Marine macroalgae will release without light some Carbon Dioxide CO2 back into the marine aquarium, whilst with light turned on it will produce oxygen. 

In a heavily macroalgae planted macroalgae refugium it is therefore recommended to keep the light on for the macroalgae anti clockwise. Which means turned on over night and turned off over the day.

For herbivore and omnivore fish species macroalgae will provide as well additional health benefits, as it will offer a natural source of live algae which they need to stay happy & healthy. Often a discoloration of some Tang fish species is a sign of an algae deficit in the food provided, where live algae can help to avoid this.