I've only looked into this a little bit, but I think one problem is that you're limited by either nitrates or phosphates, depending on whether the algae are bei

I've only looked into this a little bit, but I think one problem is that you're limited by either nitrates or phosphates, depending on whether the algae are being grown in ocean water or fresh water. Either way you have to supply some kind of nutrient, which has to be either mined or synthesized in some energy-intensive fashion.