If you think biologists have a good idea about everything that lives on the Earth, think again. Scientists say they have just now discovered an entirely new branch on the tree of life. It's made up of mysterious microscopic organisms. They are related to fungus, but they're so different you can argue they deserve their very own kingdom, alongside plants and animals. NPR's Richard Harris reports.

RICHARD HARRIS: It was just a few years ago that Tim James and his colleagues sat down and wrote the definitive scientific paper to describe the fungal tree of life.

Professor TIM JAMES (University of Michigan): We thought we knew about the major groups that existed. Many books have excellent drawings of these fungi from, you know, the last 150 years.

HARRIS: James, who is curator of fungus at the University of Michigan, says of course many are familiar.

Prof. JAMES: You've got mushrooms, yeast and molds.

HARRIS: Plenty of plant diseases like rusts and smuts. There's the stuff that grows in your shower or between your toes.

Prof. JAMES: Fungi that specialize on insects - the entomopthrelis(ph).

HARRIS: Even fungi that grow on other fungi.

Biologists figure they've probably only cataloged about 10 percent of all fungal species. But they thought that they at least knew all the major groups. Oops.

A paper being published in the journal Nature says: not so. Thomas Richards, at the Natural History Museum in London, says biologists can mostly only study microscopic fungi if they can grow them in the lab.

Dr. THOMAS RICHARDS (Natural History Museum, London): But the reality is that most of the diversity of life we can't grow in a laboratory. It exists in the environment.

HARRIS: And microscopic organisms are just about impossible to find just looking in dirt or water through a microscope. So Richards and his colleagues tried more modern means.

Dr. RICHARDS: About 10 years ago, people started using molecular approaches. So they started to try and target the DNA in the environment, specifically.

HARRIS: Using those techniques, they struck pay dirt. They found novel bits of DNA related to fungi, but clearly different from all the known varieties just about everywhere.

Dr. RICHARDS: Including pond water, lake water, fresh water sediments and marine sediments. Basically, almost everywhere we looked we found this novel group.

HARRIS: They then brought samples back to the lab and devised a technique to make the organisms containing this novel DNA glow under a microscope. So they've managed to get a few glimpses of these mysterious life forms, which they've named cryptomycota.

Dr. RICHARDS: And we know that they have at least three stages to their lifecycle. One is where they attach to a host, which we think are photosynthetic algae. Another stage where they form swimming tails so they can presumably find food. And another stage, which we call the cyst phase, where they go to sleep.

HARRIS: Now Richards and his colleagues would like to figure out how to grow them in the lab to really get to know them.

Dr. RICHARDS: At the moment it's a bit too early to be sure about what role they play in the environment. But one thing we can be certain of is that because they are so diverse, they're probably playing many, many different roles in many different environments.

HARRIS: Back at the University of Michigan, Tim James says the discovery is revolutionary. It's rocking the world of fungus phyologenetics.

Prof. JAMES: It's going to be interesting because one of the controversies is going to be: Are they really fungi or not?

HARRIS: Since they apparently lack a protein in their cell walls that is a defining feature of fungi, you could argue that they aren't actually a member of the fungus kingdom, but deserve an entire kingdom of their own. And before you get too comfortable with the idea that all these species just hang out in ponds or sediments...

Prof. JAMES: There could be some actually human parasites in here eventually discovered.

HARRIS: But fret not. Mostly fungi are doing important things, like recycling nutrients. And most of the time they seem to leave us alone.

Richard Harris, NPR News.

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INSKEEP: A little fungus music on NPR News.

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