In vivo quantification of kleptoplastic chlorophyll a content in the "solar-powered" sea slug Elysia viridis using optical methods: spectral reflectance analysis and PAM fluorometry.

Kleptoplasty is a particularly remarkable type of symbiosis, consisting of the presence of functional chloroplasts in the tissues of a host of another species. One of the most well-studied types of kleptoplasty is the association between sacoglossan molluscs (sea slugs) and algal chloroplasts. After ingestion, the chloroplasts remain photosynthetically functional and provide photosynthates to the host, therefore named as "solar-powered" sea slugs. This study evaluated the use of two optical methods, spectral reflectance analysis and in vivo Chl fluorescence, as measured by pulse amplitude modulated (PAM) fluorometry, for the in vivo quantification of kleptoplastic chlorophyll (Chl) a content in the sacoglossan Elysia viridis (Montagu, 1804) bearing chloroplasts of the macroalgae Codium tomentosum var. mucronatum (G. Hamel) Ardré. The Chl a content of E. viridis specimens was compared to a number of reflectance-based indices and to the dark-level fluorescence, F(o). Most reflectance-based indices varied linearly with the symbiosis Chl a content over the whole range of pigment content variation. Most significant correlations (P < 0.001) were found between indices using as reference the reflectance at 750 nm, with the proportion of pigment content explained by the indices varying between 63.5% and 85.9%. F(o) varied linearly with the Chl a content only for low pigment levels (below 4-6 microg Chl a per individual), above which it followed a saturation-like pattern. The use of optical methods was illustrated by monitoring the changes in Chl a content of specimens during periods of starvation and subsequent recovery. The results of this study suggest that, if basic requirements of signal detection and reproducible measuring geometry are verified, these optical methods may be readily applied to other photosynthetic symbioses.