UC Berkeley

Understanding how microorganisms contribute to nutrient transformations within their community is critical to prediction of overall ecosystem function, and thus is a major goal of microbial ecology. Communities of relatively tractable complexity provide a unique opportunity to study the distribution of metabolic characteristics amongst microorganisms and how those characteristics subscribe diverse ecological functions to co-occurring, and often closely related, species.

The microbial communities present in the low-pH, metal-rich environment of the acid mine drainage (AMD) system in Richmond Mine at Iron Mountain, CA constitute a model microbial community due to their relatively low diversity and extensive characterization over the preceding fifteen years. Here, chemoautotrophic biofilms form at the air-solution interface of the AMD solution, and carbon is fixed using energy derived from the oxidation of iron and sulfur species released from the dissolution of mineral sulfides.

The chemoautotrophic microbial communities that develop at the air-solution interface sink to the underlying sediment and degrade under microaerobic and anaerobic conditions. A transition from Bacteria- to Archaea-dominated communities coincides with this event. The Archaea identified in sunken biofilms are from the class Thermoplasmata, and in some cases, the highly divergent ARMAN nanoarchaeal lineage. Comparative community proteomic analyses showed a persistence of bacterial proteins in sunken biofilms, and evidence for amino acid modifications due to acid hydrolysis. Given the low representation of bacterial cells in sunken biofilms based on microscopy, hydrolyzed bacterial proteins were inferred to represent a population of lysed cells. These findings indicate dominance of acidophilic Archaea in degrading biofilms, and suggest that they play key roles in anaerobic nutrient cycling at low pH.

Biofilm submersion was recapitulated in microcosm experiments in which floating AMD microbial biofilms were submerged, amended with either ¹⁵NH₄⁺ or deuterium oxide (²H₂O), and proteomic stable isotope probing (protein-SIP) used to trace isotope incorporation into newly synthesized proteins of different community members. In ¹⁵N-ammonia amended experiments, different ¹⁴N/¹⁵N atom% values reflect distinct modes of nitrogen acquisition, since ¹⁴N is ultimately derived from extant organic biomass and ¹⁵N is derived from inorganic ammonia provided in the media. There were relatively few ¹⁵N-enriched archaeal proteins and all showed low ¹⁵N atom% enrichment in anaerobic iron-reducing, aerobic iron-reducing, and aerobic iron-oxidizing environments. These results are consistent with Archaea synthesizing protein using the ¹⁴N derived from recycled biomolecules. This conclusion is further supported by results of parallel experiments using ²H₂O, in which extensive archaeal protein synthesis was detected. In contrast, the bacterial species showed little protein synthesis when incubated in ²H₂O. The nearly exclusive ability of Archaea to synthesize proteins using ²H₂O may be due to archaeal heterotrophy (whereby Archaea offset deleterious effects of ²H by accessing ¹H generated by respiration of organic compounds) or differences in how archaeal versus bacterial membranes (and their associated mechanisms of energy conservation) respond to ²H₂O. In biofilms incubated with ¹⁵N-ammonium, bacteria synthesized proteins to different extents, with Sulfobacillus spp. synthesizing protein almost exclusively under iron-reducing conditions whereas Leptospirillum spp. synthesized protein in all conditions, with a clear emphasis on iron-oxidation metabolisms in the presence of Fe²⁺ and oxygen. These findings highlight distinct roles for Sulfobacillus vs. Leptospirillum in iron cycling. The greatest extent of ¹⁵N atom incorporation was detected in proteins of Leptospirillum, whereas Sulfobacillus proteins had a low extent of ¹⁵N incorporation, consistent with an autotrophic metabolism for Leptospirillum and heterotrophic metabolism for Sulfobacillus.

The role of Sulfobacillus organisms in biogeochemical cycling is poorly understood. The diversity of energy conservation and central carbon metabolism within this genus was analyzed using published Sulfobacillus genomes as well as five draft genomes of Sulfobacillus reconstructed by cultivation-independent sequencing of biofilms sampled from the Richmond Mine (AMDSBA1-5). Three of the newly sequenced species (AMDSBA1, AMDSBA2, and AMDSBA3) have no cultured representatives, and AMDSBA5 and AMDSBA4 represent strains of S. thermosulfidooxidans and S. benefaciens, respectively. Genomes were replete with pathways of sulfur oxidation, however the presence of enzymes involved with these pathways (and their copy numbers) varied considerably across the genus. Furthermore, several enzymes with putative sulfur and sulfur-compound reduction were identified, perhaps lending previously unknown anaerobic sulfur reduction capacity to Sulfobacillus species. Central carbon degradation pathways in Sulfobacillus lineages varied, with S. thermosulfidooxidans likely favoring the pentose phosphate pathway and lineages of S. acidophilus, AMDSBA1, AMDSBA2, AMDSBA3, and AMDSBA4 capable of using the semi-phosporylative Entner-Doudoroff pathway. Proteins involved in dissimilatory nitrate reduction were limited to AMDSBA3, and amongst AMDSBA genomes, only AMDSBA5 encoded nickel-iron hydrogenase proteins. AMDSBA4 (S. benefaciens) is unusual in that its electron transport chain includes a bc complex, a unique cytochrome c oxidase, and an additional succinate dehydrogenase. It is also the only Sulfobacillus species with putative carboxysome proteins. Overall, the results demonstrate diverse ecological strategies for species of Sulfobacillus within the Richmond Mine.

Metabolomics methods lag behind other omics technologies due to a wide range of experimental complexities often associated with the environmental matrix. We identified key metabolites associated with acidophilic and metal-tolerant microorganisms using stable isotope labeling coupled with untargeted, high-resolution mass spectrometry. Initially, >3,500 metabolic features were observed in extracts of AMD biofilms, although the molecular identity of these features remained unclear. Stable isotope labeling improved chemical formula prediction by >50% for larger metabolites (>250 atomic mass units), many of which were unrepresented in metabolic databases and may represent novel compounds. Taurine and hydroxyectoine were identified and likely provide protection from osmotic stress in the biofilms. Community genomic, transcriptomic and proteomic data were integrated to implicate fungi in taurine metabolism. Leptospirillum group II bacteria decrease production of ectoine and hydroxyectoine as biofilms mature, suggesting that biofilm structure provides some resistance to high metal and proton concentrations. The combination of taurine, ectoine, and hydroxyectoine may also constitute a sulfur, nitrogen, and carbon currency in the communities.

The genomic, proteomic, and metabolomic characterizations of the Richmond Mine microbial communities not only further our understanding of the physiology of acidophilic organisms but also help elucidate their functional roles within the ecosystem as a whole. Archaea dominate in anaerobic, submerged biofilms, where they synthesize protein using organic nitrogen derived from the degrading biofilm. Sulfobacillus are implicated in sulfur transformations, and encode diverse complements of proteins involved in sulfur, nitrate and hydrogen metabolisms, suggesting key niche differentiation within this genus. Metabolites that likely serve as organic nutrient sources for a variety of organisms were identified though use of stable istope labeling techniques. The development and integration of novel ‘omics’ based technologies extends our knowledge of the Richmond Mine microbial communities and will ultimately help illuminate microbial contributions to ecosystem function in more complex environments.