Anaerobic microorganisms alter heavy oil chemistry by oxygenation and sulfurization.

Over 945 days in anoxic microcosms with Pitch Lake heavy oil, sulfur-containing chemistry intensified, with CHOS-class formulas increasing and average intensity-weighted m/z rising from 344 to 366.

• Why it matters: Molecular-level sulfur evolution during anoxic heavy-oil transformation is unresolved, limiting understanding of biodegradation, persistence of recalcitrant compounds, and fate of organic sulfur species in reservoirs.
• What they did: Long-term anaerobic microcosms were incubated and analyzed by FT-ICR MS and microbial community profiling, detecting 4,499 molecular formulas and tracking compositional and microbiota shifts over time.
• The result: Samples clustered by time with parallel microbial community changes toward anaerobic, sulfur-associated taxa, and correlations linked community shifts to increasing oxidized sulfur compounds, especially CHSO3− and CHSO4− classes.