Investigation of Microbial Community using Phospholipid Ester-linked Fatty Acids from a Black Shale Weathering Profile

Studying phospholipid ester-linked fatty acid patterns throughout a 1600 cm black shale core, along with elemental analysis, may reveal the microbial community and changes in the community structure with depth. 

Black shales are organic carbon-rich, fine-grained, laminated, and dark colored rock.  The formation of black shales usually occurs in isolated basins and continental margins with highly productive surface waters and anoxic bottom waters.  Pyrite (FeS2), commonly found in black shales, along with large amounts of organic matter (OM) help maintain chemically reducing conditions in subsurface shales (Petsch et al., 2003).  When tectonic uplift exposes the black shale to a weathering environment, there is a drastic environmental change from reducing to oxidizing conditions as the rock is exposed to the atmosphere. 

Weathering is the physical, chemical, and sometimes biological breakdown of a rock when exposed to the atmosphere.  During weathering of an OM-rich sedimentary rock, oxygen is consumed from the atmosphere while carbon dioxide is released.  Weathering of black shale is thought to be a major control on the oxygen content and a secondary control on the carbon dioxide content of the atmosphere.  The activity of microorganisms in a black shale weathering profile may play a large role in controlling atmospheric composition, as oxygen is consumed and carbon dioxide is released, through different metabolic process (aerobic and anaerobic respiration).  Also as a result of weathering, there are changes in OM abundance and composition.  Under conditions of increased permeability due to exposure to oxygen and rainwater, microorganisms may infiltrate downward into the weathered rock (Petsch et al., 2003).   

It may seem surprising that microorganisms can use ancient OM from a rock.  Much ancient shale OM is macromolecular and must be sufficiently broken down for use by microorganisms.  Diagenesis occurring in the sediments before lithification degrades labile components leaving behind the refractory OM, which is unable to be readily used by microorganisms.  Weathering may however break down refractory components into labile components that microorganisms can utilize.  Microbial utilization of OM during black shale weathering is likely for three reasons: 1) there is ample organic carbon available; 2) a weathering profile is a non-sterile environment; 3) the weathering profile represents a chemical interface between oxidizing and reducing conditions (Petsch et al., 2003).

         The microbial biomass of a sediment, soil, or rock can be determined in terms of universally distributed biomarkers that are characteristic of microbes (White, 1993).  Cell membrane lipids and their fatty acids are useful biomarkers as they are essential to every living cell.  Fatty acids are long unbranched chains of carbon atoms with attached hydrogens and other groups. A carboxyl (-COOH) group at one end gives the molecule its acidic properties.

 Figure 1 – examples of different structures of fatty acids.

Phospholipids are esters that link a glycerol backbone to two fatty acids and (in place of the third fatty acid) a substituted phosphate with some other group (in figure 2 trimethylamine) attached to its other end.

Figure 2 – an example of a phospholipid structure 

Green – glycerol backbone

Red – -COOH group (ester linkage)

Blue – phosphate group

         Due to their physical characteristics, phospholipids spontaneously form a double layer in an aqueous environment to hide the hydrophobic tail regions from water while exposing the hydrophilic heads to water creating a spherical lipid bilayer shell around the cell.

Phospholipids are the major constituents of the lipid bilayer of microbial membranes and form about 50% of eukaryotic and 98% of bacterial membrane lipids (Salomonova et al., 2003). 

        The microbial community structure of a black shale weathering profile is not well known.  The use of phospholipid ester-linked fatty acids allows for the community structure to be determined.  Phospholipid analysis provides an accurate measure of viable biomass as the phospholipids have a high turnover rate, are rapidly lost after cell death, occur in all bacterial cellular membranes, and are formed over a wide range of growth conditions in bacterial cells (White, 1993, Salomonova et al., 2003).  Analysis of phospholipid ester-linked fatty acids (PLFA) is a sensitive chemical method to identify microbial communities and community structure in environments, applied here to a subsurface black shale weathering profile.  PLFA profiles represent the composition of the microbial community, however they do not provide information about the identity of specific species because different species can share various fatty acids (Steenwerth, et al., 2002).  While the PLFA profile does not give an actual species composition it does give an overall picture of the community structure (Pennanen et al., 1995).  Lipid chemistry is sufficiently diverse that it can provide quantitative estimates of the community structure and sometimes nutritional status (White, 1993). 

Preliminary Results!

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