Biology subjects

Santin, A.

Publications and source records attributed to Santin, A..

3 recordsLinked to original sources

bioRxiv2026

NRT2.2 is essential for nitrate uptake and carbon-nitrogen balance in Nannochloropsis oceanica

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.

plant biology↗

bioRxiv2026

Mapping the pathway for protein secretion in the secondary endosymbiotic alga Nannochloropsis oceanica

Microalgae are key primary producers in marine ecosystems, and their interactions with the surrounding environment rely on the secretion of intracellular metabolites and macromolecules, particularly proteins, supporting essential functions such as nutrient acquisition, environmental sensing and biotic interactions. Most abundant and ecologically relevant seawater algae are secondary endosymbionts, where multiple endosymbiotic events extensively reshaped plastids and intracellular membrane systems, requiring adaptation of protein trafficking mechanisms. This study presents the identification of signal peptides that direct protein secretion in the seawater microalga Nannochloropsis oceanica. Their expression in frame with a fluorescent tag enabled to reconstruct the protein secretion pathway in this organism. Proteins channelled for export are first targeted to the periplastidial compartment, an exclusive structure of secondary endosymbiotic algae, that acts as hub for protein trafficking. Subsequently, vesicle-mediated transport directs proteins through the endoplasmic reticulum into the periplasmic space between the cell membrane and the cell wall, from where they are released upon cell division. These findings reveal an evolutionarily remodeled protein secretion pathway, in which host- and endosymbiont-derived trafficking mechanisms merged into an integrated functional system. Significance StatementThe most abundant and ecologically relevant marine algae are secondary endosymbionts whose evolution required extensive re-adaptation of multiple cellular processes. Among them, protein secretion is essential for the interaction with external environment, and required specific re-shaping to the increased cellular complexity associated with endosymbiosis. This work uncovers protein secretory pathway in the secondary endosymbiont seawater alga Nannochloropsis oceanica showing that is does not follow a direct route, but proteins are first accumulated in the periplastidial compartment, a unique structure derived from its endosymbiotic history, before being directed for secretion. The final pathway integrated components derived from both the host and endosymbiont, highlighting how evolution was able to merge different biological modules to build an integrated and functional system.

plant biology↗

bioRxiv2025

Photosynthesis regulation impacts carbon and nitrogen assimilation in the diazotrophic cyanobacterium Anabaena sp. PCC 7120

O_LIDiazotrophic cyanobacteria fix both atmospheric carbon (C) and nitrogen (N) into biomass, but the two assimilation pathways are not compatible. Species like Anabaena sp. PCC 7120 physically separates C and N assimilation in different cell types. Even if separated, they are strongly intertwined, as N assimilation relies on the C skeletons and reducing power from photosynthesis, that in turn depends on N rich molecules as pigments and proteins. C_LIO_LIWhereas the two pathways have been extensively studied individually, here we investigate their interaction by analysing photosynthetic properties upon exposure to changes in light, CO2 and N availability, including the contribution of photosynthetic electron fluxes. C_LIO_LIGrowth depended on the availability of both light and CO2, while the N2 fixation activity mainly on the C supply. Upon diazotrophic conditions, the total photosynthetic electron transport activity increased, with a modified contribution of different electron pathways. A mutant strain affected in the vehiculation of fixed N between cell types showed that the modulation of photosynthesis depended on the metabolic connection between assimilation pathways. C_LIO_LIOverall, data showed that the regulation of photosynthetic electron fluxes is a major component of the synergic metabolic relationship between C and N assimilation pathways upon dynamic environmental conditions. C_LI

plant biology↗