Microbial Acid Sulfate Weathering of Basaltic Rocks: Implication for Enzymatic Reactions

Abstract

Two basaltic rocks were reacted in acid sulfate and non-acid sulfate solutions with an initial pH value of 2 in the presence and absence of A. ferrooxidans to determine if basalt dissolution can support the metabolically active growth of A. ferrooxidans. Similar elemental release rates (R_Si, R_Ca, R_Mg) calculated for both biotic and abiotic experiments suggest rather a negligible microbial impact on the dissolution of basaltic rocks within the acid sulfate solution. Nevertheless, in contrast with the abiotic experiments, measurements of remarkably high concentration of Fe(III)_aq in microbial experiments confirmed the bacterial metabolism. Moreover, detected cell division and increasing total cell numbers with the extent of the experiments provide further evidence for the growth of metabolically active A. ferrooxidans during the dissolution of the rocks. Formation of jarosite ((K, Na, H₃O)Fe₃(SO₄)₂(OH)₆) only in the biotic experiments is attributed to the microbially catalyzed Fe(II)_aq oxidation. Overall, our results showed that acidic solutions that reacted with basaltic rocks can sustain the growth of Fe(II)_aq oxidizing bacteria. Furthermore, identification of jarosite only in the biotic experiments emphasizes the enzymatic Fe(II) oxidation as the key step for its formation during basalt weathering at acid conditions, highlighting its biosignature potential on Earth and Earth-like planets (e.g., Mars).