Methodological concerns in measuring the lipid fraction of carbon fixation

Carbon fixation into lipid (lipid production) by phytoplankton was measured in 3 lakes on the edge of the Canadian Shield by two different extraction methods. The amount of lipid detected in the plankton samples was generally 11% lower when extracted by a ‘Folch-like’ lipid solvent (dichloromethane:methanol (2:1)) than with the lipid solvent (80% ethanol and 80% ethanol-diethyl ether) used in a sequential extraction method. The difference between methods was not due to losses of fixed carbon during extraction since the sum of the extraction fractions from both methods were not different from the amount of carbon fixed on a replicate acidified filter. Although more carbon was detected in the lipid fraction of the sequential extraction method, an additional 5% of total carbon fixed was found in the lipid extract of another fraction from the sequential method, the ‘low molecular weight’ fraction. Our results suggest that accurate comparisons of lipid production data can only occur after compensating for differences in extraction methods while comparing the LFCF determined by different lipid extraction should be avoided.