I’d certainly be cheered if saltwater algae were in the running here. Like I said, the math was done and those great rivers would be consumed with algae feeding if it was freshwater algae. There is a stoichiometric tie between the water feed and the hydrocarbon fuel output — it’s physical chemistry not just efficiency. Those hydrogens have to come from somewhere. (The carbons come from the food, and then cellular processes harness sun energy to hook them up with hydrogen and liberate oxygen.)
I'd be very surprised to learn that most naturally occurring petroleum was NOT the result of composted ocean algae.
The best species of algae for use as biofuel are those with maximum oil yield. Most of them are seawater types.