High-density biomass as a substrate for stimulating biosulfidogenesis in the deep layer of stratified acidic pit lakes

Accumulation of harmful metal(loid)s in acidic pit lakes (APLs) is a serious environmental issue in mining districts. This lab-based study evaluated a novel method to stimulate dissimilatory sulfate reduction to promote the formation of sparingly soluble metal(loid)-sulfide minerals in the permanently stratified deep layer of Cueva de la Mora (CM), an APL in the Iberian Pyrite Belt in Spain. Solid-phase biomass was selected because it can be pressed into high-density forms that are dense enough to settle into the deep layer of a lake. This “direct delivery” of electron donor overcomes the current “indirect method” to stimulate algae growth in the upper layer and wait for algae to die and settle into the deep layer. We added the microalgae Coccomyxa onubensis (predominant in CM), Euglena gracilis (another acid-tolerant microalgae), and Lemna obscura (duckweed), as well as model biocomponents (amino acids, monosaccharides, and lipids) as substrates to stimulate biological sulfide production (biosulfidogenesis). We found that compared with biocomponents, high-density biomass required a shorter lag time before it was utilized. Temporal patterns of the production of sulfide and volatile fatty acids with high-density biomass were similar to patterns with amino acids, suggesting that amino acids may be the preferred substrate among the biocomponent monomers for the microbial community. Biosulfidogenesis led to the complete removal of metal(loid)s (Zn and As) contaminants from solution, mimicking the chemical composition of the deep layer. Desulfosporosinus, the only acid-tolerant sulfate-reducing bacteria (SRB) identified in situ, was significantly enriched in the laboratory setup and presumably responsible for biosulfidogenesis.