a slight deviation from the topic but still related to dsb
http://reefkeeping.com/issues/2003-06/r ... /index.phpanyway back to the topic, the environment of denitrifying bacteria is in the anoxic regions of sand and rocks, both regions are typically absent of light.
on a more practical discussion, if you would put light on the sandbed, photosensitive species of bacteria will just move far away from the exposed areas and other species like cyanobacteria (and other life forms like diatoms and other filamentous algae) will grow there instead.