Kleptosomes: How a Sea Slug Houses Stolen Chloroplasts

The lettuce sea slug has an unusual relationship with its food. After eating algae, it retains some of their chloroplasts: the structures that capture light for photosynthesis. A 2025 study investigated how the animal houses this borrowed machinery, and why useful cargo can eventually become a meal. [8]

Corey A. H. Allard and colleagues published the peer-reviewed study online in Cell on 25 June 2025. Their experiments examined the sea slug Elysia crispata in the laboratory. [1]

The finding at a glance

• Chloroplasts from food are housed in animal-made compartments.
• The compartments help support photosynthesis.
• During prolonged starvation, retained chloroplasts can be digested.

Borrowing an organelle is different from becoming a plant

The retention of chloroplasts from food is called kleptoplasty; the retained structures are also called kleptoplasts. A chloroplast is a cellular compartment with membranes, proteins and its own genetic material. Keeping it functional after removing it from an algal cell presents a biological puzzle. The slug acquires an organelle, while losing the surrounding algal cellular system that normally supports it. [3]

This distinction gives the story its interest. A green appearance is easy to notice. Establishing what a retained structure does requires measurements of function, and explaining its persistence requires understanding how the animal manages it.

The animal provides a home for the stolen parts

The researchers combined imaging and protein analysis to investigate the membranes surrounding retained chloroplasts. They identified animal membrane markers associated with the endocytic system, including Rab7, and named the chloroplast-containing compartments kleptosomes. [1]

A kleptoplast is the retained chloroplast; a kleptosome is the host-derived compartment housing it. The names describe different parts of the arrangement. The animal therefore contributes something beyond a place for a green particle to sit: it supplies a cellular environment around the borrowed structure. [2]

The team interprets these compartments as specialized versions of the machinery cells normally use to take up and process material. That interpretation connects an unusual animal trait to familiar cellular activities, rather than requiring every component of the system to have evolved from scratch. [8]

Testing the membrane's activity

Electrical recordings from isolated kleptosomes detected ATP-sensitive ion-channel activity consistent with P2X4. A P2X4 blocker reduced photosynthetic oxygen production under the experimental conditions. These experiments link the compartment's membrane physiology with the function of its chloroplast cargo. [1]

The researchers' model is that channels responding to signals inside the compartment help maintain an environment compatible with photosynthesis. This makes the host membrane an active part of the association. [2]

The measurements answer complementary questions. Imaging identifies where structures and markers occur. Electrical recording tests membrane activity. Oxygen measurements assess a functional output. Combining them is more informative than treating any one image as proof of the entire mechanism.

When storing gives way to digesting

Retention was not permanent. During prolonged starvation, the researchers observed changes consistent with chloroplast degradation, including loss of green coloration. The study supports a model in which the animal can preserve photosynthetic cargo and later digest it as an additional nutrient source. [8]

In the featured experiments, extreme starvation lasted more than six weeks. The molecular switch governing the transition from retention to degradation remains incompletely understood. [1]

This suggests two uses for the same acquired structure: obtaining products while it functions and recovering material by eventually breaking it down. The question is therefore broader than whether a slug performs photosynthesis. It also concerns how the animal changes its treatment of foreign cellular material as its own nutritional state changes.

Keeping chloroplasts does not establish gene transfer

A separate 2021 genome study examined another kleptoplastic slug, Plakobranchus ocellatus. Taro Maeda and colleagues found no evidence of algal photosynthetic genes encoded in its nuclear genome. The animal nevertheless retained chloroplasts and benefited from illumination during starvation in that study. The result distinguishes acquiring functional organelles from acquiring algal genes in the host nucleus. [3]

That was a different species and experiment. It provides context for the inheritance question, rather than establishing every genetic detail of E. crispata. A retained organelle and an inherited change to the animal genome are different biological observations.

