Bioenergetic reprogramming plasticity under nitrogen depletion by the unicellular green alga Scenedesmus obliquus

Abstract

Simultaneous nitrogen depletion and 3,4-dichlorophenol addition induce a bioenergetic microalgal reprogramming, through strong Cyt b f synthesis, that quench excess electrons from dichlorophenol's biodegradation to an overactivated photosynthetic electron flow and H -productivity. Cellular energy management includes "rational" planning and operation of energy production and energy consumption units. Microalgae seem to have the ability to calculate their energy reserves and select the most profitable bioenergetic pathways. Under oxygenic mixotrophic conditions, microalgae invest the exogenously supplied carbon source (glucose) to biomass increase. If 3,4-dichlorophenol is added in the culture medium, then glucose is invested more to biodegradation rather than to growth. The biodegradation yield is enhanced in nitrogen-depleted conditions, because of an increase in the starch accumulation and a delay in the establishment of oxygen-depleted conditions in a closed system. In nitrogen-depleted conditions, starch cannot be invested in PSII-dependent and PSII-independent pathways for H-production, mainly because of a strong decrease of the cytochrome b f complex of the photosynthetic electron flow. For this reason, it seems more profitable for the microalga under these conditions to direct the metabolism to the synthesis of lipids as cellular energy reserves. Nitrogen-depleted conditions with exogenously supplied 3,4-dichlorophenol induce reprogramming of the microalgal bioenergetic strategy. Cytochrome b f is strongly synthesized (mainly through catabolism of polyamines) to manage the electron bypass from the dichlorophenol biodegradation procedure to the photosynthetic electron flow (at the level of PQ pool) and consequently through cytochrome b f and PSI to hydrogenase and H-production. All the above showed that the selection of the appropriate cultivation conditions is the key for the manipulation of microalgal bioenergetic strategy that leads to different metabolic products and paves the way for a future microalgal "smart" biotechnology.

A multidisciplinary review ofTetradesmus obliquus: a microalga suitable for large‐scale biomass production and emerging environmental applications

“…T. obliquus is highly tolerant to high temperature (Yang et al 2018) and irradiance (Hurtado et al 2019). Moreover, it also can grow under heterotrophic and mixotrophic conditions (Shen et al 2018;Di Caprio et al 2019), and different nutrient strategies have been evaluated (Papazi et al 2018;Qu et al 2019). Therefore, the continuous study of T. obliquus cultivation techniques improvement is an important step for the validation of a productive technological package for this species.…”

Biohydrogen from Microalgae: Production and Applications

“…The less studied condition of nitrogen (N) deprivation induces hydrogen production in Chlamydomonas reinhardtii, albeit with a delayed effect and lower yields, suggesting a different mechanism in establishing process [29]. In Scenedesmus obliquus, N deprivation modulates metabolism towards lipids accumulation to be used as an energy reserve, suggesting that this strategy is not advantageous for the purposes of hydrogen production [30]. In ma-rine Chlorella spp., where S deprivation is inapplicable due to the sulfates-rich presence in seawater, the effect of phosphorus (P) deficiency was evaluated.…”

Microalgal Hydrogen Production

“…Under nitrogen‐deficient conditions of Scenedesmus obliquus , starch cannot be invested in PSII‐dependent and PSII‐in dependent pathways to produce hydrogen, mainly because of the large decrease in cytochrome b6f complex of photosynthetic electron flow. However, after the addition of 3,4‐dichlorophenol, cytochrome b6f is strongly synthesized (mainly by catabolism of polyamines) to regenerate hydrogen …”

“…However, after the addition of 3,4-dichlorophenol, cytochrome b6f is strongly synthesized (mainly by catabolism of polyamines) to regenerate hydrogen. [186]…”