THE MICROBE-MINERAL ASSOCIATION: A FUNDAMENTAL CONTROL ON BIOMASS, COMMUNITY MEMBERSHIP, METABOLISM, AND GEOCHEMISTRY IN KARST SYSTEMS

In the typically nutrient-limited (CNP) subsurface, microbial communities depend on mineral surfaces as the primary source of limiting nutrients. These communities exert highly focused control on local geochemistry, so the individual mineral surfaces should select for populations that can take advantage of the mineral growth factors. Using laboratory and field biofilm reactors, we tested the hypothesis that specific microorganisms colonize specific minerals according to their metabolic and nutritional requirements as well as their environmental tolerances.

In a CNP-limited biofilm reactor inoculated with microbial biomat collected from a sulfidic cave stream, pH-buffering carbonates were colonized by nearly identical communities of primarily neutrophilic S-oxidizing (acid-generating) bacteria (SOB) and the carbonates were intensely corroded. Non-buffering quartz, in contrast, was colonized by a diverse community of acidophiles. Feldspars were colonized largely by aluminotolerant Gram-positive microorganisms while basalt was colonized by Thiothrix unzii (commonly found in mid-ocean ridge environments).

With the addition of acetate, heterotrophic S-reducing bacteria (SRB) outcompeted the autotrophic SOB. The metabolism of the SRB increases alkalinity and pH, resulting in carbonate precipitation; eliminating selectivity between buffering carbonates and non-buffering aluminosilicates. However, quartz was still colonized by a unique community of acidophiles and acid-tolerant microorganisms, while basalt was colonized primarily by Actinobacteria that have been previously shown to effectively weather basalts in order to access mineral bound nutrients, especially when provided a carbon source. In both the CNP limited and acetate-amended reactors significantly greater biomass accumulated on minerals with high P content. When abundant P was added to the medium at pH 8.3 biofilms on all surfaces were nearly identical.

These experiments reveal that microbial survival and biofilm community structure in unfavorable environments is highly heterogeneous and closely linked to mineral chemistry and surface reactions. Subsurface microbial communities have evolved in contact with minerals; this association is fundamental to understanding subsurface microbe-rock-water reactions.