title: 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.
authors.affiliation: Zoologisches Forschungsmuseum Alexander Koenig, Centre for Molecular Biodiversity Research (zmb), , Bonn 53113, Germany, Institute for Molecular Evolution, Heinrich Heine-University Düsseldorf, , Düsseldorf 40225, Germany, Department of Biochemistry and Cell Biology, Faculty of Veterinary Medicine, Utrecht University, , Utrecht, The Netherlands, Department of Medical Microbiology and Infectious Diseases, Erasmus University Medical Center, , Rotterdam, The Netherlands.
authors.forename: Gregor
authors.initials: G
authors.lastname: Christa
authors.forename: Verena
authors.initials: V
authors.lastname: Zimorski
authors.forename: Christian
authors.initials: C
authors.lastname: Woehle
authors.forename: Aloysius G M
authors.initials: AG
authors.lastname: Tielens
authors.forename: Heike
authors.initials: H
authors.lastname: Wägele
authors.forename: William F
authors.initials: WF
authors.lastname: Martin
authors.forename: Sven B
authors.initials: SB
authors.lastname: Gould
mesh_terms: D000818:Animals; D001835:Body Weight; D003624:Darkness; D004063:Digestion / Q000502:physiology; D004734:Energy Metabolism / Q000528:radiation effects*; D049851:Gastropoda / Q000378:metabolism / Q000502:physiology* / Q000528:radiation effects / Q000648:ultrastructure; D008027:Light; D010788:Photosynthesis / Q000187:drug effects / Q000528:radiation effects; D010802:Phylogeny; D018087:Plastids / Q000378:metabolism*; D013045:Species Specificity
keywords: Elysia; Gastropoda; Kleptoplasty; Sacoglossa; photoautotroph; photosynthetic slugs