Significance of carbon isotope discrimination between bulk carbon and extracted phospholipid fatty acids in selected terrestrial and marine environments

The development of compound-specific stable isotope analysis (CSIA) has led to many new environmental applications. One advantage of making isotopic measurements of biomarkers over that of bulk carbon is that specific components of an ecosystem can be defined isotopically. This approach has been used recently with phospholipid fatty acids (PLFAs), which can be a proxy for bacteria. We conducted a comparison of bacterial PLFAs and bulk carbon isotope ratios in diverse terrestrial, estuarine and marine environments. The isotopic discrimination between bulk carbon and PLFA 16:0 varied from +0.1 to −11.4‰ and was greatest in environments that were likely anoxic and where methane was a potential carbon source. This approach was used to estimate that methane accounted for about 15% of the carbon used by bacteria in a Gulf of Mexico cold seep site