Assessing the fertilizing potential of microalgal digestates using the marine diatom Chaetoceros muelleri

Algae-based biofuels have been explored as a sustainable alternative to first generation biofuels. However, there are several constraints to overcome in order to make this system economically viable at large scale, including eliminating or reducing the cost of fertilizers. Anaerobic digestates have shown to be a valuable source of nutrients for algae cultivation. In this study, we evaluated the fertilizing potential of algal digestates with the microalga Chaetoceros muelleri. Nutrient concentrations in the solid and liquid phase of the digestate were evaluated as well as its mineralization potential by aerobic digestion. The results demonstrate the potential for fertilizer applications. Additional nutrients were released from biosolids by aerobic digestion, further enhancing algal growth

Sugarcane bagasse hydrolysate as organic carbon substrate for mixotrophic cultivation of Nannochloropsis sp. BR2

In the present study, the mixotrophic growth of the microalga Nannochloropsis sp. BR2 in sugarcane bagasse was analyzed and compared with its photoautotrophic cultivation. Nannochloropsis cultures cultivated mixotrophically in sugarcane bagasse showed significantly higher biomass productivity, fatty acid methyl ester (FAME) and protein contents of 63.28\ua0mg\ua0L\ua0d, 170.51\ua0mg\ua0g and 35.2% of dry weight, respectively, compared to the photoautotrophic cultivations with biomass productivity, FAME and protein contents of 51\ua0mg\ua0L\ua0d, 139.21\ua0mg\ua0g and 31.6% of dry weight. Whereas, total carotenoid during photoautotrophic cultivation was 5.833\ua0mg\ua0g and decreased to 4.542\ua0mg\ua0g in mixotrophic cultures. This can be explained by the additional carbon source in the form of sugars that are metabolized to the fatty acid building block acetyl-CoA, while photosynthetic pigments were less needed. Findings of this study demonstrate that acid-pretreated hydrolysate of lignocellulosic waste from sugarcane bagasse can be developed into a potential feedstock for efficient microalgal cultivation

Efficient harvesting of Nannochloropsis microalgae via optimized chitosan-mediated flocculation

Food-grade rather than synthetic or chemical flocculants are needed for microalgae harvesting by settling, if used for food products. Chitosan is effective in harvesting freshwater microalgae, but it is expensive and typically not suitable for marine microalgae like Nannochloropsis. To minimize costs for food-grade flocculation, a number of potentially important parameters are considered, including chitosan solubility and optimized chitosan-mediated flocculation of Nannochloropsis sp. BR2 by a five-factor central composite design experiment. Results show that an optical density (440 nm) of 2 (0.23 g dry weight L-1), initial pH of 6, final pH of 10, and 22 ppm chitosan with a viscosity of 1808 cP provide optimum flocculation efficiency, which is predicted to be in the range of 97.01% to 99.93%. These predictions are verified on 4.5 and 8 L Nannochloropsis sp. BR2 cultures

Transcriptome-wide analysis of Chlorella reveals auxin-induced carotenogenesis pathway in green microalgae

Microalgae are a commercially viable route for the production of carotenoids. Chlorella sp. BR2 was treated with plant hormones indole-3-acetic acid, salicylic acid, abscisic acid and methyl jasmonate and screened for enhanced carotenoid production. Indole-3-acetic acid was the only hormone with an inductive effect on carotenoid accumulation. As such, transcriptome-wide changes following auxin treatment were profiled using RNA-Seq and expressed sequences reconstructed with de novo assembly. This revealed the active pathway components of auxin-induced carotenogenesis. Data analysis specified the differentially expressed genes involved in auxin biosynthesis and signal transduction, which hint at close, yet unique relationship to equivalent pathways in higher plants. Unlike in plants, the ancient ABP1/SCFSKP2A/IBR5-mediated pathways for auxin response likely acted as the primary signaling route in Chlorella. As carotenoids are precursors for abscisic acid and antagonists of reactive oxygen species, the findings suggest a potential link between auxin signaling and abiotic stress tolerance

Effective harvesting of Nannochloropsis microalgae using mushroom ahitosan: a pilot-scale study

For efficient downstream processing, harvesting remains as one of the challenges in producing Nannochloropsis biomass, a microalga with high-value omega-3 oils. Flocculation is an effective, low-energy, low-cost method to harvest microalgae. Chitosan has been shown to be an effective food-grade flocculant; however, commercial chitosan is sourced from crustaceans, which has disadvantages including concerns over heavy-metal contamination. Thus, this study tests the flocculation potential of mushroom chitosan. Our results indicate a 13% yield of chitosan from mushroom. The identity of the prepared chitosan was confirmed by Fourier-transform infrared (FTIR) spectroscopy. Furthermore, results show that mushroom chitosan can be an alternative flocculant with >95% flocculation efficiency when tested in 100-mL jar and 200-L vertical column photobioreactor. Applications in a 2000-L raceway pond demonstrated that thorough mixing of mushroom chitosan with the algal culture is required to achieve efficient flocculation. With proper mixing, mushroom chitosan can be used to produce food-grade Nannochloropsis biomass suitable for the production of vegan omega-3 oils as a fish oil alternative

