Eleonora Sforza

Adjusted Light and Dark Cycles Can Optimize Photosynthetic Efficiency in Algae Growing in Photobioreactors

Biofuels from algae are highly interesting as renewable energy sources to replace, at least partially, fossil fuels, but great research efforts are still needed to optimize growth parameters to develop competitive large-scale cultivation systems. One factor with a seminal influence on productivity is light availability. Light energy fully supports algal growth, but it leads to oxidative stress if illumination is in excess. In this work, the influence of light intensity on the growth and lipid productivity of Nannochloropsis salina was investigated in a flat-bed photobioreactor designed to minimize cells self-shading. The influence of various light intensities was studied with both continuous illumination and alternation of light and dark cycles at various frequencies, which mimic illumination variations in a photobioreactor due to mixing. Results show that Nannochloropsis can efficiently exploit even very intense light, provided that dark cycles occur to allow for re-oxidation of the electron transporters of the photosynthetic apparatus. If alternation of light and dark is not optimal, algae undergo radiation damage and photosynthetic productivity is greatly reduced. Our results demonstrate that, in a photobioreactor for the cultivation of algae, optimizing mixing is essential in order to ensure that the algae exploit light energy efficiently.

Acclimation of Nannochloropsis gaditana to different illumination regimes: Effects on lipids accumulation

Algae are interesting potential sources of biodiesel, although research is still needed to develop efficient large scale productions. One major factor affecting productivity is light use efficiency. The effect of different light regimes on the seawater alga Nannochloropsis gaditana was accessed monitoring growth rate and photosynthetic performances. N. gaditana showed the capacity of acclimating to different light intensities, optimizing its photosynthetic apparatus to illumination. Thanks to this response, N. gaditana maintained similar growth rates under a wide range of irradiances, suggesting that this organism is a valuable candidate for outdoor productions in variable conditions. In the conditions tested here, without external CO(2) supply, light intensity alone was not found to be a major signal affecting lipids accumulation showing the absence of a direct regulatory link between the light stress and lipids accumulation. Strong illumination can nevertheless indirectly influences lipid accumulation if combined with other stresses or in the presence of excess CO(2).

Excess CO2 supply inhibits mixotrophic growth of Chlorella protothecoides and Nannochloropsis salina

Mixotrophy can be exploited to support algal growth over night or in dark-zones of a photobioreactor. In order to achieve the maximal productivity, however, it is fundamental also to provide CO(2) in excess to maximize photosynthetic activity and phototropic biomass production. The aim of this paper is to verify the possibility of exploiting mixotrophy in combination with excess CO(2). Two species with high biomass productivity were selected, Nannochloropsis salina and Chlorella protothecoides. Different organic substrates available at industrial scale were tested, and glycerol chosen for its ability to support growth of both species. In mixotrophic conditions, excess CO(2) stimulated photosynthesis but blocked the metabolization of the organic substrate, thus canceling the advantages of mixotrophy. By cultivating microalgae under day-night cycle, organic substrate supported growth during the night, but only if CO(2) supply was not provided. This represents thus a possible method to reconcile CO(2) stimulation of photosynthesis with mixotrophy.

Cultivation of Scenedesmus obliquus in liquid hydrolysate from flash hydrolysis for nutrient recycling

The production of biofuels from microalgae is associated with high demands of nutrients (nitrogen and phosphorus) required for growth. Recycling nutrients from the residual biomass is essential to obtain a sustainable production. In this work, the aqueous phase obtained from flash hydrolysis of Scenedesmus sp. was used as cultivation medium for a microalga of the same genus, to assess the feasibility of this technique for nutrient recycling purposes. Batch and continuous cultivations were carried out, to determine growth performances in this substrate compared to standard media, and verify if a stable biomass production could be obtained. In continuous experiments, the effect of hydrolysate inlet concentration and of residence time were assessed to optimize nutrient supply in relation to productivity. Results obtained show that nutrient recycling is feasible by treating biomass with flash hydrolysis, and Scenedesmus is capable of recycling large amounts of recovered nutrients.

Evaluation of maintenance energy requirements in the cultivation of Scenedesmus obliquus: effect of light intensity and regime

Most relevant factors influencing microalgae growth are light availability and utilization. In view of maximizing the photosynthetic efficiency of a photobioreactor, the evaluation of maintenance energy is essential to understand the energy requirement for cell growth and it is also a key parameter of most mathematical growth models. In this work, steady-state continuous experiments were carried out in a flat-bed photobioreactor, at different light intensities and irradiation regimes, measuring growth rate, productivity, lipid content, and photosynthetic efficiency of Scenedesmus obliquus at non-limiting N and P condition. Energy balance was applied to measure the maintenance term as a function of irradiation, under both continuous and simulated seasonal irradiation at middle latitudes. The energy requirement for maintenance was found to be greater under high irradiances as a result of the photoinhibition effect. Experimental data were correlated following a substrate inhibition model. The nutrient uptakes were found to depend on growth rate and light intensity and to be affected by the maintenance requirement.

