Some sea slugs are able to steal functional chloroplasts (kleptoplasts) from their algal food sources, but the role and relevance of photosynthesis to the animal host remain controversial. While some researchers claim that kleptoplasts are slowly digestible 'snacks', others advocate that they enhance the overall fitness of sea slugs much more profoundly. Our analysis shows light-dependent incorporation of 13C and 15N in the albumen gland and gonadal follicles of the sea slug Elysia timida, representing translocation of photosynthates to kleptoplast-free reproductive organs. Long-chain polyunsaturated fatty acids with reported roles in reproduction were produced in the sea slug cells using labelled precursors translocated from the kleptoplasts. Finally, we report reduced fecundity of E. timida by limiting kleptoplast photosynthesis. The present study indicates that photosynthesis enhances the reproductive fitness of kleptoplast-bearing sea slugs, confirming the biological relevance of this remarkable association between a metazoan and an algal-derived organelle.

Elysia crispata is a sacoglossan sea slug that retains intracellular, functional chloroplasts stolen from their macroalgal food sources. Elysia crispata juveniles start feeding on the algae following metamorphosis, engulfing chloroplasts and turning green. In laboratory-reared animals, we report one juvenile "albino" specimen unable to retain chloroplasts. Within 6 weeks post-metamorphosis, the aposymbiotic sea slug was significantly smaller than its chloroplast-bearing siblings. This evidence highlights that chloroplast acquisition is required for the normal development of E. crispata.

Some sacoglossan sea slugs steal functional macroalgal chloroplasts (kleptoplasts). In this study, we investigated the effects of algal prey species and abundance on the growth and photosynthetic capacity of the tropical polyphagous sea slug Elysia crispata. Recently hatched sea slugs fed and acquired chloroplasts from the macroalga Bryopsis plumosa, but not from Acetabularia acetabulum. However, adult sea slugs were able to switch diet to A. acetabulum, rapidly replacing the great majority of the original kleptoplasts. When fed with B. plumosa, higher feeding frequency resulted in significantly higher growth and kleptoplast photosynthetic yield, as well as a slower relative decrease in these parameters upon starvation. Longevity of A. acetabulum-derived chloroplasts in E. crispata was over twofold that of B. plumosa. Furthermore, significantly lower relative weight loss under starvation was observed in sea slugs previously fed on A. acetabulum than on B. plumosa. This study shows that functionality and longevity of kleptoplasts in photosynthetic sea slugs depend on the origin of the plastids. Furthermore, we have identified A. acetabulum as a donor of photosynthetically efficient chloroplasts common to highly specialized monophagous and polyphagous sea slugs capable of long-term retention, which opens new experimental routes to unravel the unsolved mysteries of kleptoplasty.

Background: Microphytobenthos (MPB) are the main primary producers of many intertidal and shallow subtidal environments. Although these coastal ecosystems are particularly vulnerable to anthropogenic activities, little is known on the effects of climate change variables on the structure and productivity of MPB communities. In this study, the effects of elevated temperature and CO2 on intertidal MPB biomass, species composition and photosynthetic performance were studied using a flow-through experimental life support system. Results: Elevated temperature had a detrimental effect on MPB biomass and photosynthetic performance under both control and elevated CO2. Furthermore, elevated temperature led to an increase of cyanobacteria and a change in the relative abundance of major benthic diatom species present in the MPB community. The most abundant motile epipelic species Navicula spartinetensis and Gyrosigma acuminatum were in part replaced by tychoplanktonic species (Minidiscus chilensis and Thalassiosira cf. pseudonana) and the motile epipelic Nitzschia cf. aequorea and N. cf. aurariae. Elevated CO2 had a beneficial effect on MPB biomass, but only at the lower temperature. It is possible that elevated CO2 alleviated local depletion of dissolved inorganic carbon resulting from high cell abundance at the sediment photic layer. No significant effect of elevated CO2 was detected on the relative abundance of major groups of microalgae and benthic diatom species. Conclusions: The interactive effects of elevated temperature and CO2 may have an overall detrimental impact on the structure and productivity of intertidal MPB, and eventually in related ecosystem services.

