The Transition Toward Nitrogen Deprivation in Diatoms Requires Chloroplast Stand-By and Deep Metabolic Reshuffling

Abstract

Microalgae have adapted to face abiotic stresses by accumulating energy storage molecules such as lipids, which are also of interest to industries. Unfortunately, the impairment in cell division during the accumulation of these molecules constitutes a major bottleneck for the development of efficient microalgae-based biotechnology processes. To address the bottleneck, a multidisciplinary approach was used to study the mechanisms involved in the transition from nitrogen repletion to nitrogen starvation conditions in the marine diatom Phaeodactylum tricornutum that was cultured in a turbidostat. Combining data demonstrate that the different steps of nitrogen deficiency clustered together in a single state in which cells are in equilibrium with their environment. The switch between the nitrogen-replete and the nitrogen-deficient equilibrium is driven by intracellular nitrogen availability. The switch induces a major gene expression change, which is reflected in the reorientation of the carbon metabolism toward an energy storage mode while still operating as a metabolic flywheel. Although the photosynthetic activity is reduced, the chloroplast is kept in a stand-by mode allowing a fast resuming upon nitrogen repletion. Altogether, these results contribute to the understanding of the intricate response of diatoms under stress.

Bioinformatics-Based Screening Approach for the Identification and Characterization of Lipolytic Enzymes from the Marine Diatom Phaeodactylum tricornutum

“…It is well-known that nitrogen starvation induces TAG accumulation in diatoms, mainly through a recycling of polar lipids [ 75 ], however the role of TAG lipases in this accumulation process is mostly unknown. As their main function is to degrade TAG, expression of these proteins is expected to be downregulated during a nitrogen stress, and to be upregulated when nitrogen is resupplied.…”

“…To identify in our dataset which candidate lipases are responsive to stress conditions, the regulation of their transcripts, expressed as a log2 value, was retrieved from three experiments that specifically investigated the response of the whole cell to nitrogen starvation over time [ 75 , 76 , 77 ]. The three datasets exposed P. tricornutum cultures to a shift from standard nitrate-containing f/2 medium to nitrate deprived f/2.…”

“…The three datasets exposed P. tricornutum cultures to a shift from standard nitrate-containing f/2 medium to nitrate deprived f/2. Those of Matthijs et al (2016) [ 77 ] and Alipanah et al (2015) [ 76 ] describe the response of the Pt1 ecotype (CCAP 1055/1) to a nitrogen stress in batch cultures during respectively 4 to 20 h and 48 to 72 h. The transcriptomic dataset of Scarsini et al (2022) [ 75 ] monitors the response of the Pt4 ecotype (UTEX 646) during 15 days in a progressive transition to nitrogen deprivation.…”

“…[ 78 ], was also down-regulated (log2 −1.44). This is surprising because it has been previously described that during a nitrogen starvation in P. tricornutum , galactolipids are degraded and their acyl chains transferred to newly formed TAG [ 55 , 75 ].…”

“…It seems paradoxical that some putative TAG lipases are upregulated during neutral lipid storage. However, it has already been reported that lipases can also hydrolyze other substrates such as polar lipids from which acyl chains can be incorporated into newly formed neutral lipids in a process of recycling [ 55 , 75 , 79 ]. Indeed Nomaguchi et al (2018) have found in the diatom Fistulifera solaris that some lipases are upregulated, while others are downregulated under lipid degradation conditions [ 52 ].…”