Algal Remodeling in a Ubiquitous Planktonic Photosymbiosis

Photosymbiosis between single-celled hosts and microalgae is common in oceanic plankton. However, the functioning of this ecologically important cell-cell interaction and the subcellular mechanisms allowing the host to accommodate and benefit from its microalgae remain enigmatic. Here, using a combination of quantitative single-cell structural and chemical imaging techniques, a collaboration of researchers show that the structural organization, physiology, and trophic status of the algal symbionts (the haptophyte Phaeocystis) significantly change within their acantharian hosts compared to their free-living phase in culture. In symbiosis, algal cell division is blocked, photosynthesis is enhanced, and cell volume is increased by up to 10-fold with a higher number of plastids (from 2 to up to 30) and thylakoid membranes. This study unveils an unprecedented morphological and metabolic transformation of microalgae following their integration into a host, and it suggests that this widespread symbiosis is a farming strategy wherein the host engulfs and exploits microalgae.

The IBS-ISBG electron microscopy platform was involved in the preparation of the planctonic or cultured samples for electron microscopy imaging. An optimized sample preparation was also set up for correlative imaging between structural (TEM, SEM, FIB-SEM) and chemical imaging (X-ray fluorescence microscopy, SIMS).

Algal Remodeling in a Ubiquitous Planktonic Photosymbiosis. Decelle J, Stryhanyuk H, Gallet B, Veronesi G, Schmidt M, Balzano S, Marro S, Uwizeye C, Jouneau PH, Lupette J, Jouhet J, Maréchal E, Schwab Y, Schieber NL, Tucoulou R, Richnow H, Finazzi G, Musat N. Current Biology ; doi : 10.1016/j.cub.2019.01.073