The role of surface layer proteins in the degradation of a photosynthetic prokaryote, the cyanobacterium Synechococcus sp.

The accumulation of refractory prokaryotic cell membranes has been suggested as a possible source of both dissolved and particulate organic matter in the deep ocean. A surface layer protein (S-layer) is widely found as part of the cell envelope in both Eubacteria and Archaea and is made up of a monomolecular layer of glycoproteins. This heavily glycosylated protein covers the outermost cell surface in a regularly ordered planar crystalline structure. With special attention to the possible geochemical importance of S-layer protein in seawater, we studied the degradation of two species of marine cyanobacteria, Synechococcus sp. CCMP2370 and CCMP1334, with and without an S-layer structure, respectively, after they had been treated with buffers that can strip the S-layer from the cell surface. Based on evidence from bulk chemical and molecular analysis as well as electron microscopy, stripped cells of CCMP2370 lost their cell membrane structure and degraded more rapidly and became more enriched in d-amino acids, while stripped CCMP1334 cells maintained their membrane structures, and degradation was not enhanced. These results suggest that S-layer glycoproteins have a limited contribution to selective preservation of proteinaceous materials during the degradation of the cyanobacterium Synechococcus, but like peptidoglycan, S-layer structure functions as a defensive barrier on the cell membrane to prevent cell lysis and slow degradation of cyanobacterial cellular materials. Our results imply that selective preservation of glycoprotein is not as important as physical protection by S-layer in regulating the degradation of cellular materials of cyanobacteria.