Seaweed is simple

Seaweed grows by photosynthesis, as terrestrial plants do, but the photosynthetic efficiency (fraction of light energy converted into chemical energy) of seaweed is higher than plants grown on land.

Seaweed grows faster than terrestrial plants, and do so at low temperatures. The bulk of seaweed forests are formed by large brown seaweed (kelp) and are found in temperate to cold waters, where the temperature varies far less than on land. Having a slight negative buoyancy, the seaweeds are almost weightless when fully submerged. They need no tissue re-enforcement to keep them growing upright as with land plants, which makes seaweed biomass easier to brake down in the bioconversion process.

Dissolved nutrients like nitrate and phosphate are easily absorbed through the surface area of the entire plant. Totally immersed in water, seaweed wastes hardly any energy for uptake and transport of neither water nor nutrients. A definite advantage of growing plants in seawater is, of course, that they need no freshwater what so ever nor deep roots to access it.

The life cycle of large brown seaweeds is initiated when mature plants (the sporophytes) produce microscopic spores, a process that now can be conducted in a laboratory. In hatcheries, each of the huge number of microscopic spores grows into a microscopic sexual stage, with 50 % each of males and females. These quite rapidly yield the next generation of macroscopic plants that can be harvested, and the life cycle is repeated.