Quarterdeck Magazine Online -- Texas A&M Oceanography

Texas A&M University
Department of Oceanography

Summer 1999 - Vol. 7, No. 1

Biomarkers indicate source of carbon

Monitoring microbial degradation through stable carbon isotope analysis of phospholipid fatty acids

by Gregory Salata

In recent years, scientists have used lipid biological marker compounds, or biomarkers, to determine what happens to the organic compounds in an ecosystem as a result of biogeochemical processes, such as bacterial degradation. One particularly useful subset of microbial biomarkers are phospholipid fatty acids (PFAs), which are chemicals found in all living cells which are produced during the bacterial degradation of organic material.
Because phospholipids have a relatively rapid turnover rate in living cells, and are not found in man-made contaminants, they are ideal indicators of the current source of organic carbon in pollution or ecosystem studies. When multiple sources of carbon are available in an environment, it is often difficult to identify which carbon source the bacteria are using. One solution to this problem is to measure the ratio of two stable carbon isotopes, 13C to 12C (defined as del13C), in both the available organic carbon and certain PFAs, and show similarities between these ratios. Provided that the del13C is not altered during bacterial degradation, this technique can be used to identify which of two (or more) sources of carbon the bacteria are utilizing, provided the organic carbon sources have differing del13C values.
To investigate the applicability of this technique, a single strain of bacteria, Sphingomonas paucimobilis, was grown on a variety of carbon sources with differing del13C. The bacterial PFAs for these cells were then extracted and analyzed to determine the del13C of the individual phospholipids. The results of this experiment clearly showed that the difference between the del13C of the carbon source and the del13C of the PFA is dependent upon the chemistry of the carbon source, and can be used to identify the source of carbon being utilized. For example, certain molecules, such as simple sugars and organic compounds with similar functional groups throughout the molecule, showed minimal differences between the del13C of the carbon source and that of the PFAs. In contrast, bacteria growing on a single carbon compound (methanol) showed large variations between the del13C of the carbon source and PFA, due to the preferential utilization of the molecules containing 12C. Thus, a large negative discrepancy between the PFA and carbon source del13C is indicative of the presence of methane degraders (methanotrophs), suggesting an anaerobic environment, while a minimal difference between these two values is indicative of the degradation of larger carbon molecules, or an aerobic environment.
This technique may also be useful in the assessment and subsequent cleanup of a marine system, such as an estuary, impacted by a pollutant such as oil. In particular, the method can show whether the oil is being degraded by the natural bacteria, greatly influencing cleanup decisions. For example, a bacterial PFA 16:0 del13C similar to that of the hydrocarbon contaminant would indicate the removal of the contaminant by natural mechanisms, reducing or alleviating the need for a costly cleanup strategy. By contrast, if the bulk and PFA 16:0 del13C are dissimilar, removal of the contaminant by mechanical means may be the only solution.

Gregory Salata graduated with a Ph.D. in May 1999, and now works for B&B Laboratories in College Station. His e-mail address is GregorySalata@TDI-BI.com.

Figure:
Changes in isotopic discrimination with respect to substrate. A represents amino acids with multifunctional groups. B represents a variety of simple and complex organic molecules. C represents single carbon substrates.

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