text: Most companies pursuing algae as a source of biofuels pump nutrient-rich water through plastic or borosilicate glass tubes (called "bioreactors" ) that are exposed to sunlight (and so-called photobioreactors or PBR).
  Running a PBR is more difficult than using an open pond, and costlier, but may provide a higher level of control and productivity. In addition, a photobioreactor can be integrated into a closed loop cogeneration system much more easily than ponds or other methods.