Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea

Rising oxygen levels in the Earth's early atmosphere marked the end of a 2.5-billion-year period dominated by oceans with low levels of oxygen. But geochemical evidence suggests that for the following billion years the oceans remained largely devoid of oxygen. The discovery of molecular fossils (hydrocarbon biomarkers) in 1.6-billion-year-old sedimentary rocks from a marine basin in northern Australia now offers insights into the marine ecosystem at the time. The biomarkers record an anoxic and sulphidic world hostile to to many forms of life but supporting blooms of sulphide-breathing green and purple bacteria.