Rapid induction and disappearance of electron-dense organelles following sulfide exposure in the marine annelid Branchioasychis americana

. Electron-dense organelles (EDOs) are a characteristic feature of marine annelids adapted to hydrogen sulfide, but little is known about their origin or fate or the mechanism by which they are formed. In this study, transmission electron microscopy was used to determine the appearance, quantity, and size distribution of EDOs in body wall epithelium of the sulfide-tolerant annelid Branchioasychis americana. EDOs were enclosed by either one or two lipid bilayers with a typical diameter in the range of 0.4–1.2 μm. The contents included amorphous electron-dense material, lamellated membranes, and distinct membrane-bounded structures. In animals fixed immediately after collection from the mudflat, the EDO quantity was 0.148 μm−2. This decreased by one-third to 0.100 μm−2 after 2 weeks under sulfide-free conditions. Subsequent exposure to 250–400 μmol L−1 sulfide for ≤24 h increased the EDO quantity by 2.4-fold to 0.240 μm−2, which reverted to control levels after 24 h under sulfide-free conditions. The results show that EDOs are transient structures that are rapidly induced by sulfide, which is consistent with the hypothesis that sulfide-induced EDOs represent the upregulated autophagic degradation of mitochondria or other cellular constituents damaged by sulfide exposure.