For decades, Jon Zehr was haunted by an organism he knew was there — but couldn’t see. It all started in the ‘90s on a research boat in the middle of the ocean. Zehr was an oceanographer studying nitrogen-fixing bacteria — simple, microbial life forms that could pull the element straight from the air, making it bioavailable to plants and animals. Scientists at the time had only seriously studied one species of nitrogen-fixing bacteria in the entire ocean, but Zehr wanted to change that. His plan...