The benefit depends on what an experiment measures

An earlier study, published online in November 2013 with a 2014 journal issue, examined Elysia timida and Plakobranchus ocellatus. The slugs fixed carbon dioxide in light, yet their survival during the tested starvation conditions did not require photosynthesis. The work showed why photosynthetic activity and dependence on photosynthesis must be evaluated separately. [4]

Surviving a period without food is a specific endpoint. It does not reveal every possible contribution to growth, reproduction or performance in another environment. An experiment can demonstrate activity without showing that the activity supplies all nutritional needs. Conversely, an animal's survival in darkness does not establish that photosynthesis has no useful role under other conditions.

In a 2021 study of E. timida, Paulo Cartaxana and colleagues traced labelled carbon and nitrogen into reproductive tissues that lacked chloroplasts. They also reported reduced fecundity when kleptoplast photosynthesis was limited. Their findings support a contribution to reproductive fitness, extending the question beyond short-term survival. [5]

The combined lesson is to specify the species, conditions and measured outcome. The 2025 kleptosome study addresses cellular housing and regulation. Earlier nutritional experiments ask what the retained machinery contributes to the animal. Those questions inform each other, but their answers are not interchangeable.

Capturing light also creates a protection problem

Photosynthetic machinery must cope with excessive illumination. A 2024 study by Luca Morelli and colleagues compared E. crispata and E. timida carrying chloroplasts from the same green alga, Acetabularia acetabulum. Their analyses found different responses to high light, highlighting the importance of the host-donor combination. [6]

This adds another condition to successful retention. Borrowed machinery needs an appropriate environment as well as an energy source. Testing whether a chloroplast remains functional under one light regime does not establish how it will perform under all regimes. Compatibility needs to be measured, rather than assumed from uptake alone.

An early engineering parallel in cultured cells

A separate 2024 study by Ryota Aoki and colleagues introduced chloroplasts from the red alga Cyanidioschyzon merolae into cultured Chinese hamster ovary cells, CHO-K1. The incorporated chloroplasts retained photosystem II electron-transport activity for at least two days after incorporation. This was an early cell-culture result, not evidence of a photosynthetically self-sufficient mammal. [7]

Photosystem II activity measures part of the light-driven machinery. It does not, by itself, establish that an animal cell obtains all the energy and materials required to grow from sunlight. The experiment illustrates a tractable engineering problem: uptake can be demonstrated while durable maintenance and useful metabolic integration remain separate goals. [7]

For future work, useful questions include how a host controls the surrounding membrane, protects cargo from damage and decides when to digest it. These are proposed research priorities, rather than additional benefits demonstrated in humans. The sea slug offers a natural system for investigating those questions at the level of living cells.

Sources and references

1. Allard CAH et al. A host organelle integrates stolen chloroplasts for animal photosynthesis. Cell 188(19), 5266–5277.e13 (2025). Published online 25 June 2025; issue date 18 September 2025. Peer-reviewed research.
2. Harvard Medical School, Department of Cell Biology. Allard Lab uncovers animal organelle that sustains photosynthesis in stolen chloroplasts. Original research-team account, 27 June 2025.
3. Maeda T et al. Chloroplast acquisition without the gene transfer in kleptoplastic sea slugs, Plakobranchus ocellatus. eLife 10, e60176 (2021).
4. Christa G, Zimorski V, Woehle C, Tielens AGM, Wägele H, Martin WF, Gould SB. Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive. Proceedings of the Royal Society B 281, 20132493 (2014). Published online 20 November 2013.
5. Cartaxana P et al. Photosynthesis from stolen chloroplasts can support sea slug reproductive fitness. Proceedings of the Royal Society B 288, 20211779 (2021).
6. Morelli L, Havurinne V, Madeira D, Martins P, Cartaxana P, Cruz S. Photoprotective mechanisms in Elysia species hosting Acetabularia chloroplasts shed light on host-donor compatibility in photosynthetic sea slugs. Physiologia Plantarum 176(2), e14273 (2024).
7. Aoki R et al. Incorporation of photosynthetically active algal chloroplasts in cultured mammalian cells towards photosynthesis in animals. Proceedings of the Japan Academy, Series B 100(9), 524–536 (2024).
8. Pattison K. Stealing a ‘superpower’. Harvard Gazette, original research-institution report, 25 June 2025.