BACTERIAL COMMUNITY STRUCTURES ARE SPATIALLY DISTINCT IN TROPICAL SAPROLITE NEAR THE GRANODIORITE WEATHERING FRONT

Microbial activity at the interface between bedrock and overlying regolith can promote mineral weathering and contribute to early stages of soil formation. Here we describe the structures of bacterial communities in deep saprolite (4.5-4.9 m) sampled from two cores one meter apart on a ridgetop (LG-1 site) in the Rio Icacos Watershed, Luquillo Experimental Forest in Eastern Puerto Rico. In previous studies of microbial distribution with depth, microbial abundance was observed to increase immediately above the saprock (slightly weathered, spheroidally fractured bedrock) at 5 m, relative to microbial densities observed between 0.5- and 4.5 m. The granodiorite is of early tertiary age and is dominated by quartz and plagioclase with lesser amounts of biotite, hornblende, and K-feldspar, which weather predominantly to kaolinite, halloysite, and goethite. We propose that microbial biofilm communities on minerals of deep saprolite contribute to weathering processes. Aseptically collected subsamples from interior portions of core segments from depths 4.5, 4.7, and 4.9 m were used to construct 16S rRNA clone libraries from PCR-amplified bacterial DNA. No archaeal PCR products were obtained. Sequence analyses indicated a wide diversity of source bacteria with representatives from ten phyla. Across all clone libraries, Acidobacteria and Proteobacteria were major contributors. Acidobacteria were dominant in the LG-1 S core, with 29, 50, and 38% at 4.5, 4.7, and 4.9 m respectively. Gamma-proteobacterial sequences dominated libraries from the two lower depths of the LG-1 N core (70 and 53% at 4.7 and 4.9 m respectively). Library comparison analysis using the LibShuff program indicated that all communities were significantly different from each other except for the two 4.5 m depth core samples . Differences in 16S rRNA libraries imply that bacterial communities in unsaturated saprolite are spatially segregated and that development of bacterial community structure reflects micro-heterogeneity in mineral composition and nutrient availability.