In the Arctic, thawing permafrost creates many ponds that release greenhouse gases. We collected water from 15 ponds in the Canadian Arctic and conducted a 188–day experiment to see how bacteria use carbon in summer. They consumed about one-third of the available carbon in 100 days, and adding nutrients made no difference. Since they prefer fresh matter from plants and algae, longer growing seasons could boost gas emissions from recently fixed carbon as the Arctic landscape continues to change.