I'll answer your last question first..."...500,000 oil wells"....That is an incredible amount of infrastructure that we have built up over the last century (and more) to harvest/pump oil from the ground. An incredible amount of steel, carbide, physical effort went into creating this massive infrastructure...And land, yes, LOTS of land, maybe an area equal to (or greater than) the land mass of Delaware. I am sure that someone (yourself for example) would state BEFORE it was created...that this couldn't practically or economically be accomplished...yet it was!!!

You state that it would take huge amounts of land and "plastic"...and even pull a figure for that "plastic" out of the air.

I took your whole point to be that algae would never be practical as a replacement for petroleum because it required "TOO MUCH" land or "TOO MUCH PLASTIC" (and the plastic would be too expensive anyway).

Yet we have an INCREDIBLE amount of STEEL and LAND devoted to the harvesting/transport/refining of oil and its end products.

So let's consider that the REAL question is COMPARED TO PETROLEUM is the algae infrastructure greater/lesser/equal to the ALREADY EXISTING infrastructure for petroleum that it would eventually replace?

My thesis is that when you compare the two....the algae infrastructure will be somewhat but not prohibitively greater than the already existing petroleum infrastructure.