Specificity of bacterial attachment sites on the filamentous diatom Navicula confervacea

The association of bacteria with the frustule of Navicula confervacea was examined with scanning electron microscopy (SEM). 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 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. 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. SEM of cocci, rods, vibrio and spirilla suggests that these bacteria initially prefer mucilage-free attachment sites near openings likely to leak cellular nutrients.