Effects of Bacterial Growth on the Environment 177 



in fact, they can be photographed by their own hght. The glow 

 of decaying wood sometimes observed in forests is caused as a rule 

 by fungi, such as certain toadstools and mushrooms. The term 

 "fox-fire" is applied to this luminescence. 



Light is emitted during respiration of these organisms under 

 aerobic conditions, rather than being absorbed as occurs in photo- 

 synthesis. Part of their waste energy is given off as heat and part 

 as light. Luminescence, apparently, is an unnecessary side-effect 

 of metabolism, since the organisms are capable of growing well 

 without emitting light. 



Fig. 30. Luminous bacteria photographed by their own light. 

 (Courtesy of A. C. Giese, Stanford University.) 



The cold light given off by these microscopic lamps appears to 

 be the result of the action of luciferase enzyme acting upon its sub- 

 strate luciferin. While a single cell gives off insufficient light to 

 be detected by the usual measuring devices, the cumulative effect 

 of an active culture containing billions of cells is bright enough to 

 permit the reading of a newspaper when a few liters of the culture 

 are actively aerated in a dark room. Those persons who have 

 had an opportunity to be on boats in salt water at night have 

 undoubtedly observed on occasion streaks of light in the water at 

 the stern of the boat as the propeller whipped air into the salt 

 water and activated the bioluminescent bacteria. Some waters are 

 richer in these organisms than are other bodies of water, and 



