In the ocean acidification thread, gncp stated
If you grind through all the energy you need to use to produce fuel from ocean biomass, and do it honestly by not neglecting things like the need to rinse all the salt out of the biomass before processing, you will find it is an energy-losing proposition (similar to how producing alcohol from corn uses more energy than it produces).
This topic needs more discussion. At least for freshwater, harvesting algae for fuel seems to be feasible. The Holtzapple group in Texas has a mixed alcohols process that may also be applicable. Further 2/3 partners have signed on and commited research grants to start a new research project pertaining to the use of algae to clean waste water. I am interested in uses for the algae that will be produced.