Algae, iron and carbon dioxide

Studies in the 90s

There were a couple of studies in the nineties where iron, which can be a limiting factor in certain environments, was spread across a large expanse of ocean. It increased the amount algae, which in turn caused more carbon dioxide to be used in photosynthesis. The debate centers around the merits and possible pitfalls of the 'fertilizing' very large amounts of sea to increase the amount of carbon fixing. What would be the ecological consequences, etcetera?

In the 2007-2008, new research pointed to the fact that additional carbon from algal blooms may not sink to ocean deeps to be removed from the short-term carbon cycle. But my question is does the carbon used during photosynthesis end up being carbon dioxide again? Or does some of the carbon become stored in the structure of the algae?

Hypothetical Amount of carbon dioxide removed from system in algal biofuel

How would it affect global warming? The flow of carbon from CO2, into some kind of oil, to a break down of the oil into carbon dioxide is pretty much a zero sum game? (Minus the amount of algal material that isn't combusted by the engine)

1. Stop release of future CO2 from petrofuel.
2. A part of CO2 would be sequestered in the non combusted part of the algae and oil(?)
3. If the amount of oil use in the world remained constant, then for each gallon/liter produced, a gallon/liter would be used. It would a zero sum equation, I guess?

Depending upon the amount of carbon sequested in each 'cycle' of the algae fuel harvesting, there could be a significant removal of carbon dioxide from the atmosphere. This could eventually be a problem, but not in the near term forseeable future. And it is probably easier to add carbon dioxide to the atmosphere than to remove it.