How do symbiotic algae give some freshwater sponges a green color?

Freshwater sponges of the family Spongillidae derive their characteristic green color from symbiotic algae that live within their cells or among them.1

This relationship is mutually beneficial: the sponge provides the algae with protection from environmental stressors and metabolic waste products, while the algae serve as a source of oxygen and nutrients for the sponge. These algae can be transmitted to new generations through the sponge's gemmules. In some cases, the sponge may also consume dying algae as a food source.1