BIOGEOCHEMISTRY OF OXALATE IN THE ANTARCTIC CRYPTOENDOLITHIC LICHEN-DOMINATED COMMUNITY

Cryptoendolithic (hidden in rock) lichen-dominated microbial communiti es from the Ross Desert of Antarctica were shown to produce oxalate (o xalic acid). Oxalate increased mineral dissolution, which provides nut rients, creates characteristic weathering patterns, and may ultimately influence the biological residence time of the community. Oxalate was the only organic acid detectable by HPLC, and its presence was verifi ed by GC/MS. Community photosynthetic metabolism was involved in oxala te production since rates of C-14-oxalate production from (CO2)-C-14 w ere higher in light than in dark incubations. Flaking of the sandstone at the level of the lichen-dominated zone a few millimeters beneath t he rock surface can be explained by dissolution of the sandstone cemen t, which was enhanced by Si, Fe, and Al oxalate complex formation. Add ed oxalate was observed to increase the solubility of Si, Fe, Al, P, a nd K. Oxalate's ability to form soluble trivalent metal-oxalate comple xes correlated with the observed order of metal oxide depletion from t he lichen-dominated zone (Mn > Fe > Al). Thermodynamic calculations pr edict that Fe oxalate complex formation mobilizes amorphous Fe oxides (feriihydrite) in the lichen-dominated zone, and where oxalate is depl eted, ferrihydrite should precipitate. Hematite, a more crystalline Fe oxide, should remain solid at in situ oxalate concentrations. Oxalate was not a carbon source for the indigenous heterotrophs, but the micr obiota were involved in oxalate mineralization to CO2, since oxalate m ineralization was reduced in poisoned incubations. Photooxidation of o xalate to CO2 coupled with photoreduction of Fe(III) may be responsibl e for oxalate removal in situ, since rates of C-14-oxalate mineralizat ion in dark incubations were at least 50% lower than those in the ligh t. Removal of oxalate from Si, Fe, and Al complexes should allow free dissolved Si, Fe, and Al to precipitate as amorphous silicates and met al oxides. This may explain increased siliceous crust (rock varnish or desert varnish) formation near the surface of colonized rocks were li ght intensity is greatest.