Halloysite Nanotubes Associated with Bacterial Cells in Deep Tropical Saprolite Formed From Granodiorite Weathering.

Microbes in unsaturated subsurface regions of the critical zone tend to receive less research attention than microbes in surface soils or in deeper saturated zones because of their relatively lower densities and presumed lower activities. Here we describe a subsurface environment that supports the formation of natural nanotubes of halloysite. In previous studies of 5-m soil profiles developed from granodiorite in the Luquillo Experimental Forest in eastern Puerto Rico, we observed increased microbial abundance at the base of the profile in unsaturated saprolite samples immediately above saprock (slightly weathered, spheroidally fractured bedrock).  The granodiorite is of early tertiary age and is dominated by quartz and plagioclase with lesser amounts of biotite, hornblende, and K-feldspar. These primary minerals weather predominantly to kaolinite, halloysite, and goethite, and we propose that microorganisms in the deep saprolite contribute to weathering and the formation of natural nanotubes of halloysite. Subsamples from interior portions of deep core segments were collected and stored aseptically for environmental scanning electron microscopy (ESEM), enrichment cultures, and DNA analysis. Bacterial cell-like structures were observed on the edges and interiors of biotite particles undergoing weathering. We propose that the halloysite particles precipitate from dissolved Al and Si using the cell surfaces as nucleation sites. 16S rRNA clone libraries were constructed from DNA extracts obtained from the same sample depths used for ESEM. DNA sequences of clones indicated a wide diversity of source bacteria, including, Acidithiobacillus ferrooxidans which has been reported to cause release of metal cations into solution. Enrichment cultures also yielded strains of Stenotrophomonas maltophilia. This research can lead to better understanding of mechanisms of natural nanotube formation in the unsaturated subsurface.