Plastid-bearing sea slugs fix CO2 in the light but do not require photosynthesis to survive

Abstract

Several sacoglossan sea slugs (Plakobranchoidea) feed upon plastids of large unicellular algae. Four species--called long-term retention (LtR) species--are known to sequester ingested plastids within specialized cells of the digestive gland. There, the stolen plastids (kleptoplasts) remain photosynthetically active for several months, during which time LtR species can survive without additional food uptake. Kleptoplast longevity has long been puzzling, because the slugs do not sequester algal nuclei that could support photosystem maintenance. It is widely assumed that the slugs survive starvation by means of kleptoplast photosynthesis, yet direct evidence to support that view is lacking. We show that two LtR plakobranchids, Elysia timida and Plakobranchus ocellatus, incorporate (14)CO2 into acid-stable products 60- and 64-fold more rapidly in the light than in the dark, respectively. Despite this light-dependent CO2 fixation ability, light is, surprisingly, not essential for the slugs to survive starvation. LtR animals survived several months of starvation (i) in complete darkness and (ii) in the light in the presence of the photosynthesis inhibitor monolinuron, all while not losing weight faster than the control animals. Contrary to current views, sacoglossan kleptoplasts seem to be slowly digested food reserves, not a source of solar power.

Keywords: Elysia; Gastropoda; Kleptoplasty; Sacoglossa; photoautotroph; photosynthetic slugs.

Figure 1.

Overview of phylogenetic relationships of Plakobranchoidea (Sacoglossa). Blue rectangles indicate photosynthetic activity over at least two weeks of starvation (based on pulse–amplitude–modulation (PAM) measurements), whereas grey squares indicate species that immediately digest plastids. Slugs showing photosynthetic activities over two months are highlighted in green and the respective pictures of the species and food alga are provided on the right. Photo of E. chlorotica with permission of M. Rumpho, and that of Vaucheria litorea with permission of C. F. Carter. Phylogeny based on Wägele et al. [8].

Figure 2.

Light-dependent incorporation of ¹⁴CO₂ by E. timida and P. ocellatus. CO₂ incorporation in E. timida is almost completely blocked when the slugs were kept in the dark. In P. ocellatus, we additionally blocked photosynthesis using monolinuron, which led to an 87% decrease of CO₂ incorporation. Owing to the size difference, four E. timida specimens (always representing an equal amount of weight) and only one P. ocellatus were used for each individual time point measured. Only for the 120 min values of P. ocellatus, the mean of two individual measurements is shown (values (nmol per incubation) of these 120 min incubations of P. ocellatus were D: 0.42 and 0.47; L: 24.5 and 32.2; M: 3.14 and 4.35); for all others a single measurement was carried out. Orange, light; black, dark; blue, light + monolinuron.

Figure 3.

PAM measurements of E. timida. The maximum quantum yields of slugs kept in the dark (black) were compared to slugs kept under low- (orange) and high-light (red) conditions. Those kept under high light show the strongest decrease over the three months measured, whereas the linear regression of those kept in the dark runs in parallel to that of those kept under low-light conditions. Six specimens were used for each condition tested. The error bars present the standard deviation.

Figure 4.

Influence of photosynthesis inhibition on P. ocellatus. (a) PAM measurements of monolinuron-treated slugs in a 12 L : 12 D cycle (25 µmol quanta m⁻² s⁻¹; blue) in comparison to those kept in the dark (black) and under at a 12 L : 12 D cycle (25 µmol quanta m⁻² s⁻¹; red). Two specimens were used for each condition tested and the error bars present the standard deviation. (b) Weight measurements of the P. ocellatus specimens shown in (a). (c) Exemplary images of P. ocellatus specimens. Image (i) shows a slug kept in the light and which was regularly fed, hence best representing natural conditions. Image (ii) shows a slug after 55 days of starvation in the dark.