Nannochloropsis limnetica (this may be the most nerdy thing you read today)

Nannochloropsis limnetica is a type of algae that thrives in cold water with low nutrient levels. Like other Nannochloropsis algae, it accumulates fats-and-oils which is rare for aquatic vegetation. Nannochloropsis have been measured as having between 7% and 38% fats-and-oils on a dry-weight basis.

Most species in the genus Nannochloropsis are found in salty and brackish waters. N. limnetica is found in fresh-water.

Another entertaining factoid is that "fish oil", specifically the EPA and DHA Omega-3 components are not created by the fish. EPA and DHA are synthesized by a few select species of algae and concentrated with each step through the food chain, much like certain toxins bio-accumulate.

Fish and crustaceans that feed in waters where those algae species thrive are rich in EPA and DHA. Fish and crustaceans that feed in waters where those algaes' needs are not met are low in EPA and DHA.

Nannochloropsis are one of the few genus of algae that are primary producers of EPA.

Hurdles to cultivating Nannochloropsis

It seems like cultivating Nannochloropsis would be a no-brainer.

But hold on, Virginia, there are complications.

Algae needs liquid-water, carbon-dioxide, light and dissolved nutrients. The fact that algae comes in so many forms is due to the fact that those needs exist in various proportions and a form that is optimal in one ecosystem is sub-optimal in others and will be out-competed by forms that are closer to optimum.

Nannochloropsis are optimized for low-nutrient, cold, open waters. Envision crystal-clear, high altitude trout waters or the middle of the Bering Sea.

Nannochloropsis are tiny! They run between 2-and-5 microns (1e-6 meters). Airborne pollens like grass and pine-trees average about 20 microns. 

The smaller the organism the greater the ratio between surface area and volume. Nannochloropsis have an enormous amount of surface area relative to their volume, the better to absorb nutrients.

This poses a problem with culturing them. Adding fertilizer will advantage larger forms of algae relative to Nannochloropsis.

Filamentous algae can anchor to reeds and the mixing effect of waves will expose them to washing that will bring nutrients to them. Nannochloropsis do not have that benefit. They are so tiny they move with the water. Nutrients come via diffusion and Brownian motion of the organism in the water.

That is a problem from the standpoint of the Nannochloropsis because even tiny bodies of water have currents. They have up-wellings of (relatively) nutrient rich water and down-welling of depleted water with high populations of suspended algae...elevators down into the Stygian depths.

In warmer waters, a clump of algae cells might be able to create their own personal flotation device with the oxygen they generate with photosynthesis, oxygen being much less soluble in warm water than in cold water. No such luck with the Nannochloropsis in their native haunts.

Nannochloropsis are like bloggers and are not disposed to "clumping". Furthermore, they live in COLD waters that eagerly absorb oxygen so the odds of a bubble or film of oxygen forming on the organism is nill.

BUT...

Those buoyancy issues might explain the existence of those fats-and-oils. They must confer some advantage. They come at a very high metabolic price and Nannochloropsis  would have been quickly out-competed into extinction unless if offered some advantage.

One might speculate that those fats-and-oils add buoyancy that load-the-dice and help keep the Nannochloropsis in the light-rich upper levels of the water column.

Weebles wobble and they don't fall down. The chaotic currents of the sea conspire to sink Nannochloropsis  and hydrocarbons within them help them rise above that.

---Aside---

One could presumably sort for high-oil Nannochloropsis by placing a small amount of mixed origin Nannochloropsis culture in the bottom of a test-tube and covering with a column of water.

Since it is almost impossible to accomplish that without inducing turbulence and mixing, the smart money would not use pure water, they would use water with a gelling agent to increase the media's viscosity. Viscosity is what dissipates the energy of turbulence. Increased viscosity would also make the "race" to the top longer and provide more resolution in the population. 

It would also favor Nannochloropsis with larger diameters as smaller particles have relatively larger viscous drag than larger ones.

I am not convinced that mid-Western waters need to be inoculated with Nannochloropsis  cultures because the factors that favor Nannochloropsis  algae occur every spring with snow-melt. They are probably ubiquitous and it is not documented because nobody went looking for and categorizing all of the species of 2-to-5 micron algae.

---End Aside---

Commercial exploitation

These issues conspire to make commercial production of Nannochloropsis not economical. The fact that the organisms are highly dispersed means that they need to be harvested by filtering large volumes of water.

Their small size means that the filters would need a very fine mesh and will clog quickly.

Unlike corn plants which stand still and gain mass over time, algae levels (if centrifuged out flat) never get much more than paper-thin from a mass-standpoint. Their large, annual production is due to their rapid replacement rates as existing algae gets consumed. That means frequent harvesting...which means high energy costs.

At this time, the most promising method to gain benefit from their ability to produce edible fats-and-oils and EPA is to use biological processes to harvest them and then to harvest those fish and mussels.

If one were inclined to tip the scales toward Nannochloropsis  production one might make the water less transparent to favor the floating algae in the well-lit water closest to the surface (the most buoyant algae). They could also eliminate features that provide anchorage for filamentous algae and if they fertilize to do so with a very light hand.

Bonus images

Locations of major up-welling currents in red

Distribution of phytoplankton (algae) in the oceans.

Guess where the fish are.