Background: Microphytobenthos (MPB) are the main primary producers of many intertidal and shallow subtidal environments. Although these coastal ecosystems are particularly vulnerable to anthropogenic activities, little is known on the effects of climate change variables on the structure and productivity of MPB communities. In this study, the effects of elevated temperature and CO2 on intertidal MPB biomass, species composition and photosynthetic performance were studied using a flow-through experimental life support system. Results: Elevated temperature had a detrimental effect on MPB biomass and photosynthetic performance under both control and elevated CO2. Furthermore, elevated temperature led to an increase of cyanobacteria and a change in the relative abundance of major benthic diatom species present in the MPB community. The most abundant motile epipelic species Navicula spartinetensis and Gyrosigma acuminatum were in part replaced by tychoplanktonic species (Minidiscus chilensis and Thalassiosira cf. pseudonana) and the motile epipelic Nitzschia cf. aequorea and N. cf. aurariae. Elevated CO2 had a beneficial effect on MPB biomass, but only at the lower temperature. It is possible that elevated CO2 alleviated local depletion of dissolved inorganic carbon resulting from high cell abundance at the sediment photic layer. No significant effect of elevated CO2 was detected on the relative abundance of major groups of microalgae and benthic diatom species. Conclusions: The interactive effects of elevated temperature and CO2 may have an overall detrimental impact on the structure and productivity of intertidal MPB, and eventually in related ecosystem services.

Changes in biomass and photosynthesis of a diatom-dominated microphytobenthos (MPB) intertidal community were studied over a diel emersion period using a combination of O2 and scalar irradiance microprofiling, variable chlorophyll (Chl) fluorescence, and pigment analysis. The MPB biomass in the photic zone (0-0.5 mm) of the sediment exposed to low irradiance (150 μmol photons m(-2) s(-1)) showed a >2-fold increase during the first hours of the emersion period, reaching >0.2 mg Chl a cm(-3). Concentrations of Chl a started to decrease half-way through the emersion period, almost 2 h before tidal inundation. Similarly, O2 concentrations and volumetric gross photosynthesis in the photic zone increased during the first half of the emersion period and then decreased toward the timing of incoming tide/darkness. The results suggest that intertidal MPB community-level photosynthesis is mainly controlled by changes in the productive biomass of the photic zone determined by cell migration. A diel pattern in the photosynthesis vs. irradiance parameters α (photosynthetic efficiency at limiting irradiance) and ETR max (photosynthetic capacity at saturating irradiance) was also observed, suggesting photoacclimation of MPB. Under high light exposure (2000 μmol photons m(-2) s(-1)), lower α, ETR max and sediment O2 concentrations were observed when cell migration was inhibited with the diatom motility inhibitor latrunculin A (Lat A), showing that migration is also used by MPB to maximize photosynthesis by reducing exposure to potentially photoinhibitory light levels. A higher de-epoxidation state in sediment treated with Lat A indicates that the involvement of the xanthophyll cycle in physiological photoprotection is more relevant in MPB when cells are inhibited from migrating. In the studied diatom-dominated MPB intertidal community, cell migration seems to be the key factor regulating photosynthesis over a diel emersion period and upon changes in light exposure.

