IRON PHOSPHATE PRECIPITATION BY EPILITHIC MICROBIAL BIOFILMS IN ARCTIC CANADA

On Ellesmere Island, in the Canadian Arctic, dark-colored biofilms proliferate on moist surfaces, including exposed granodiorite outcrops. Transmission electron microscopy of these biofilms indicates that complex epilithic microbial communities developed, consisting of cyanobacteria and fungi symbiotically associated in a lichen, along with a consortium of free-living algae and gram-negative bacteria. The epilithic cyanobacteria and bacteria were shown to extensively precipitate phosphatic minerals, ranging from relatively large polyphosphate granules (approximately 250 nm in diameter) within their cytoplasmic membranes to smaller iron phosphate grains (generally less than 50 nm in diameter) associated with the periplasmic space and encompassing capsule. Complete encrustation of some bacterial cells by the iron phosphates was observed. Energy-dispersive X-ray spectroscopy suggested that these grains are compositionally similar to the mineral strengite (FePO4 . 2H2O). This study clearly indicates that the Arctic supports a thriving microbial community that influences the biogeochemical cycling of PO4 in an environment of low nutrient availability. Nutritional requirements by the microorganisms were actively maintained through a relatively closed recycling mechanism, which restricted the immediate loss of phosphorus from the biofilm.