How can sacoglossan sea slugs perform photosynthesis – a process usually associated with plants?
Kleptoplasty describes a special type of endosymbiosis where a host organism retain photosynthetic organelles from their algal prey. Kleptoplasty is widespread in ciliates and foraminifera; however, within Metazoa animals (animals having the body composed of cells differentiated into tissues and organs, and usually a digestive cavity lined with specialized cells), sacoglossan sea slugs are the only known species to harbour functional plastids. This characteristic gives these sea slugs their very special feature.
The “stolen” chloroplasts are acquired by the ingestion of macro algal tissue and retention of undigested functional chloroplasts in special cells of their gut. These “stolen” chloroplasts (thereafter named kleptoplasts) continue to photosynthesize for varied periods of time, in some cases up to one year.
In our study, we analyzed the pigment profile of Elysia viridis in order to evaluate appropriate measures of photosynthetic activity.
The pigments siphonaxanthin, trans and cis-neoxanthin, violaxanthin, siphonaxanthin dodecenoate, chlorophyll (Chl) a and Chl b, ε,ε- and β,ε-carotenes, and an unidentified carotenoid were observed in all Elysia viridis. With the exception of the unidentified carotenoid, the same pigment profile was recorded for the macro algae C. tomentosum (its algal prey).
In general, carotenoids found in animals are either directly accumulated from food or partially modified through metabolic reactions. Therefore, the unidentified carotenoid was most likely a product modified by the sea slugs since it was not present in their food source.
Pigments characteristic of other macro algae present in the sampling locations were not detected inthe sea slugs. These results suggest that these Elysia viridis retained chloroplasts exclusively from C. tomentosum.
In general, the carotenoids to Chl a ratios were significantly higher in Elysia viridis than in C. tomentosum. Further analysis using starved individuals suggests carotenoid retention over Chlorophylls during the digestion of kleptoplasts. It is important to note that, despite a loss of 80% of Chl a in Elysia viridis starved for two weeks, measurements of maximum capacity of performing photosynthesis indicated a decrease of only 5% of the photosynthetic capacity of kleptoplasts that remain functional.
This result clearly illustrates that measurement of photosynthetic activity using this approach can be misleading when evaluating the importance of kleptoplasts for the overall nutrition of the animal.
Finally, concentrations of violaxanthin were low in C. tomentosum and Elysia viridis and no detectable levels of antheraxanthin or zeaxanthin were observed in either organism. Therefore, the occurrence of a xanthophyll cycle as a photoregulatory mechanism, crucial for most photosynthetic organisms, seems unlikely to occur in C. tomentosum and Elysia viridis but requires further research.