Representative FEG-SEM images of organic structures observed in the fossil terrace layers (a-c) compared to a fresh sinter rim sample from the active pool rim(d): (a) large organic matter cluster that showed no distinguishing morphological microbial features even at higher resolution (b). Only one structure could be interpreted as possible microbial remnants (c). All these structures when analyzed under EDS exhibited a stronger C peak than the calcite substrate; in addition these organic clusters were easily degraded under the electron beam (beam damage). For comparison (d) shows filamentous microbes in a sinter piece from the active part of the main terrace. Scales (a) and (d) 20 μm; (b) 1 μm; (c) 2 μm.