The distribution of lipids and sterols in cell types from the marine sponge Pseudaxinyssa sp.

The sponge Pseudaxinyssa sp., unique in sterol and fatty acid composition, was cellularly dissected into fractions enriched in each of the major cell types present in the sponge: microbial symbionts (cyanobacteria), small sponge cells (pinacocytes and choanocytes), and large sponge cells (archeocytes and cyanophytes). Three phototrophic microbial symbionts were also isolated from the cell fractions and grown in culture. An unsymmetrical distribution of fatty acids and sterols was observed for the sponge cells: small cells contained larger quantities of long chain fatty acids (greater than C24) and smaller quantities of sterols than were present in the larger sponge cells. Moreover, the rare sterols 24-isopropylcholesterol predominated in the smaller sponge cells, whereas its 22-dehydro analog predominated in the larger sponge cells. Long chain fatty acids and sterols were not detected in the cultured microbial symbionts. This constitutes the first report of lipid variability according to cell type for this most primitive group of Metazoa.