Question: how is pond scum useful?

Daphne Ng answered on 13 Mar 2012:

Pond scum is a general term for microalgae, the stuff that turns ponds and fish tanks green. Like tiny plants, microalgae photosynthesize and take in carbon dioxide from the air. The microalgae can then use this captured carbon dioxide to make useful products such as vitamins, proteins and colourings that we use. In fact, some of the food dyes that we use are made by microalgae. Of course, too much pond scum can be a nuisance as it clogs ponds and makes fish tanks ugly. But as long as we don’t let them grow out of hand, they can be very useful to us!