EVIDENCE FOR ONLY MINOR CONTRIBUTIONS FROM BACTERIA SEDIMENTARY ORGANIC-CARBON

BECAUSE their molecular signatures are often prominent in extracts of sediments, bacteria are thought to be important contributors to petrol eum source beds(1). It has been shown recently(2,3), however, that abu ndances of biomarkers do not always reflect relative contributions to sedimentary organic carbon (C-org). The contribution of photosynthetic green sulphur bacteria to sediments can be assessed effectively becau se the diagenetic products of distinctive carotenoids from these organ isms occur widely(4-11) and their biomass is isotopically labelled, be ing enriched in C-13 (refs 11, 13). We show here that, although sedime nts and oils from the Western Canada and Williston basins contain prom inent biomarkers of photosynthetic bacteria, the absence of C-13 enric hment in the total C-org requires that the bacterial contribution is i n fact minimal. Although the importance of bacterial reworking of sedi mentary debris cannot be doubted(14), we argue that our findings, when considered in conjunction with those from other settings, suggest tha t bacterial biomass may commonly represent only a minor component of t otal C-org in carbonaceous rocks.