Restrictions on the Production of Multi-Wall Carbon Nanotubes and Nanofibers by Gallionella sp.

ABSTRACT

The enzymatic oxidization of dissolved Fe(II) to Fe(III) by neutrophilic Fe-oxidizing bacteria plays a significant role in biological cycling of iron by inducing the precipitation of Fe(III) oxyhydroxide in aqueous environments. Among the diverse neutrophilic Fe-oxidizing bacteria, the genus Gallionella has received wide attention for its production of unique twisted extracellular stalks. Hallberg and Tai (2014) recently reported the detection of multi-wall carbon nanotubes on the twisted-stalks, and they viewed those carbon nanotubes as being biologically produced by Gallionella. We scrutinized Gallionella-produced biofilms collected from natural environments by scanning electron microscopy and high-resolution transmission electron microscopy. Ferrihydrite and lepidocrocite were the only nano-scaled minerals observed on the stalk, while there were nanometer-sized sheet-like graphitic contaminants on the grid in the vicinity of the sample which showed the same morphology as Hallberg and Tai (2014) observed. Moreover, similar materials on an empty grid and a grid loaded with randomly selected synthesized materials were also observed. Based on the current knowledge of carbon nanotube syntheses, none of the three known synthesizing methods including root-growth, rolling-up and bottom-up could be biochemically produced by any life because of the significant kinetic and energy obstacles. The carbon nanomaterials reported by Hallberg and Tai (2014) were clearly contaminations from amorphous carbon film on the grids for holding samples for transmission electron microscopic observations.

KEYWORDS:

• carbon nanotube
• Gallionella
• grids for transmission electron microscopy
• sheet-like carbonaceous contaminants

Acknowledgments

We thank Mr. Tan-Wei Li of the University of Science and Technology of China and Mr. Frankie Chan of the University of Hong Kong for their kind help in electron microscopic analyses.

Funding

This study was supported by Hong Kong Research Grants Council (HKU 702913P).