The pristine state of unpolluted waterways may be their downfall, according to new research. A species of freshwater algae that lives in streams and rivers, called Didymo for Didymosphenia geminata, is able to colonize and dominate the bottoms of some of the world’s cleanest waterways — precisely because they are so clear. Didymo does so with a little help from its friends — in this case, bacteria — which allow it to make use of nutrients like phosphorus.