In situ localization of sulphur in the thioautotrophic symbiotic model Lucina pectinata (Gmelin, 1791) by cryo-EFTEM microanalysis

Background information. Lucina pectinata is a large tropical lucinid known to harbour sulphide-oxiclizing bacteria in specialized gill cells. Conventional TEM (transmission electron microscopy) has shown that bacteriocytes also harbour visibly 'empty' vesicles whose chemical content remains, to date, only roughly determined. Results. In the present study, L. pectinata gill tissues were cryo-fixed as fast as possible by performing high-pressure freezing before a freeze-substitution process and finally performing a cryo-embedding in Lowicryl. Ultrathin sections were then used for a cryo-EFTEM (where EFTEM stands for energy-filtered TEM) microanalysis. Results show that bacteriocytes within the gill tissues contain elemental sulphur in small vesicles produced by the host itself. In instances of sporadic depletion of sulphur in the environment, such structures may act as energy sources for bacterial endosymbionts. Conclusions. The cryo-EFTEM techniques represent (i) the only method used to date to locate and preserve sulphur at the cellular level and (ii) a powerful tool for sulphur metabolism analysis in thioautotrophic symbiont relationships.