Abstract
A sandstone outcrop exposed to freshwater seepage supports a diverse assemblage of photosynthetic microbes. Dominant taxa are two cyanophytes (Oscillatoria sp., Rivularia sp.) and a unicellular green alga (Palmellococcus sp.). Less abundant taxa include a filamentous green alga, Microspora, and the desmid Cosmarium. Biologic activity is evidenced by measured levels of chlorophyll and lipids. Bioassay methods confirm the ability of these microbes to dissolve and metabolize Fe from ferruginous minerals. Chromatographic analysis reveals citric acid as the likely chelating agent; this low molecular weight organic acid is detectable in interstitial fluid in the sandstone, measured as 0.0756 mg/mL. Bioassays using a model organism, Synechoccus elongates strain UTEX 650, show that Fe availability varies among different ferruginous minerals. In decreasing order of Fe availability: magnetite > limonite > biotite > siderite > hematite. Biotite was selected for detailed study because it is the most abundant iron-bearing mineral in the sandstone. SEM images support the microbiologic evidence, showing weathering of biotite compared to relatively undamaged grains of other silicate minerals.
Highlights
Relationships between living organisms and the lithosphere have received considerable attention from geoscientists because organisms are known to have occurred in association with rocks and sediments for more than three billion years of the Earth’s history
The rock surface is saturated with freshwater seepage from overlying unconsolidated sediment, an environment that provides a habitat for a variety of surface-dwelling microorganisms, resulting in a nearly continuous biofilm
This study is significant because it demonstrates the ability of epilithic algae to obtain Fe from their mineral substrate; chelating abilities of bacteria, lichen, and fungi are well-documented, but few data have been available for algae
Summary
Relationships between living organisms and the lithosphere have received considerable attention from geoscientists because organisms are known to have occurred in association with rocks and sediments for more than three billion years of the Earth’s history. This report describes observations and laboratory investigations of surface-dwelling (epilithic) microbes that are able to dissolve iron from ferruginous minerals, and to metabolize this element. The studies focus on the weathering of biotite, the dominant iron-bearing mineral in the sandstone bedrock at the study site, located in northwest Washington, USA. At this locality, the rock surface is saturated with freshwater seepage from overlying unconsolidated sediment, an environment that provides a habitat for a variety of surface-dwelling microorganisms, resulting in a nearly continuous biofilm. This study is significant because it demonstrates the ability of epilithic algae to obtain Fe from their mineral substrate; chelating abilities of bacteria, lichen, and fungi are well-documented, but few data have been available for algae. The research demonstrates the value of the bioassay method for studying bioweathering and iron metabolism
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