Green Algae and Carbon Dioxide.

So I just recently started playing seablock and am still in the early stages. I am very confused as I see people making Green Algae with Carbon Dioxide and mineral water? In my game I appear to make green algae with just regular water. Also I'm not sure I understand why I would use charcoal. It says wood pellets have 8.0 MJ fuel value and charcoal has 4.0 MJ fuel value. Also it ends up taking 2 pellets per charcoal and that's after using some fuel to smelt the wood bricks that you turned the pellets into. I'm sure I'm missing something, but I just can't figure it out.