In this paper, we study the ergodic capacity of free space optical communication systems over Gamma-Gamma atmospheric turbulence fading channels with perfect channel state information at both the transmitter and the receiver. In our framework, we mainly focus on the low signal-to-noise ratio range and show that the ergodic capacity scales proportionally to S NR log4(1/S NR). We show also that one-bit CSI feedback at the transmitter is enough to achieve this capacity using an on-off power control scheme.