Blue light enhances astaxanthin biosynthesis metabolism and extraction efficiency in Haematococcus pluvialis by inducing haematocyst germination

Haematococcus pluvialis accumulates large amounts of astaxanthin during its haematocyst stage. Cyst germination is a biological pretreatment method for improved astaxanthin extraction with potential to replace energy-intensive homogenizer-based mechanical cell cracking methods. The present study demonstrates effects of different LED light wavelengths on haematocyst germination and on its astaxanthin biosynthesis pathway. Blue light enhanced the germination efficiency and slowed the nitrogen consumption rate, resulting in significantly higher astaxanthin content and improved extractability compared with white and red lights, that also had lower germination rates. After 5 days, the total astaxanthin extractability under blue light was 4.0 and 6.7 times higher than for white and red lights, respectively. The blue light receptor gene phot was significantly induced which upregulated the biosynthesis pathway genes psy and pds, as well as dgat1 and dgat2d. Hence, blue light triggers germination and astaxanthin biosynthesis, providing a strategy for improved extraction while modulating higher biosynthesis during germination

Biogas production coupled to repeat microalgae cultivation using a closed nutrient loop

Anaerobic digestion is an established technology to produce renewable energy as methane-rich biogas for which microalgae are a suitable substrate. Besides biogas production, anaerobic digestion of microalgae generates an effluent rich in nutrients, so-called digestate, that can be used as a growth medium for microalgal cultures, with the potential for a closed nutrient loop and sustainable bioenergy facility. In this study, the methane potential and nutrient mobilization of the microalga Scenedemus dimorphus was evaluated under continuous conditions. The suitability of using the digestate as culture medium was also evaluated. The results show that S. dimorphus is a suitable substrate for anaerobic digestion with an average methane yield of 199 mL g−1 VS. The low level of phosphorus in digestate did not limit algae growth when used as culture medium. The potential of liquid digestate as a superior culture medium rather than inorganic medium was demonstrated

Growth-promoting bacteria double eicosapentaenoic acid yield in microalgae

High-yielding microalgae present an important commodity to sustainably satisfy burgeoning food, feed and biofuel demands. Because algae-associated bacteria can significantly enhance or reduce yields, we isolated, identified and selected highly-effective “probiotic” bacterial strains associated with Nannochloropsis oceanica, a high-yielding microalga rich in eicosapentaenoic acid (EPA). Xenic algae growth was significantly enhanced by co-cultivation with ten isolated bacteria that improved culture density and biomass by 2.2- and 1.56-fold, respectively (1.39 × 10 cells mL; 0.82 g L). EPA contents increased up to 2.25-fold (to 39.68% of total fatty acids). Added probiotic bacteria possessed multiple growth-stimulating characteristics, including atmospheric nitrogen fixation, growth hormone production and phosphorous solubilization. Core N. oceanica-dominant bacterial microbiomes at different cultivation scales included Sphingobacteria, Flavobacteria (Bacteroidetes), and α, γ-Proteobacteria, and added probiotic bacteria could be maintained. We conclude that the supplementation with probiotic algae-associated bacteria can significantly enhance biomass and EPA production of N. oceanica

Gene expression profiling of astaxanthin and fatty acid pathways in Haematococcus pluvialis in response to different LED lighting conditions

Haematococcus pluvialis is a green microalga of major interest to industry based on its ability to produce large amounts of astaxanthin. Biosynthesis of astaxanthin and its mono- and di-esters was significantly stimulated under 150 μmol m s of white LED (W-150) compared with lower light intensities, but the highest astaxanthin amounts were produced under 70 μmol m s of blue LED (B-70). Transcripts of astaxanthin biosynthesis genes psy, crtO, and bkt2 were upregulated under W-150, while psy, lcy, crtO, and crtR-B were upregulated by B-70. Total fatty acid content and biosynthesis genes fata and all dgat genes were induced under W-150, while C18:3n6 biosynthesis and dgat2a expression were specifically stimulated by B-70 which was correlated to astaxanthin ester biosynthesis. Nitrogen starvation, various LEDs and the identified upregulated genes may provide useful tools for future metabolic engineering to significantly increase free astaxanthin, its esters and fatty acid precursors in H. pluvialis