Influence of Nitrogen Limitation on Lipid Accumulation and EPA and DHA Content in Four Marine Microalgae for Possible Use in Aquafeed

Picture a salmon farm somewhere off the Norwegian coast. The fish are fed pellets, and those pellets contain oil — specifically, oil rich in eicosapentaenoic acid and docosahexaenoic acid, the omega-3 fatty acids that make farmed salmon worth eating in the first place. That oil mostly comes from wild-caught fish. And here is the problem: the wild fisheries supplying that oil are under pressure, aquaculture is growing faster than the supply can follow, and vegetable oils can't fill the gap because they simply don't contain the right fats. Somewhere in that squeeze, a group of researchers in Norway asked a question worth taking seriously: can marine microalgae — the organisms that make these fatty acids in the first place, the original source at the bottom of the marine food chain — be grown at scale to replace fish oil in aquafeeds? Wang and colleagues investigated four species of marine microalgae and a specific biological lever for pushing them to make more fat. The lever is nitrogen limitation. When microalgae run short of nitrogen, they stop investing carbon in protein and start stockpiling it as fat — specifically, as neutral lipids in the form of triacylglycerol, or TAG. It's a metabolic switch. The cell shifts from building itself to saving for later. Wang and colleagues cite earlier work by Jia and colleagues showing that in Nannochloropsis oceanica alone, glycerolipids jumped from around eight percent of dry weight to forty-four percent when nitrogen was removed entirely, and eicosapentaenoic acid — EPA — stored in TAG rose from 3.6 to 21 micromoles per gram dry weight. But here is the complication that runs through the entire paper. The fatty acids most useful for fish — eicosapentaenoic acid and docosahexaenoic acid, or DHA — are nutritionally more available when they sit inside polar lipids, the membrane-bound fats, than when they're locked into TAG. Nitrogen starvation fills the cell with TAG. That's exactly what makes this trade-off worth examining carefully. To stress the algae in two different ways, Wang and colleagues used two cultivation setups. Batch culture creates strong nitrogen limitation — the nutrient depletes and the cells have to cope. Semi-continuous culture maintains moderate, ongoing limitation, keeping growth going while still elevating fat content. The distinction turns out to matter enormously for what you actually get out of the system. The four species the team studied — Phaeodactylum tricornutum, Isochrysis aff. galbana clone T-Iso, Rhodomonas baltica, and Nannochloropsis oceanica — each responded to nitrogen stress with a distinct strategy, and the contrasts between them are where the paper gets interesting. Phaeodactylum tricornutum, a diatom, responded to strong nitrogen limitation by converting its membrane lipids into TAG, enriching that storage pool with monounsaturated fats — particularly 16:1n-7 — while the fraction of polyunsaturated fatty acids dropped by forty-three percent. Eicosapentaenoic acid, the fatty acid that Phaeodactylum tricornutum is known for, fell as a proportion of total fatty acids under batch conditions. But under semi-continuous culture, eicosapentaenoic acid production actually doubled compared to batch. The lesson from this species is that the percentage of a valuable fatty acid and the amount you can harvest are different things, and strong stress can move them in opposite directions. Rhodomonas baltica was the most nitrogen-sensitive of the four. It was the only species to carry both eicosapentaenoic acid and docosahexaenoic acid in meaningful amounts under normal conditions, but strong nitrogen limitation devastated its polyunsaturated fatty acid content — the team documents a fifty-one to seventy-seven percent decrease in all n-3 polyunsaturated fatty acids under batch conditions. Monounsaturated fats rose to take their place, with 18:1n-9 approximately doubling. This is a species with a rich nutritional profile that simply can't maintain it under severe stress. T-Iso, the Isochrysis clone, broke the pattern in a way that deserves a pause. It was the only species in which the docosahexaenoic acid fraction actually increased with stronger nitrogen limitation — a sixteen percent rise in docosahexaenoic acid, and a thirty percent increase in the percentage of docosahexaenoic acid in TAG under stronger limitation. Total polyunsaturated fatty acid content rose under nitrogen stress, not fell. This is the opposite of what the other three species did. The reason isn't fully resolved, but the result is practically significant. T-Iso under moderate nitrogen limitation delivered the highest docosahexaenoic acid productivity of any species in the study, primarily because it maintained strong biomass growth while increasing its docosahexaenoic acid fraction. Docosahexaenoic acid productivity doubled in T-Iso when comparing semi-continuous to batch culture. Then there is Nannochloropsis oceanica, which committed hardest to the storage-lipid strategy. Under moderate nitrogen limitation, it accumulated fifty-one percent of its dry weight as lipid — a remarkable number. Up to eighty-seven percent of its fatty acids were localized in TAG across both culture modes. It also delivered the highest lipid productivity of the four species in semi-continuous culture, at seven point five one milligrams per liter per day. By comparison, Phaeodactylum tricornutum and Rhodomonas baltica in the same conditions reached only one point four three and one point five zero milligrams per liter per day, respectively. Nannochloropsis oceanica also showed the highest eicosapentaenoic acid productivity among all four species — a consequence Wang and colleagues attribute directly to the sheer volume of lipid it produces, even though eicosapentaenoic acid's fractional share of those lipids is modest. Now comes the finding that reframes everything. Total lipid productivity showed no increase in batch culture despite the fact that stronger nitrogen limitation led to lipid accumulation. The cells got fatter, but the culture got less productive. Biomass productivity fell by sixty to sixty-seven percent in batch conditions across all species except Nannochloropsis oceanica. More fat per cell, fewer cells. The math doesn't help you. This is the central tension of the paper, and it has a direct practical consequence: if you're trying to produce eicosapentaenoic acid and docosahexaenoic acid at scale, what matters is productivity — the amount of fatty acid you harvest per liter per day — not what fraction of each cell is fat. Nannochloropsis oceanica partly escaped this trap because its lipid accumulation was so large that even under moderate limitation it dominated the productivity rankings. T-Iso escaped it differently: by holding biomass growth steady under moderate limitation while increasing its docosahexaenoic acid fraction. Both paths lead to the same result — high polyunsaturated fatty acid productivity — but through entirely different biology. Wang and colleagues land on a practical conclusion that is carefully stated. Nannochloropsis oceanica and T-Iso are the two most promising candidates for sustainable n-3 long-chain polyunsaturated fatty acid production under moderate nitrogen limitation. Nannochloropsis oceanica's extreme TAG accumulation also makes it a candidate for biodiesel — a second use case that might improve the economics of growing it at scale. T-Iso earns its place through docosahexaenoic acid: its unusual ability to increase docosahexaenoic acid under nitrogen stress, combined with robust biomass, makes it the standout for that specific fatty acid. But the paper doesn't let optimism run ahead of the data. The honest limitation is this: as nitrogen limitation intensifies and more fatty acids migrate into TAG, the fraction of eicosapentaenoic acid and docosahexaenoic acid sitting in polar lipids — where they are most bioavailable to fish — decreases. The very conditions that make microalgae fatter also make that fat nutritionally less accessible. Wang and colleagues frame the open question plainly: can cultivation strategies be tuned to keep lipid content high while preserving the polar lipid fraction? That might mean moderate rather than extreme nitrogen stress, as this study suggests. It might mean strain selection or engineering the metabolism of the cell itself. What this paper establishes is the shape of the problem. Microalgae can make eicosapentaenoic acid and docosahexaenoic acid in quantities that matter. The right species under the right nitrogen regime can do it productively. But productivity and bioavailability pull against each other, and the path from a laboratory flask to a salmon feed pellet will require holding both in mind at once. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.