NRT2.2 is essential for nitrate uptake and carbon-nitrogen balance in Nannochloropsis oceanica
- 1University of Padova
Abstract
Nitrate is the dominant nitrogen source for marine microalgae, yet how it is taken up remains poorly understood in many ecologically and biotechnologically relevant species. Here we show that NITRATE TRANSPORTER 2.2 (NRT2.2) is the principal nitrate transporter of the oleaginous marine microalga Nannochloropsis oceanica. N. oceanica encodes only two NRT2 proteins and, unlike land plants and green algae, lacks a recognizable homolog of NITRATE ASSIMILATION-RELATED 2 (NAR2), the partner protein that green-lineage NRT2s require for activity and plasma membrane targeting. CRISPR knock-outs and confocal imaging showed that NRT2.2 is located at the plasma membrane and is required for nitrate uptake, whereas its paralog NRT2.1 cannot compensate for its loss. When nitrate was resupplied after nitrogen starvation, nrt2.2 mutants did not take up nitrate and did not resume growth, while they recovered normally with ammonium. Their photosystem II efficiency fell below that of fully nitrogen-starved wild-type cells, and chlorophyll content was reduced, while total protein remained stable. Fixed carbon was redirected into storage, with lipids and carbohydrates accumulating above the levels of all strains after 3 days of nitrogen starvation. Loss of NRT2.2 therefore mimics a prolonged nitrogen starvation, even though nitrate is available. Because nitrate was present but could not be used, recovery from starvation depends on the capacity to take up nitrate, not only on its availability. In the ocean, where nitrate is depleted for long periods and resupplied episodically, NRT2.2 is thus likely to set how quickly Nannochloropsis can use new nitrate inputs and resume growth.
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BibTeX
@misc{santin2026nrt22,
title = {{NRT2.2 is essential for nitrate uptake and carbon-nitrogen balance in Nannochloropsis oceanica}},
author = {A. Santin and L. Verdesca and A. Bellan and M. Minio and c. Gerotto and T. Morosinotto and G. Perin and A. Alboresi},
howpublished = {bioRxiv},
year = {2026},
month = nov,
doi = {10.64898/2026.11.02.763162},
url = {https://www.biorxiv.org/content/10.64898/2026.11.02.763162v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - NRT2.2 is essential for nitrate uptake and carbon-nitrogen balance in Nannochloropsis oceanica
AU - A. Santin
AU - L. Verdesca
AU - A. Bellan
AU - M. Minio
AU - c. Gerotto
AU - T. Morosinotto
AU - G. Perin
AU - A. Alboresi
PY - 2026
DA - 2026/11/02
PB - bioRxiv
DO - 10.64898/2026.11.02.763162
UR - https://www.biorxiv.org/content/10.64898/2026.11.02.763162v1
AB - Nitrate is the dominant nitrogen source for marine microalgae, yet how it is taken up remains poorly understood in many ecologically and biotechnologically relevant species. Here we show that NITRATE TRANSPORTER 2.2 (NRT2.2) is the principal nitrate transporter of the oleaginous marine microalga Nannochloropsis oceanica. N. oceanica encodes only two NRT2 proteins and, unlike land plants and green algae, lacks a recognizable homolog of NITRATE ASSIMILATION-RELATED 2 (NAR2), the partner protein that green-lineage NRT2s require for activity and plasma membrane targeting. CRISPR knock-outs and confocal imaging showed that NRT2.2 is located at the plasma membrane and is required for nitrate uptake, whereas its paralog NRT2.1 cannot compensate for its loss. When nitrate was resupplied after nitrogen starvation, nrt2.2 mutants did not take up nitrate and did not resume growth, while they recovered normally with ammonium. Their photosystem II efficiency fell below that of fully nitrogen-starved wild-type cells, and chlorophyll content was reduced, while total protein remained stable. Fixed carbon was redirected into storage, with lipids and carbohydrates accumulating above the levels of all strains after 3 days of nitrogen starvation. Loss of NRT2.2 therefore mimics a prolonged nitrogen starvation, even though nitrate is available. Because nitrate was present but could not be used, recovery from starvation depends on the capacity to take up nitrate, not only on its availability. In the ocean, where nitrate is depleted for long periods and resupplied episodically, NRT2.2 is thus likely to set how quickly Nannochloropsis can use new nitrate inputs and resume growth.
ER -