Minerals as Ecosystems in the Nutrient-Limited Subsurface

Abstract:

A majority of microorganisms in dark, nutrient-poor, subsurface habitats live in biofilms attached to mineral surfaces. As a result, microorganisms have likely adapted and evolved to take advantage of specific minerals that support a variety of biogeochemical processes. Using biofilm reactors inoculated with a diverse microbial biomat from a sulfidic cave, we found that specific microorganisms colonize specific minerals according to their metabolic/nutritional requirements as well as their environmental tolerances in order to increase survival in unfavorable environments.

In a neutral pH, carbon (C) and phosphate (P)-limited (unfavorable) reactor, highly-buffering carbonates were colonized by nearly identical communities of neutrophilic sulfur-oxidizing (acid-generating) bacteria (SOB), which intensely corroded the carbonates. Non-buffering quartz was colonized by acid-generating acidophiles, while feldspars (containing potentially toxic aluminum) were colonized largely by aluminotolerant microbes. The SOB Thiothrix unzii demonstrated a clear affinity for basalt, and it is commonly found on basaltic rocks in mid-ocean ridge environments.

In an identical reactor amended with acetate, heterotrophic sulfur-reducing bacteria (SRB) dominated on most surfaces. The metabolism of the SRB causes an increase in both alkalinity and pH, nearly eliminating the need for buffering minerals and resulting in carbonate precipitation. However, SRB were not dominant on quartz, which was again colonized by acidophiles and acid-tolerant microorganisms or basalt which hosted a complex consortium similar to those found on natural basalt outcrops. These organisms have been shown to weather basalts to access mineral nutrients, especially when provided a carbon source.

In both the C&P-limited and acetate-amended reactors significantly greater biomass accumulated on minerals with high P content. When abundant P was added and the pH was buffered to 8.3, mineral selectivity was eliminated and every surface accumulated similar total biomass and nearly identical communities (primarily SOB and alkalitrophs). These experiments suggest that in unfavorable environments microbial survival, growth, and community structure is closely linked to mineral chemistry and reactions at the microbe/mineral interface.