Thanks for posting, of course it makes us think, so is highly valuable.
The reason I struggle with the idea of creating algae as being directly "viable", is that it appears to have no built-in clean energy capability, that might supply energy to the industry needed to carry out the work.
In other words, the energy needed for the industry to create the algae, if conventionally supplied, might generate more even more C02, which would cancel out the benefits intended.
I agree with Stephen’s response, I added a comment to his, with more details.