Specificity of bacterial attachment sites on the filamentous diatom Navicula confervacea (Bacillariophyceae)

Abstract

The association in soil–water cultures of bacteria with the siliceous wall (frustule) of Navicula confervacea was examined with scanning electron microscopy. This diatom is filamentous through serial valve–face unions. Thus, the valve mantle and girdle bands are exposed to the aqueous environment and its bacterial flora, whereas the internally isolated adhering valve–face surfaces are not. Secretion of diatom mucilage strands occurred largely from parallel slits in the valve mantle and from valve–band and band–band sutures. These strands appeared to create a surface environment that discouraged bacterial adhesion. However, as the diatoms divided, their newly exposed mucilage-free bands provided a substratum for bacterial attachment. Dense aggregates of bacteria sometimes occurred at frustule apices, where filaments most easily bend and where recently formed adjacent hypovalves emerge. Along mucilage-free segments of the diatom filament, bacterial development occasionally engulfed the frustules; more commonly, bacteria were sparse but in specific locations. Bacterial attachment structures were mostly straight fibers, even on cocci, whereas strands from diatoms were contorted. Attachment by rods and spirilla was most often by their apices, with or without obvious lateral anchoring fibers. Scanning electron microscopy of cocci, rods, vibrio, and spirilla suggests that these bacteria initially prefer mucilage-free attachment sites near openings likely to leak cellular nutrients, i.e., slits of the mantle and band sutures rather than on the substratum between them, where presumably, other nutrients would have been available through their adsorption from the medium. Key words: bacterial adhesion, diatom girdle, mucilage, trail fibers.