Chloroplast incorporation and long-term photosynthetic performance through the life cycle in laboratory cultures of Elysia timida (Sacoglossa, Heterobranchia)

Valerie Schmitt,William F Martin,Sven B Gould,Katharina Händeler,Susanne Gunkel,Heike Wägele,Diedrik Menzel,Marie-Line Escande

doi:10.1186/1742-9994-11-5

Abstract

IntroductionThe Mediterranean sacoglossan Elysia timida is one of the few sea slug species with the ability to sequester chloroplasts from its food algae and to subsequently store them in a functional state in the digestive gland cells for more than a month, during which time the plastids retain high photosynthetic activity (= long-term retention). Adult E. timida have been described to feed on the unicellular alga Acetabularia acetabulum in their natural environment. The suitability of E. timida as a laboratory model culture system including its food source was studied.ResultsIn contrast to the literature reporting that juvenile E. timida feed on Cladophora dalmatica first, and later on switch to the adult diet A. acetabulum, the juveniles in this study fed directly on A. acetabulum (young, non-calcified stalks); they did not feed on the various Cladophora spp. (collected from the sea or laboratory culture) offered. This could possibly hint to cryptic speciation with no clear morphological differences, but incipient ecological differentiation. Transmission electron microscopy of chloroplasts from A. acetabulum after initial intake by juvenile E. timida showed different states of degradation — in conglomerations or singularly — and fragments of phagosome membranes, but differed from kleptoplast images of C. dalmatica in juvenile E. timida from the literature. Based on the finding that the whole life cycle of E. timida can be completed with A. acetabulum as the sole food source, a laboratory culture system was established. An experiment with PAM-fluorometry showed that cultured E. timida are also able to store chloroplasts in long-term retention from Acetabularia peniculus, which stems from the Indo-Pacific and is not abundant in the natural environment of E. timida. Variations between three experiment groups indicated potential influences of temperature on photosynthetic capacities.ConclusionsE. timida is a viable laboratory model system to study photosynthesis in incorporated chloroplasts (kleptoplasts). Capacities of chloroplast incorporation in E. timida were investigated in a closed laboratory culture system with two different chloroplast donors and over extended time periods about threefold longer than previously reported.

Highlights

The Mediterranean sacoglossan Elysia timida is one of the few sea slug species with the ability to sequester chloroplasts from its food algae and to subsequently store them in a functional state in the digestive gland cells for more than a month, during which time the plastids retain high photosynthetic activity (= long-term retention)
The finding that juvenile E. timida fed directly on the adult diet A. acetabulum is eminent in the light of future analyses, since only a single food source system has to be provided during complete life cycles
We demonstrated for the first time that E. timida is able to perform long-term retention in culture with an alternative algal chloroplast donor

Summary

Introduction

The Mediterranean sacoglossan Elysia timida is one of the few sea slug species with the ability to sequester chloroplasts from its food algae and to subsequently store them in a functional state in the digestive gland cells for more than a month, during which time the plastids retain high photosynthetic activity (= long-term retention). The phenomenon of “long-term retention” of functional chloroplasts from food algae with ongoing photosynthesis inside the slugs’ cells over longer time periods than a month only occurs in very few sea slug species [1,2,3,4,5]. Among animals, such kleptoplasty is only known for sacoglossan sea slugs [4,5,6]. Curtis and coworkers [12] raised slugs hatched from egg masses laid in the laboratory by adult sea slugs, which were originally collected from the sea and fed the offspring after metamorphosis, but only for a limited period of time for subsequent experiments

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