Microbe Foam

seafoam
(Ecomare via Wikimedia Commons)

Algae make a chemical defense called DMSP. It is a sulfur-rich compound potentially used to ward off predators, as antifreeze, or even sunscreen. It is also a great resource of sulfur for microbes. The bubbly foam in this photo was made by a microbe breaking down DMSP. A gas is released in this process, and when enough microbes are present foam forms.