Identifying Fossil Biofilms In Carbonate Pool Speleothems Using the SEM: Criteria and Pitfalls

Living biofilm is composed of a complex mixture of living organisms, extracellular polysaccharides (EPS), gradients (oxygen, nitrite, nitrate, ammonium, pH, sulfide, and methane), trapped sediments, and precipitated minerals. When fossilized, cells are often not preserved, leaving behind only lithified EPS to reveal the former community. There are a number of components, including clays and added materials such as epoxy, which can be confused with biofilm. Criteria are needed to distinguish biofilm from imposters.

Using the scanning electron microscope (SEM), we have identified a variety of fossil biofilm fabrics in carbonate pool speleothems from caves in the Guadalupe Mountains, NM, USA. To distinguish fossil biofilm from modern contaminants, we etch samples to get below the surface into the unexposed mineral. Since we are specifically looking for organic material, we coat samples with Au-Pd. In addition, many samples are thin sections that allow correlation of submicroscopic features with macroscopic and microscopic petrographic fabrics.

Fossil biofilm has a smooth to fibrous surface and is carbon-rich when analyzed with EDX. Fibers typically vary in diameter along their length, from <0.1 to several microns. This contrasts with microbial filaments that are much more consistent in diameter. Biofilm often takes on the lacy appearance of a spider web or coalesces to form a smooth ropy texture. In addition, fibers often split and rejoin like twisted ropes.

Various clay minerals such as halloysite can mimic biofilm. In this case, EDX is useful since biofilm contains C instead of the Si, Al, Mg, or K of clays. Epoxy can also mimic the smooth biofilm and contains abundant C but it lacks the fibers.

The work reported here clarifies the distinction between true biofilm in mineralogically fossilized material and non-biogenic mineral structures that mimic biofilm.