A missing nitrate transporter leaves marine algae starved amid plenty

A marine microalga can remain nitrogen-starved even when its surroundings contain abundant nitrate. In Nannochloropsis oceanica, researchers found that disrupting a single gene, NRT2.2, prevented nitrate uptake and recovery from nitrogen starvation. The affected cells stopped multiplying, lost chlorophyll and accumulated more lipids and carbohydrates than nitrate-fed controls.

The study, available as a bioRxiv preprint, has not been peer reviewed. Its experiments provide causal evidence that NRT2.2 is required for nitrate-supported recovery under the conditions tested. They also reveal accompanying metabolic changes, although some proposed mechanisms and ecological consequences remain interpretations rather than direct measurements.

Testing the route into the cell

The researchers identified two NRT2-family genes in N. oceanica: NRT2.1 and NRT2.2. Using CRISPR-Cas12a editing, they generated two independent knockout lines for each gene and confirmed that the corresponding transcripts were no longer detectable.

Cells first spent three days without nitrogen to deplete their reserves. The team then supplied either nitrate or ammonium as the sole nitrogen source and followed recovery over four days, with at least three biological replicates per condition.

When supplied with 882 micromolar nitrate, wild-type cells and NRT2.1 knockout cells reduced the nitrate remaining in the medium to below 100 micromolar and roughly tripled their cell density. Neither NRT2.2 knockout line measurably depleted nitrate or increased in cell number. The same qualitative defect occurred at 250 micromolar nitrate.

Crucially, wild-type cultures consumed about one-fifth of the supplied nitrate during the first day, before cell numbers increased. That timing argues against failed uptake simply being a consequence of failed growth. NRT2.2 mutants also recovered normally when given ammonium, narrowing the defect to nitrate utilization rather than a general inability to grow after starvation.

Fluorescently tagged NRT2.2 formed a ring around the cell periphery, separate from chlorophyll fluorescence. This imaging supports localization at the plasma membrane, consistent with an entry route for external nitrate. Together, the genetic, nutrient-resupply and imaging results identify NRT2.2 as the dominant nitrate transporter in these experiments. NRT2.1 did not compensate for its loss.

Growth stalls while storage increases

The uptake block had consequences beyond cell division. After nitrate resupply, NRT2.2 knockout cells had maximum photosystem II efficiencies of 0.43–0.49, compared with about 0.65 in wild-type cells. This fluorescence-based measure assesses photosynthetic performance; it is not a direct measurement of carbon fixation. Chlorophyll content was reduced by up to approximately half relative to nitrate-fed controls, while total protein per cell remained stable.

Infrared spectroscopy showed that the mutants’ lipid-to-protein band ratio was about 3.7 times the wild-type value, and their carbohydrate-to-protein ratio about 1.4 times higher. An independent fluorescent staining assay for neutral lipids produced a signal approximately three times higher than in wild-type cells supplied with nitrate.

These measurements support increased carbon-rich storage in cells unable to acquire nitrate. The authors interpret this as carbon being redirected away from nitrogen-dependent growth and toward lipids and carbohydrates. However, the study did not directly track carbon flux through those pathways, and spectroscopy and staining signals should not be equated with absolute production yields.

The nitrogen measurements also require care. Nitrogen as a fraction of dry weight fell below the assay’s 4% detection threshold in five of six mutant samples. That does not mean the cells contained no nitrogen. Because protein was measured per cell and nitrogen per dry weight, accumulating carbon-rich material could lower the measured nitrogen fraction without a corresponding loss of total protein per cell.

What the experiments leave open

The mutants’ condition resembled prolonged nitrogen starvation, but comparisons with the starvation controls involved different durations. By the final nitrate-resupply measurement, NRT2.2 mutants had effectively spent seven days without usable nitrogen, whereas the initial starvation samples represented three days. Their poorer photosynthetic performance cannot therefore be attributed solely to a distinctive effect of transporter loss rather than longer deprivation.

Other questions concern the transporter itself. Although NRT2 proteins are associated with high-affinity uptake in plants and green algae, this study used nitrate concentrations well above those typical of nutrient-poor seawater. NRT2.2’s affinity remains unmeasured, and NRT2.1 could have a role at lower concentrations or under other conditions.

Genome searches found no recognizable NAR2 partner protein, which supports NRT2 activity in plants and green algae. That leaves open whether NRT2.2 functions alone or with an unidentified partner. The authors also propose that its activity could influence recovery after oceanic nitrate pulses, but they did not test natural populations or community responses. The demonstrated result is narrower: in these laboratory cultures, nitrate availability could not restore growth without the machinery needed to take it up.