They bubble air through the water. You can see the bubbles in the picture. They actually developed something like this years ago and could actually charge batteries from the photosynthesis of the algae… Not a lot of energy but enough to power some lights for a while. They were floated as a way to light walkways and stuff at night while also sucking up tons of CO2. The original designs were just tubular tanks and supposedly one tank would capture as much CO2 in a year as a tree would over it’s entire lifespan. Or so they claimed.