug, it’s accomplishing this in a single lifetime. Could this transition evolve into a more enduring relationship over an immense timespan? Perhaps.”

The ancient occurrences of endosymbiosis transpired billions of years ago, and the evidence has been obscured by time. In the case of sea slugs, the biologists observed the organelle thieves in action — allowing them to examine endosymbiosis in real-time.

Elysia are not the solitary sea slugs recognized for appropriating organelles. In his lab at the Medical School, Allard is investigating another category of sea slugs from the genus Berghia that consume sea anemones, navigate the material through their digestive systems, and attach the venomous barbs to their own backs as a defense against predators.

Even more remarkably, the slug hosts can link these appropriated organelles to their own nervous systems to deploy what Allard described as a “bag filled with spear guns.”

Allard believes that the implications may extend beyond slugs. Understanding organelle regulation could have relevance for neurodegenerative diseases or lysosomal storage disorders, which are metabolic conditions where the body cannot efficiently process waste materials.

“In many instances regarding these disorders, the lysosomes either fail to form correctly or operate inefficiently,” clarified Allard, “and it somewhat resembles what the slugs have adapted to accomplish in certain ways.”