similar storage of oxygen in the vacuoles would not be possible since the lipid membranes enclosing cells and vacuoles are permeable to gases. the thioplocas thus move up and down, recharging with nitrate at the surface and oxidizing sulfide at depth, therefore storing elemental sulfur globules as an energy reserve. thioploca and beggiatoa although the thioplocas typically live in sheaths in bundles ranging from a few up to a hundred filaments per sheath, many were found at the sediment surface apparently without a sheath. at the bay of concepcion on the chilean coast, there was a transition between an apparently pure beggiatoa community inside the bay to a mixed community of both genera at the entrance of the bay to pure thioploca outside. in the mixed community it was not possible to discriminate beggiatoas from thioplocas by simple microscopy but only by analyzing statistically their diameter distributions. the tapered ends of filaments, characteristic of thioploca but absent in beggiatoa, was not a consistent character of the thioplocas. future changes in classification of thioploca and beggiatoa are likely. the range of strains over which the genus designation beggiatoa is used is overly broad. more importantly, the differentiation between thioploca and beggiatoa is currently based on the formation of a common sheath surrounding filament bundles, a characteristic that might vary in response to environmental conditions. in the absence of pure cultures, it may be impossible to prove or disprove whether any natural population of vacuolated beggiatoa will form sheath bundles in some specific environment. the clade comprised three thioploca strains, two beggiatoa strains, and a thiomargarita strain is united by the possession of a large central vacuole. this feature currently appears to be the best morphological candidate to replace sheath formation as a marker in a revised taxonomy of the group beggiatoa–thioploca. this marker, in addition to being consistent with 16s rrna phylogeny, appears to be universally connected to intracellular nitrate accumulation, presumably in the vacuole, for nitrate respiration enabling sustained anaerobic metabolism. a future revision of the genus thioploca, based on the vacuolated, nitrate-respiring phenotype and corresponding 16s rrna clade, might include these gliding filaments regardless of whether they occur in sheathed bundles.