Continuous microalgal cultivation in a laboratory-scale photobioreactor under seasonal day–night irradiation: experiments and simulation

In this work, the production of Scenedesmus obliquus in a continuous flat-plate laboratory-scale photobioreactor (PBR) under alternated day–night cycles was tested both experimentally and theoretically. Variation of light intensity according to the four seasons of the year were simulated experimentally by a tunable LED lamp, and effects on microalgal growth and productivity were measured to evaluate the conversion efficiency of light energy into biomass during the different seasons. These results were used to validate a mathematical model for algae growth that can be applied to simulate a large-scale production unit, carried out in a flat-plate PBR of similar geometry. The cellular concentration in the PBR was calculated in both steady-state and transient conditions, and the value of the maintenance kinetic term was correlated to experimental profiles. The relevance of this parameter was finally outlined.

Effect of specific light supply rate on photosynthetic efficiency of Nannochloropsis salina in a continuous flat plate photobioreactor

In this work, Nannochloropsis salina was cultivated in a continuous-flow flat-plate photobioreactor, working at different residence times and irradiations to study the effect of the specific light supply rate on biomass productivity and photosynthetic efficiency. Changes in residence times lead to different steady-state cell concentrations and specific growth rates. We observed that cultures at steady concentration were exposed to different values of light intensity per cell. This specific light supply rate was shown to affect the photosynthetic status of the cells, monitored by fluorescence measurements. High specific light supply rate can lead to saturation and photoinhibition phenomena if the biomass concentration is not optimized for the selected operating conditions. Energy balances were applied to quantify the biomass growth yield and maintenance requirements in N. salina cells.

An Identifiable State Model To Describe Light Intensity Influence on Microalgae Growth

Despite the high potential as feedstock for the production of fuels and chemicals, the industrial cultivation of microalgae still exhibits many issues. Yield in microalgae cultivation systems is limited by the solar energy that can be harvested. The availability of reliable models representing key phenomena affecting algae growth may help designing and optimizing effective production systems at an industrial level. In this work the complex influence of different light regimes on seawater alga Nannochloropsis salina growth is represented by first principles models. Experimental data such as in vivo fluorescence measurements are employed to develop the model. The proposed model allows description of all growth curves and fluorescence data in a reliable way. The model structure is assessed and modified in order to guarantee the model identifiability and the estimation of its parametric set in a robust and reliable way.

Biochemical characterization and genetic identity of an oil-rich Acutodesmus obliquus isolate

Microalgae provide one of the more promising strategies for the production of renewable biodiesel and liquid fuels. Coupling biodiesel production and wastewater treatment based on the use of microalgae can be an effective approach for achieving sustainable production. Several pilot programs aimed at lipid production are currently working with Acutodesmus obliquus, and companies offering algal inocula are increasing in number. A. obliquus strain RL01 was isolated from a pretreated leachate, and preliminary analyses indicated a high growth rate, a final biomass concentration of 1.9 g∙L-1 and a lipid content of 47% DW. With the aim of strictly differentiating and unequivocally identifying the new strain in comparison with other A. obliquus lipid producers available in culture collections, we employed a DNA barcoding approach, targeting conventional and non-conventional genome regions. Genomic analyses were performed using two internal transcribed spacers (nuclear ITS1 and ITS2 sequences) and three selected chloroplast regions (the trnL genic intron and the rpl20-psbA and psbM-psbZ intergenic spacers). Our findings raise some concerns about the discriminant power of the ITS2 nuclear target and place particular emphasis on the psbM-psbZ plastid DNA barcode, which is characterized by high polymorphism, for use in studies addressing intraspecific genetic diversity.

Exploitation of urban landfill leachate as nutrient source for microalgal biomass production

Increasing concern over climate change and the impact of greenhouse gas emissions as well as diminishing global oil reserves has pushed research into alternative energy sources. Among these options, reducing the cost of microalgae cultivation, a promising source for renewable energy, is a key step in commercializing biodiesel production. Microalgae also have a two-fold advantage thanks to the capability to treat waste streams by reducing contaminants via bioremediation.In this work, the possibility of exploiting nutrients from landfill leachate was tested. Some microalgal species are able to grow in the leachate as a substrate, but the chemical composition of this waste stream often leads to growth inhibition. Thus, using strain directly isolated from the target environment could avoid problems of inhibition, in order to obtain higher biomass productivity and nutrient, i.e. pollutant, removal.Acutodesmus obliquus is a microalgal species isolated from a pond containing pretreated leachate from an urban landfill located in Lazio (Italy). This strain was cultivated in leachate from a landfill located in Istrana (TV), Italy. The leachate was diluted with distilled water and the growth of the species was measured. Several dilution ratios were tested and the maximum growth rate was obtained with 10 % of leachate in water, even though an acceptable growth was also observed with 1:2 dilution. Lipid content of about 38 – 48 % dry weight was measured. Nutrients uptake in the form of ammonia from the various leachate dilutions ranged from 30 - 97 % and, generally, it was >90 % in the case of phosphorus, which was assessed as the limiting nutrient for growth.The outcome of this work has established the real potential of utilizing ammonia-N enriched leachate as the nutrient source for yielding high lipid content algal biomass, by using native strains. Based on experimental results, preliminary calculations on photobioreactor design to treat leachate from urban landfill were carried out.