Bacterial Associations with Weathering Minerals at the Regolith-Bedrock Interface, Luquillo Experimental Forest, Puerto Rico

Abstract

Microbe-mineral associations in regolith overlying granodiorite bedrock (4.6–4.9 m depth) from the Luquillo Experimental Forest, Puerto Rico, were imaged with confocal scanning laser microscopy at a novel scale of 400X magnification. After adding BacLight™ stain, proportionally more surface area of minerals (quartz, biotite, and mixed opaque kaolinite/goethite) emitted fluorescence from cell-impermeant propidium iodide than from cell-permeant SYTO 9, which suggested greater coverage of minerals by extracellular DNA or DNA in non-intact cells than by intact cells. Microscopic observations of predominantly non-intact cell material in deep saprolite were consistent with the abundance of rRNA sequences related to heterotrophic bacteria in clone libraries prepared from community DNA. A few sequences were affiliated with bacteria recognized to produce siderophores, oxidize Fe(II), or fix N₂. Bacterial DNA in deep regolith from two boreholes 1.5 m apart yielded libraries with high diversity and taxa specific for each borehole. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental files.

Keywords:

• regolith, confocal microscopy, iron oxidation, saprolite, 16S rRNA

Acknowledgments

We thank J. Williams for field support; N. Zembower, E. Kunze, and S. Magargee from the Huck Institute's Cytometry lab for assistance with the CLSM; and D. Grove at the Genomics Core Facility. Research was funded by DOE grant DE-FG02-05ER15675, with some logistical support contributed by the NSF-supported Luquillo Critical Zone Observatory (EAR 0722476). M. L. Minyard acknowledges fellowship support from the Penn State Biogeochemical Research Initiative for Education (BRIE, NSF-IGERT grant DGE-9972759) and the DoD Science Mathematics and Research for Transformation (SMART) Program.