Do the algae growing schemes involve farming or collecting from the oceans? Most of the time in the ocean it's pretty thinly distributed compared to what you see in a nutrient rich pond.
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I think that at this point, most plans do assume land-based, 'algaculture' systems.
See, for example, the crisp design concepts for algaculture high rises and balloons (yes, balloons) featured at BLDGBLOG:
<http://bldgblog.blogspot.com/2007/11/algae-power.html>
I expect that if these became viable it would only be a matter of time before clever souls figured out how to maximize the use of the oceans as a harvesting site. Perhaps by re-engineering some of the land systems for ocean applications.
Which would put us in the enclosure situation John Gulick mentioned.
.d.