Stress induced lipid production in Chlorella vulgaris: Relationship with specific intracellular reactive species levels
Microalgae have significant potential to be an important alternative energy source, but the challenges to the commercialization of bio-oil from microalgae need to be overcome for the potential to be realized. The application of stress can be used to improve bio-oil yields from algae. Nevertheless, the understanding of stress effects is fragmented due to the lack of a suitable, direct quantitative marker for stress. The lack of understanding seems to have limited the development of stress based strategies to improve bio-oil yields, and hence the commercialization of microalgae-based bio-oil. In this study, we have proposed and used the specific intracellular reactive species levels (siROS) particularly hydroxyl and superoxide radical levels, separately, as direct, quantitative, markers for stress, irrespective of the type of stress induced. Although ROS reactions are extremely rapid, the siROS level can be assumed to be at pseudo-steady state compared to the time scales of metabolism, growth and production, and hence they can be effective stress markers at particular time points. Also, the specific intracellular (si-) hydroxyl and superoxide radical levels are easy to measure through fluorimetry. Interestingly, irrespective of the conditions employed in this study, that is, nutrient excess/limitation or different light wavelengths, the cell concentrations are correlated to the siROS levels in an inverse power law fashion. The composite plots of cell concentration (y) and siROS (x) yielded the correlations of y=k1·x-0.7 and y=k2·x-0.79, for si-hydroxyl and si-superoxide radical levels, respectively. The specific intracellular (si-) neutral lipid levels, which determine the bio-oil productivity, are related in a direct power law fashion to the specific hydroxyl radical levels. The composite plot of si-neutral lipid levels (z) and si-hydroxyl radical level (x) yielded a correlation of z=k3·x0.65. More interestingly, a nutrient shift caused a significant change in the sensitivity of neutral lipid accumulation to the si-hydroxyl radical levels. © 2013 Wiley Periodicals, Inc.
Sustainable Diesel Feedstock: a Comparison of Oleaginous Bacterial and Microalgal Model Systems
The key to sustainable and commercially viable biodiesel production relies primarily on species selection. Oleaginous species with high biomass productivity, lipid content, and lipid productivity are desirable. High growth rate of the species results in high biomass productivity, which leads to high lipid productivity. It is known that algal oil technology lacks commercial feasibility predominantly due to low biomass productivity and other factors. The use of a faster-growing organism, such as oleaginous bacteria, could offset this major disadvantage. Thus, the current study analyzes two model oleaginous systems: Rhodococcus opacus PD630 (a bacterium) and Chlorella vulgaris NIOT5 (a microalga) for their growth rate and lipid productivity. It was found that the bacterial growth rate was 25-fold the microalgal growth rate. The bacterium also showed 57-fold higher biomass productivity and 75-fold higher biodiesel productivity. Further, the analysis of a large number of literature data from relevant studies under different cultivation conditions showed that R. opacus PD630 has productivities far higher than various autotrophic microalgae. Similarly, a frequency distribution of data collected from the literature showed that Rhodococcus sp. has productivities in the higher range as compared to heterotrophic microalgae. Thus, bacteria could serve as a better alternative to microalgae toward developing a commercially viable biofuel technology. Further, the biodiesel characterization study showed that the quality of diesel from the bacterium was better than that from the microalga.
Photon up-conversion increases biomass yield in Chlorella vulgaris
Photon up-conversion, a process whereby lower energy radiations are converted to higher energy levels via the use of appropriate phosphor systems, was employed as a novel strategy for improving microalgal growth and lipid productivity. Photon up-conversion enables the utilization of regions of the solar spectrum, beyond the typical photosynthetically active radiation, that are usually wasted or are damaging to the algae. The effects of up-conversion of red light by two distinct sets of up-conversion phosphors were studied in the model microalgae Chlorella vulgaris. Up-conversion by set 1 phosphors led to a 2.85 fold increase in biomass concentration and a 3.2 fold increase in specific growth rate of the microalgae. While up-conversion by set 2 phosphors resulted in a 30% increase in biomass and 12% increase in specific intracellular neutral lipid, while the specific growth rates were comparable to that of the control. Furthermore, up-conversion resulted in higher levels of specific intracellular reactive oxygen species in C. vulgaris. Up-conversion of red light (654 nm) was shown to improve biomass yields in C. vulgaris. In principle, up-conversion can be used to increase the utilization range of the electromagnetic spectrum for improved cultivation of photosynthetic systems such as plants, algae, and microalgae.
Simultaneous increases in specific growth rate and specific lipid content of Chlorella vulgaris through UV-induced reactive species
A challenge in algae-based bio-oil production is to simultaneously enhance specific growth rates and specific lipid content. We have demonstrated simultaneous increases in both the above in Chlorella vulgaris through reactive species (RS) induced under ultraviolet (UV) A and UVB light treatments. We postulated that the changes in photosystem (PS) stoichiometry and antenna size were responsible for the increases in specific growth rate. UVB treatment excited PSII, which resulted in a twofold to sevenfold increase in PSII/PSI ratio compared to control. An excited PSII caused a 2.7-fold increase in the specific levels of superoxide and a twofold increase in the specific levels of hydroxyl radicals. We have established that the increased specific intracellular RS (si-RS) levels increased the PSII antenna size by a significant 10-fold as compared to control. In addition, the 8.2-fold increase in specific lipid content was directly related to the si-RS levels. We have also demonstrated that the RS induced under UVA treatment led to a 3.2-fold increase in the saturated to unsaturated fatty acid ratio. Based on the findings, we have proposed and demonstrated a UV-based strategy, which achieved an 8.8-fold increase in volumetric lipid productivity. © 2013 American Institute of Chemical Engineers.