Bacterial and Mineral Associations at the Saprolite Interface with Quartz Diorite Bedrock, Luquillo Experimental Forest, Puerto Rico

The Luquillo Experimental Forest in eastern Puerto Rico is underlain by quartz diorite bedrock which, due to warm air temperatures (22 °C) and high annual precipitation (4200 mm), has been transformed into a 5- to 8-m-thick layer of saprolite beneath the forest soil. Microbial abundance increased 100-fold within a distance of 50 cm immediately above the saprolite–bedrock interface. We hypothesize that with low organic matter availability at this depth, microbial growth would depend on other electron donors such as Fe(II) from bedrock minerals. The objective of this study was to determine whether Fe(II)-oxidizing lithoautotrophs (FeOB) are present at the saprolite-bedrock interface and their proportion within the total community . A second objective was to gain understanding about the involvement of FeOB in bedrock weathering. Community analysis by 16S rRNA clone libraries showed a diverse community at 4.6 m depth composed of gamma-proteobacteria, including the known FeOB, Acidithiobacillus ferrooxidans; acidobacteria; chloroflexi; and firmicutes. Confocal scanning laser microscopy of formaldehyde-preserved samples showed bacterial cells associating with smaller, more weathered minerals and mineral aggregates that were not light-transparent. Quartz particles, distinguishable because of their transparency, were virtually devoid of bacteria. At a higher magnification, environmental scanning electron microscopy showed bacteria were located on weathered particles and they were nearly always located with a meshwork of tube-like minerals about 1000 nm in length and 50 nm wide. Transmission electron microscopy and energy dispersive X-ray spectroscopy of these minerals showed a distinct tube morphology that consisted mainly of aluminum, silicon, and oxygen and had diffraction patterns consistent with the secondary mineral halloysite. Consistent microbial association with the halloysite-coated minerals suggests their involvement in secondary mineral transformations and saprolite formation.