The Role of Phosphorus in the Enhancement of Microalgal Biodiesel Productivity Under Nitrogen Starvation

Abstract

Microalgal biodiesel is gaining recognition worldwide as an alternative fuel to replace conventional fuels because of its advantages on renewability, sustainability and cleaner emissions. Nitrogen (N) deprivation or limitation has taken prominence in recent years due to the ability to stimulate the increase of lipid content in microalgal cells. Nevertheless, the higher lipid content obtained was always offset by lower biomass production, resulting in not much of an increase in lipid productivity. Therefore, many researchers deemed biodiesel production in complete nutrient media to be more economical. However, the role of phosphorus (P) may be ignored since algal cells usually assimilate more P under stress conditions.

To investigate the role of P in lipid production under N starvation conditions, five types of media possessing different N and P concentrations or their combination were prepared to culture Chlorella vulgaris FACHB-1072. The biomass production under N deficient condition with sufficient P supply was similar to that of the control (with sufficient nutrients), resulting in a maximum lipid productivity of 58.39 mg L^-1 day^-1. Meanwhile, ³¹P NMR showed that P in the medium was transformed and accumulated as polyphosphate in cells. The uptake rate of P in augmented cells was 3.8 times higher than the uptake rate of the control. These results demonstrated that P plays an important role in lipid production of C. vulgaris under N deficient conditions.

In addition, two other significant parameters (CO₂ concentration and light intensity) with respect to the biodiesel productivity and P uptake rate of C. vulgaris were optimized, and the optimized conditions were 4% CO₂ concentration and 200 μmol photons m^-2 s^-1 light intensity. The maximum biodiesel productivity was 34.56 mg L^-1 day^-1; 2.7 times higher than the control (nutrient sufficient condition). P was accumulated as polyphosphate and its maximum uptake rate was 2.08 mg L^-1 day^-1; twice that of the control. After optimization, the performances under N deficiency were significantly better compared with those under N sufficiency, which has rarely been reported in the literature.

Furthermore, the utilization of polyphosphates and the regreening process in N resupplying conditions, especially for biodiesel reviving, were also investigated. This regreening process was completed within approximately 3-5 days. Polyphosphates were first degraded within 3 days in the degreening process with and without an external P supply and the degradation was prior to the assimilation of phosphate in the media with an external P offering. Nitrate assimilation was dramatically influenced by the starvation of P after polyphosphates were exhausted in the medium without external phosphates, and then the reviving process of biomass and biodiesel production was strictly impeded. It is thus reasonable to assume that to provide external N and P simultaneously is essential for overall biodiesel production reviving during the regreening process.

In order to study the effect of P on biodiesel production under N deficiency conditions, another common green algae, Scenedesmus obliquus, was applied. Six types of media with combinations of N repletion/depletion and P repletion/limitation/depletion were investigated, and N starvation compared to N repletion enhanced biodiesel productivity. Moreover, biodiesel productivity was further strengthened by varying the supply level of P from depletion, limitation, through to repletion. The maximum fatty acid methyl esters productivity was obtained in N depletion with P repletion, which was twice as high as that in nutrient complete medium. More P was accumulated in cells under N starvation with sufficient P, but no polyphosphate was formed.

This study indicated that N starvation plus a sufficient P supply might be the real “lipid trigger” for many algal species. Furthermore, results of the current study suggest a potential application for utilizing microalgae to combine P removal from wastewater with biodiesel production.

Date of Award	12 Jun 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kwan Sing Paul LAM (Supervisor)