STABLE CARBON ISOTOPIC CORRELATION OF INDIVIDUAL BIOLIPIDS IN AQUATIC ORGANISMS AND A LAKE BOTTOM SEDIMENT

Isotope ratio monitoring GC-MS (irm-GCMS) analyses of individual hydrocarbons isolated from the surface sediment of a small eutrophic lake (Priest Pot, U.K.) and a phytoplankton tow sample from the upper water column have been undertaken in order to assign sources of sedimentary lipids. The deltaC-13 values for the sedimentary n-alkanes showed no clear distinction between lower molecular weight components (C-17-C19) of presumed algal origin and higher members (C23-C33) of presumed higher plant origin, thus indicating a limitation to the use of compound specific isotope analyses in the freshwater system studied. In contrast, the deltaC-13 values of sedimentary hopenes were markedly depleted (by over 20 parts per thousand) compared with those in the phytoplankton sample from the upper water column. Hop-22(29)-ene in the surficial sediment had a deltaC-13 value of -55.2 parts per thousand, which contrasts with the value of the phytoplankton component (-32.7 parts per thousand), attributed to cyanobacteria. These values indicate that a highly C-13-depleted source provides a major contribution to the sedimentary component. It is proposed that the sedimentary hopenes originate in part from methanotrophic bacteria, demonstrating the presence of a bacterial methane cycle in this environment.