I've been reading a bit about algae fuel. It sounds like people did some work and then decided it was too difficult. This link mentions Exxon's work:
And this next link shows the ex-CEO of Exxon talking about how they wanted an algae genetically modified to secrete oil, so they didn't have to kill and harvest it. He then says they encountered a low-level problem with the basic science, but doesn't elaborate.
Anyone know what that problem was?