News Oceans Before altering the air, microbes oxygenated large swaths of the sea The finding suggests cyanobacteria were prevalent much earlier than previously thought Stromatolites (shown) are layered formations constructed mostly by microorganisms called cyanobacteria, the first photosynthesizers on Earth. Cyanobacteria helped oxygenate Earth’s atmosphere 2.4 billion to 2.1 billion years ago. Totajla/iStock/Getty Images By Nikk Ogasa 53 seconds ago Ancient oxygen-making microbes may have oxygenated large swaths of Earth’s seafloor hundreds of millions of years before the element filled the atmosphere. Geochemical analysis of sediments deposited roughly 2.6 billion years ago reveals that pulses of oxygen may have swept through large regions of the ocean, researchers report April 26 in Nature Geoscience. The findings suggest that cyanobacteria, the microorganisms responsible for oxygenating Earth’s atmosphere, were more widespread at the time than previously believed. This shows that not only had cyanobacteria already evolved, but they were around in vast numbers and had even oxygenated the seafloor, says geochemist Kurt Konhauser of the University of Alberta in Edmonton, Canada, who was not involved in the study. And that, he says, means aerobic organisms might have evolved on the seabed long before oxygen permeated the sky. Sign up for our newsletter We summarize the week's scientific breakthroughs every Thursday.