Biology subjects

Bellan, A.

Publications and source records attributed to Bellan, 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↗

bioRxiv2024

The Xanthophyll Cycle balances Photoprotection and Efficiency in the seawater alga Nannochloropsis oceanica

Photosynthetic reactions require continuous modulation to respond to highly dynamic environmental conditions. Regulation of photosynthesis involves various mechanisms, which differ across phylogenetic groups. One such mechanism, found widespread in photosynthetic eukaryotes, is the xanthophyll cycle, which involves the reversible light-dependent conversion between the carotenoids violaxanthin, antheraxanthin, and zeaxanthin. In this study, we investigated the impact of the xanthophyll cycle in Nannochloropsis oceanica, a seawater microalga member of Eustigmatophyta that features a peculiarly high content of xanthophylls. We generated and characterized lines with increased levels of the enzymes involved in the xanthophyll cycle, i.e. violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZEP) and demonstrated that their content is a main factor in controlling the overall reaction rates and the dynamics of the xanthophyll cycle. Subsequent differences in the xanthophyll profile affect the activation of photoprotection mechanisms such as non-photochemical quenching and tolerance to reactive oxygen species. Interestingly, overexpression of VDE expands the limits of high light tolerance, whereas the increased content of ZEP facilitates faster recovery after exposure to light but also heightened photosensitivity under some conditions. These findings underscore the critical role of the xanthophyll cycle in the regulation of photosynthesis in Nannochloropsis, where it is not simply a mechanism to respond to excess illumination, but plays a central role in modulating photosynthesis, fulfilling the complex task of balancing photoprotection and light-use efficiency under different environmental conditions.

plant biology↗

bioRxiv2022

Modulation of xanthophyll cycle impacts biomass productivity in the marine microalga Nannochloropsis

Life on earth depends on photosynthetic primary producers that exploit sunlight to fix CO2 into biomass. Approximately half of global primary production is associated with microalgae living in aquatic environments. Microalgae also represent a promising source of biomass to complement crop cultivation, and they could contribute to the development of a more sustainable bioeconomy. Photosynthetic organisms evolved multiple mechanisms involved in the regulation of photosynthesis to respond to highly variable environmental conditions. While essential to avoid photodamage, regulation of photosynthesis results in dissipation of absorbed light energy, generating a complex trade-off between protection from stress and light-use efficiency. This work investigates the impact of the xanthophyll cycle, the light-induced reversible conversion of violaxanthin into zeaxanthin, on the protection from excess light and on biomass productivity in the marine microalgae of the genus Nannochloropsis. Zeaxanthin is shown to have an essential role in protection from excess light, contributing to the induction of Non-Photochemical Quenching and scavenging of reactive oxygen species. On the other hand, the overexpression of Zeaxanthin Epoxidase, enables a faster re-conversion of zeaxanthin to violaxanthin that is shown to be advantageous for biomass productivity in dense cultures in photobioreactors. These results demonstrate that zeaxanthin accumulation is critical to respond to strong illumination, but it may lead to unnecessary energy losses in light-limiting conditions, and accelerating its re-conversion to violaxanthin provides an advantage for biomass productivity in microalgae. Significance StatementThis work investigates the impact of the xanthophyll cycle in marine microalgae on the trade-off between photoprotection and light-use efficiency. Our results demonstrate that whilst zeaxanthin is essential for photoprotection upon exposure to strong illumination, it leads to unnecessary energy losses in light-limiting conditions and thus accelerating its re-conversion to violaxanthin provides an advantage for biomass productivity in microalgae.

plant biology↗