In article ,
Peter Jason wrote:

And what about the idea for a genetically modified floating seaweed - like
the notorious Water Hyacinth - to float on the surface of the world's oceans
producing some sort of edible or industrial oil which is easily cropped my
mechanical harvesters. No fertilizers or soils needed.
Surely some one is working on this idea.

Interesting notion. One problem with it is that to get a vascular plant to
cope with that much salinity would be quite a challenge. Another is that
the main limit to growth in marine ecosystems is basic nutrients like
nitrogen and phosphorus, which are at an extremely low level -- they are
rapidly taken up and incorporated into biomass.

On the other hand, floating and emersed aquatic plants are extremely
productive in fresh waters, where plant growth is limited by dissolved
CO2 and by available light since the water isn't often clear to much depth.
By having surfaces exposed to the air, these plants get around these
limitations, and in the presence of plenty of nutrients grow explosively.
They are excellent for removing nutrients from polluted waters, and there
are already installations where artificial wetlands are used to purify
water from livestock operations, sewage plants and storm sewer outflow.

Another important application of this practice is in fish farming, which
can generate huge amounts of pollution. By partly closing the system,
this can be greatly reduced, and if the fish are herbivores, like tilapia
and carp, the floating plants can be fed back to them. This is particularly
valuable in arid or heavily populated regions.

Duckweed (Lemna spp) is very high in good quality protein and is already
being used in aquaculture and wastewater treatment to produce a protein
feed supplement for fish, poultry and pigs. Here's a URL with links:
http://ecosyn.us/ecocity/Links/My_Li...ckweed_01.html

People are already considering the use of duckweed for "molecular farming",
in which genetically engineered plants produce pharmaceuticals, much as
genetically engineered bacteria have for decades. I have strong reservations
about these practices because of the risk of contaminating the gene pool of
crop and fodder plants with genes for toxic substances. Once they escape,
you'll never get them back, and they could have disastrous consequences for
the food supply.