I again sugest growing plants in the sea to be used to make biofuel
A study of arctic climatology reported that azolla may have had a
significant role in reversing a greenhouse effect that occurred 55
million years ago that caused the region around the north pole to turn
into a hot tropical environment. This research conducted by the
Institute of Environmental Biology at Utrecht University claims that
large dense patches of azolla growing around freshwater lakes formed by
the climate change eventually consumed enough carbon dioxide for the
greenhouse effect to reverse.
"Leaf protein isolate from water hyacinth (Eichornia crassipes) was
prepared and the chemical composition was studied. It contained 49.6%
protein, 16.0% total lipids, 26.9% total carbohydrates, 1.7% fibre
and 5.8% ash."

T. M. Abo Bakr, N. M. El-Shemi and A. S. Mesallam (1983,
January).Isolation and chemical evaluation of protein from water
hyacinth. Retrieved May 30, 2007, from the SpringerLink website:

I guess the question would be: what are the chain lengths of the
resultant oils, and whether sufficient bulk of material could be
produced cheaply enough to make up for lower lipid concentration.

Warning, this is the water plant from hell. I grew up in Florida and
it choked the waterways down there. Yeah it grows fast, and God help
you if it gets out into native waterways in a tropical/subtropical
region. I did see a controlled usage at Walt Disney World in
Kissimmee where it was used as a part of the wastewater treatment
facility there.

I don't know about north america, but the sugar beet is used elsewhere. An ethanol producer here in Sweden is suggesting sugar beets. Looks better than say wheat, not to mention corn. Best efficiency is achived when the ethanol plant is operationg in tandem with a biogas (methane) plant, generation methane for the by-product. Or the by-product can be used as a high value animal feed.

France has substantial production from sugar beets, according to them one hectar of beets produces in excess of 8000 liters of ethanol. Or more than ~855 gallons per acre.

http://www.roulonspropre-roulonsnature.com/2007/04/02/comment-fabrique-t-on-de-l’ethanol-a-partir-de-betteraves/

The link i sin french, but google does a servicable translation.