Algae grow much faster when fed by carbon dioxide concentrations far above even the elevated levels in our atmosphere (which amount to ~.04% CO2), such as those of generator exhaust (~12%, about 300x higher). Other pollutants (such as NOx) are also captured by eaten by the algae. So there is most definitely a point to having a genny in the picture.

This is not meant to be an excuse for buying more generators, just a way to clean up the many existing exhaust sources while creating a variety of "green" products.