Algae farms in the ocean

Futuristic-tier research allowing to create massive algae farms on the strategic layer.

The farms would require phosphorus, manpower, and ship fuel, and produce biomass that can be processed into biofuel (spirit) or foodstuff (not very tasty).

Phosphorus in turn can be mined (guano etc sediments).

Real-life background:
Algae (cyanobacteria) are much more efficient at photosynthesis than land-based plants. They live in water with 50x higher CO2 concentration than air, and they don't need expensive vertical transport systems (roots/leafs).
Currently algae are grown in tanks or ponds, but it's a bit expensive.
It would be very efficient to grown them directly in the ocean in net-encircled farms with areas of 1-50 km2, but there are a few unsolved issues.
1. There's not enough phosphorus in the water, need supplements
2. Primitive marine life loves to snack on algae, need filtration
3. Risk of GM escape/mutation and ecosystem impact

See e.g. https://www.nature.com/articles/ncomms14724