invention are realized by not sparging gas or running a pump continuously but by duty cycling their operations based on the needs of the algae culture for mixing ensuring suspension of the culture and providing adequate co 2 as illustrated in fig1 and 3 the photobioreactor pbr assembly 10 includes a first bank of a plurality of vertical transparent cylindrical tubes 12 i e a phototube array a second bank of a plurality of vertical transparent tube not shown may also be provided wherein the banks are offset such that the center of each tube 12 of the first bank is aligned with space between tubes 14 of the second bank an upper horizontal manifold 16 and a lower horizontal manifold 18 are connected to the ends of the first bank of tubes 12 the lower horizontal manifold 18 is connected to a gas inlet source 24 at one end 26 and a tube fill line 28 and a tube drain line 30 at the other end 32 the upper horizontal manifold 16 is connected to an overflow and or gas exchange line 34 the overflow line 34 enables gas originally in the empty tubes 12 14 to be transferred back to the tank 36 during full cycles.