In thin sections of resin-embedded samples of glutaraldehyde- and osmi
um tetroxide-fixed tissue from five genera of marine sponges, Stromato
spongia, Astrosclera, Jaspis, Pseudoceratina and Axinyssa, cells of a
bacteria-like symbiont microorganism which exhibit a membrane-bounded
nuclear region encompassing the fibrillar nucleoid have been observed
within the sponge mesohyl. The nuclear region in these cells is bounde
d by a single bilayer membrane, so that the cell cytoplasm is divided
into two distinct regions. The cell wall consists of subunits analogou
s to those in walls of some Archaea. Cells of the sponge symbionts obs
erved here are similar to those of the archaeal sponge symbiont Cenarc
haeum symbiosum. (C) 1998 Federation of European Microbiological Socie
ties. Published by Elsevier Science B.V. All rights reserved.