Some Sacoglossa sea slugs are capable of stealing and maintaining functional intracellular chloroplasts—kleptoplasts—from their macroalgal prey for periods of up to several months, a process known as kleptoplasty. Although the cultivation of these marine invertebrates under laboratory conditions is crucial for research in various fields (e.g., endosymbiosis, animal physiology, discovery of new marine natural products), rearing protocols are scarce. This study presents a standardized protocol for the laboratory rearing of large numbers of the sacoglossan tropical sea slug Elysia crispata. The detailed protocol successfully facilitated embryonic development, larval metamorphosis, and juvenile-to-adult transition, allowing the rearing of multiple generations. Two groups, characterized by acquiring different kleptoplasts, were obtained by feeding the sea slugs with two different prey macroalgae: Bryopsis sp. and Acetabularia acetabulum. Usually referred to as lettuce sea slug among marine aquarium hobbyists, E. crispata is a highly valued organism for its striking appearance and ability to control nuisance algal growth in reef aquariums. This protocol allows experimental reproducibility and access to specimens under different development stages, potentially boosting research on kleptoplasty while also contributing to reducing the impact of the marine aquarium trade on natural populations.

Some sea slugs are able to steal functional chloroplasts (kleptoplasts) from their algal food sources, but the role and relevance of photosynthesis to the animal host remain controversial. While some researchers claim that kleptoplasts are slowly digestible 'snacks', others advocate that they enhance the overall fitness of sea slugs much more profoundly. Our analysis shows light-dependent incorporation of 13 C and 15 N in the albumen gland and gonadal follicles of the sea slug Elysia timida, representing translocation of photosynthates to kleptoplast-free reproductive organs. Long-chain polyunsaturated fatty acids with reported roles in reproduction were produced in the sea slug cells using labelled precursors translocated from the kleptoplasts. Finally, we report reduced fecundity of E. timida by limiting kleptoplast photosynthesis. The present study indicates that photosynthesis enhances the reproductive fitness of kleptoplast-bearing sea slugs, confirming the biological relevance of this remarkable association between a metazoan and an algal-derived organelle.

Kleptoplasty, the process by which a host organism sequesters and retains algal chloroplasts, is relatively common in protists. The origin of the plastid varies, as do the length of time it is retained in the host and the functionality of the association. In metazoa, the capacity for long-term (several weeks to months) maintenance of photosynthetically active chloroplasts is a unique characteristic of a handful of sacoglossan sea slugs. This capability has earned these slugs the epithets "crawling leaves" and "solar-powered sea slugs." This Unsolved Mystery explores the basis of chloroplast maintenance and function and attempts to clarify contradictory results in the published literature. We address some of the mysteries of this remarkable association. Why are functional chloroplasts retained? And how is the function of stolen chloroplasts maintained without the support of the algal nucleus?

Background: Some Sacoglossa sea slugs integrate intracellular chloroplasts derived from the algae they feed on, a process termed kleptoplasty. The stolen chloroplasts – kleptoplasts – can maintain their functionality up to several months and support animal metabolism. However, chloroplast longevity can vary depending on sea slug species and algal donor. In this study, we focus our attention on Elysia viridis, a polyphagous species that is mostly found associated to the macroalga Codium tomentosum, but that was reported to eat other macroalgae, including Chaetomorpha sp. Methods: We have investigated the changes in E. viridis physiology when provided with the two different food sources to evaluate to which extent the photosynthetic and photoprotective mechanisms of the algae chloroplasts matched those of the plastids once in the animal cells. To perform the study, we rely on the evaluation of chlorophyll a variable fluorescence to study the photophysiologic state of the integrated kleptoplasts and the high-performance liquid chromatography (HPLC) to study variations in the photosynthetic pigments. Results: We observed that the photosynthetic efficiency of E. viridis specimens is similar to the one of the respective algal donor but specimens fed with Chaetomorpha tend to have lower performances. Significant differences appear in the non-photochemical quenching (NPQ) abilities. While sea slugs fed with C. tomentosum react to high-light stress as the algae, E. viridis hosting Chaetomorpha chloroplasts are unable to properly recover from photoinhibition or perform a functional xanthophyll cycle (XC). Conclusions: Our results showed that, even if the sea slugs fed with the two algae show photosynthetic activities like the respective algal donors, not all the photoprotective mechanisms present in Chaetomorpha can be maintained in E. viridis. This indicates that the functionality of the kleptoplasts does not depend solely on their origin but also on the degree of compatibility with the animal species integrating them.