Arsenic Accumulation in Microbial Biomass and the Interpretation of Signals of Early Arsenic-Based Metabolisms

Area of Science:

• Geobiology and Astrobiology
• Early Earth Environments
• Microbial Metabolism

Background:

• Ancient microbialites (~3.5 Ga) contain carbonaceous particles enriched in arsenic (As), potentially tracing early microbial arsenic cycling.
• The role of biological activity versus passive postmortem binding in arsenic accumulation within these ancient environments remains uncertain.
• Volcanically influenced, arsenic-rich settings are key to understanding early microbial-arsenic interactions.

Purpose of the Study:

• To investigate the mechanisms of arsenic accumulation in microbial biofilms, distinguishing between biological activity and passive binding.
• To evaluate arsenic speciation, concentration, and detectability in both active and heat-killed biofilms.
• To assess the potential for reconstructing early microbial interactions with arsenic in geological records.

Main Methods:

• Exposure of active and autoclaved biofilms (cyanobacteria, anoxygenic photosynthetic microbes) to environmentally relevant arsenic concentrations (As(III) and As(V)).
• Analysis of arsenic concentrations and speciation within biomass using X-ray based spectroscopy techniques (X-ray fluorescence, electron probe micro-analyzer with wavelength-dispersive spectrometry).
• Genomic and metagenomic analysis to identify genes involved in arsenic metabolism.

Main Results:

• Microbial biomass accumulates arsenic in a concentration-dependent manner, with a preference for As(V) over As(III).
• Autoclaved (inactive) biomass shows stronger arsenic accumulation than active biomass, indicating passive binding is significant.
• Active biofilms actively metabolize arsenic, oxidizing and reducing it, while inactive biofilms primarily bind arsenic.
• Arsenic enrichments are detectable using standard geological analytical techniques, enabling fossil analysis.

Conclusions:

• Both active biological processes and passive postmortem binding contribute to arsenic accumulation in microbial biomass.
• The strong binding of arsenic by dead biomass suggests it can act as a significant sink for arsenic in geological settings.
• These findings provide a framework for interpreting arsenic enrichments in ancient microbialites as evidence of past microbial activity and diagenetic processes.