BIOFILMS AND CALCITE PRECIPITATION WITHIN A CAVE AND ESTEVELLE SPRING IN THE UPPER FLORIDAN AQUIFER – CITRUS COUNTY, FLORIDA

Iron oxidizing bacteria are ubiquitous to the surface realm. We present herein an example from a cave in west-central Florida, Thornton’s Cave, where bacteria morphologically similar to Leptothrix spp. manifest on the water surface as thin biofilms and on the cave walls as fibrous membranes. Some membranes have crystalline overgrowths, particularly at the mean water table, giving a “corn flake” appearance.

Thornton’s Cave comprises a maze of 250 m of shallow, orange-stained, fracture-oriented passages formed within the upper third of the Ocala Limestone. The cave behaves as an estevelle. During normal flow, water rises from a narrow pool at least 35 m deep and discharges into a cypress swamp adjacent to the Withlacoochee River. During high flow, water from the river recharges the cave and the upper Floridan aquifer. During low flow in the spring of 2009, Fe_diss ranged between 3-4 mg/L. SO₄ and Cl at this same time exceeded 20 mg/L and 10 mg/L, respectively. Following the onset of summer rainfall, TDS levels decreased precipitously.

SEM images of the biofilms and fibrous membranes reveal a mat of hollow tubes of biological origin that are 1 μm in diameter. SEM images of the corn flakes illustrate that this same mat of tubes are overlain by and intercalated with rhomboidal crystals. EDS and XRD demonstrate the mineralogy of the tube sheaths and the crystal overgrowth to be FeO_x and CaCO₃, respectively. It appears that the biofilm acts as a nucleation site for calcite precipitation.

Two calcite tablets, one exposed directly to the cave water and the second encased in a chamber with a 0.45 μm filter, were left in the cave for almost 5 months during the winter of 2007-08. Neither sample lost weight during the course of the experiment, nor were there any etchings or borings detected using the SEM in the sample exposed to microbial activity. In fact, SEM images of this unfiltered sample following fixation and dehydration document microbial filaments and calcite overgrowths.