COMPARATIVE METAGENOMIC AND CULTURE-BASED ANALYSIS OF EXTREMELY ACIDOPHILIC ACIDITHIOBACILLUS SPP. FROM SULFIDIC CAVE BIOFILMS

Caves form by sulfuric acid speleogenesis when anoxic fluids bearing hydrogen sulfide (H₂S) encounter air-filled pores or oxygenated groundwater. Substantial cave enlargement occurs above the water table where hydrogen sulfide degasses into the cave atmosphere and oxidizes to sulfuric acid on the moist cave walls. This sulfide gas represents the primary energy source for extreme acidophiles that colonize subaerial surfaces, where they produce pendulous, highly acidic (pH 0-2) “snottite” biofilms and contribute to cave formation by rapidly oxidizing sulfide to sulfuric acid.

The numerically dominant microorganisms in these biofilms are members of the genus Acidithiobacillus. Previous research has shown that sulfidic caves in Italy contain diverse strains of At. thiooxidans that have distinct sulfur oxidation pathways and exhibit growth at pH values as low as 0.2 (Jones et al., 2016). However, detailed physiological studies of these strains have not yet been performed. We are evaluating the pH tolerance and sulfur utilization capabilities of extremely acidophilic At. thiooxidans from the Frasassi and Acquasanta cave systems, and relating observed physiological differences to genomic properties of the strains. Additionally, snottites from sulfidic caves in Mexico are also dominated by Acidithiobacillus spp., but contain different microbial assemblages (D’Auria et al., 2018). We used single molecule long-read sequencing (Pacific Biosciences) to generate a metagenomic dataset from a snottite sample collected from Cueva de Villa Luz in 2013. Based on this metagenomic evidence, Acidithiobacillus spp. from Cueva de Villa Luz are autotrophs with pathways for inorganic sulfur compound oxidation that include a partial SOX system, heterodisulfide reductase, and multiple sulfide:quinone oxidoreductases. We will also present results on new Acidithiobacillus strains isolated on elemental sulfur media from Cueva de Villa Luz and describe how Acidithiobacillus spp. from this North American cave system differ from snottite-forming At. thiooxidans from the Italian caves.

Jones, D. S., Schaperdoth, I., & Macalady, J. L. (2016). ISME J, v. 10, p. 2879.

D'Auria, G., Artacho, A., Rojas, R. A., Bautista, J. S., Méndez, R., Gamboa, M. T., Gamboa, J. R., Gómez-Cruz, R. (2018). Genes, 9(1), 55. doi: 10.3390/genes9010055.