Kleptoplasts mediate nitrogen acquisition in the sea slug Elysia viridis

Abstract

Kleptoplasty is a remarkable form of symbiosis whereby photosynthetically functional algal chloroplasts are retained by the host organism. Certain sacoglossan sea slugs form such associ- ations and some species such as Elysia viridis (Montagu, 1804) can survive for months without access to their food algae. We report evidence for light-dependent assimilation of ammonium, nitrite, and urea (but not nitrate) in E. viridis with functional kleptoplasts. N uptake was decreased in slugs with reduced chloroplast densities, and after exposure to inhibitors of glutamine synthetase and glutamate synthetase. Incorporation of 15 N-labeled substrates was traced into individual hydrolyzable amino acids, with highest incorporation in glutamine-glutamate. N assimilation by kleptoplasts in multi- cellular hosts constitutes an additional mechanism which may contribute to the unique and long-lived functionality of these symbioses.