Nannobacteria: What is the evidence?
Keywords: micrometerorites, nannobacteria, nanobacteria, NASA, nonbiological precipitates, silica vapor deposits, Stardust Mission.
RALPH P. HARVEYDepartment of Geological Sciences, Case Western University, Cleveland, OH 44106-7216, USA, firstname.lastname@example.org
Received June 4, 1997, published June 6, 1997
Summary: It has been claimed (Folk 1997) that the abundant rounded or elongated objects, less than 0.2 microns in diameter, that are visible in scanning electron micrographs of samples of terrestrial soils, sediments, minerals and rocks are extremely small microorganisms, or “nannobacteria.” Here, the evidence for this claim is examined.
Dr. Folk’s curiosity (Folk 1997) has been captured by the rounded, sometimes elongated sausage-shaped forms shown in very high magnification scanning electron photo-micrographs. Whereas few would suggest that these shapes aren’t really there, many scientists differ with Folk as to what they represent. Folk calls these forms “nannobacteria,” implying that they are living things (or their remnants). What’s wrong with calling them “nannobacteria”? Simply put, Folk has no evidence that shows they are bacteria, or living organisms of any kind. As a result, his article is mainly conjectural.
In the very first words of his article, Folk provides us with his working definition of nannobacteria. He describes them as “dwarf forms of bacteria, mostly 0.05 to 0.2 micrometers (in diameter).” This definition suggests that size and shape are all you need to identify living organisms like bacteria. However, shape and size are only useful as identifiers of bacteria when applied to organisms whose identity has been previously confirmed by other studies; the organisms have to be cultured, chemically analyzed, their typical living and reproductive cycle studied, and their interactions with the environment examined, so that all the possible forms, and the range of shapes and forms seen, are well characterized. None of these critical pieces of analysis have been done on the features Folk sees, and thus their true nature is completely unknown. They have as much likelihood, if not more, of being inorganic as biological.
I’ll offer a single example of how misleading shape and size can be. Figure 1 is a scanning electron microscope image exhibiting shapes similar to those shown by Folk; a set of little sausages arranged on a rough surface exhibiting some “globby” characteristics. In this case, however, we know what these features really are. They are silica vapor deposits on the rear side of a small glass bead fired at nearly 10 kilometers a second into aerogel during an experiment simulating the capture of micrometeorites in space, prepared for NASA’s upcoming Stardust mission (D. Brownlee, pers. comm.). If we were to use Folk’s definition of “nannobacteria,” then these must be living creatures that were born, lived, died and were fossilized in nanoseconds, in a vacuum, while undergoing accelerations of about 50,000 g.
Hopefully the distinction between Folk’s interpretive method and that used to evaluate Figure 1 is clear. The nature of the features seen in Figure 1 is known not from their shape and size, but from our knowledge of the conditions under which they formed. In contrast, there is so little supplemental data provided with the features Folk sees that their origin must remain enigmatic. As noted recently by one microbiologist, better to call them “nannothingies” than “nannobacteria,” acknowledging our lack of data (K. Nealson, pers. comm.).
So what are these “nannothingies”? In fact, they COULD be living organisms (or their remains) in some cases. Some very small parasitic organisms have been isolated and cultured (e.g., Kajander et al. 1994), and may deserve the name nannobacteria. However, most of the things Folk sees are vastly smaller than those cultured organisms or the “ultramicrobacteria” originally described by Torella and Morita (1981), which are all around 0.2 micrometers in size and whose viability is extremely difficult to prove. Many of the features Folk describes are 20 to 30 nanometers across, a factor of 10 smaller than the cultured organisms (Figure 2). They are, in fact, probably too small to be independently living, viable organisms, simply because the realities of chemistry set limits of size on the machinery of life. Given minimal estimates of the thickness of a rigid cell wall necessary for preservation of their form, many reported “nannothingies,” including the Martian “nannofossils” reported by McKay et al. (1996), are too small to contain a single molecule of ribosomal RNA! In order to attribute life to these forms, one must invoke an entire new chemistry of life, where cell walls are atomically thin, energy is made available without enzyme catalysis, and reproduction requires no chemical blueprint.
A possible alternative explanation for many of the forms Folk sees lies in the environments where they are found. Many of Folk’s images come from environments where chemical disequilibrium is, or was, actively promoting the exchange of elements from one phase to another, where the mobility of these elements allows the production of new minerals from old as conditions change. He sees his forms where metal is oxidizing, but does not consider that newly formed mineral grains such as iron oxides (rust), aluminum oxides or copper oxides (green stuff) often have filamentous or bulbous shapes. He sees his forms in carbonate assemblages where minerals are actively precipitating from a fluid rich in organics, but does not consider that filamentous and bulbous shapes are common in minerals formed during hydration or dehydration reactions. He sees his forms within meteorites that have been severely aqueously altered,where a virtual cornucopia of phases, minerals and amorphous materials have formed, dissolved and reacted. Unfortunately, the significant possibility that many “nannothingies” are nothing more than non-biological precipitates has not been explored by Folk.
In summary, while the microscopic features Folk has photographed certainly exist, to date there is no evidence that they are life forms or their remnants. Folk’s failure to critically discuss plausible alternatives to his “nannobacteria” hypothesis, such as secondary mineral growth, indicates his apparent unwillingness to accept the burden of proof in support of his case. As a result, his interpretations must remain nothing more than conjecture.