Microalgae can be used as a source of alternative food, animal feed, biofuel, fertilizer, cosmetics, nutraceuticals and for pharmaceutical purposes. The extraction of organic constituents from microalgae cultivated in the different nutrient compositions is influenced by microalgal growth rates, biomass yield and nutritional content in terms of lipid and fatty acid production. In this context, nutrient composition plays an important role in microalgae cultivation, and depletion and excessive sources of this nutrient might affect the quality of biomass. Investigation on the role of nitrogen and phosphorus, which are crucial for the growth of algae, has been addressed. However, there are challenges for enhancing nutrient utilization efficiently for large scale microalgae cultivation. Hence, this study aims to highlight the level of nitrogen and phosphorus required for microalgae cultivation and focuses on the benefits of nitrogen and phosphorus for increasing biomass productivity of microalgae for improved lipid and fatty acid quantities. Furthermore, the suitable extraction methods that can be used to utilize lipid and fatty acids from microalgae for biofuel have also been reviewed.

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Influence of Nitrogen and Phosphorus on Microalgal Growth, Biomass, Lipid, and Fatty Acid Production: An Overview.

Harvesting of microalgae is a crucial step in microalgae-based mass production of different high value-added products. In the present work, magnetic harvesting of Chlorella vulgaris was investigated using microwave-synthesized naked magnetite (Fe3O4) particles with an average crystallite diameter of 20 nm. Optimization of the most important parameters of the magnetic harvesting process, namely pH, mass ratio (mr) of magnetite particles to biomass (g/g), and agitation speed (rpm) of the C. vulgaris biomass-Fe3O4 particles mixture, was performed using the response surface methodology (RSM) statistical tool. Harvesting efficiencies higher than 99% were obtained for pH 3.0 and mixing speed greater or equal to 350 rpm. Recovery of magnetic particles via detachment was shown to be feasible and the recovery particles could be reused at least five times with high harvesting efficiency. Consequently, the described harvesting approach of C. vulgaris cells leads to an efficient, simple, and quick process, that does not impair the quality of the harvested biomass.

https://www.mdpi.com/2079-4991/11/6/1614/pdf

Optimization of Microalga Chlorella vulgaris Magnetic Harvesting.

Nannochloropsis oceanica represents a preferred oleaginous alga for producing lipids. Here we found that phosphorus deprivation (PD) caused a severe decrease in protein and a considerable increase in lipids including triacylglycerol (TAG), yet it had little effect on the carbohydrate level and biomass production of N. oceanica. The combinatorial analysis by integrating physiological, biochemical, and transcriptomic data unraveled the molecular mechanisms underlying PD-induced lipid accumulation. Albeit attenuating the Calvin-Benson cycle, PD stimulated the C4-like pathway to maintain CO2 fixation for biomass production. PD attenuated nitrogen utilization and enhanced protein catabolism thus leading to protein decrease, from which the carbon was likely salvaged into the stimulated tricarboxylic acid cycle for supplying lipid synthesis with carbon precursors. The impairment of TAG catabolism by downregulating certain lipases rather than the stimulation of TAG assembly pathways contributed to PD-boosted TAG increase. These findings provide novel insights into PD-induced lipogenesis without compromising biomass production by N. oceanica.

Novel Insights into Phosphorus Deprivation Boosted Lipid Synthesis in the Marine Alga Nannochloropsis oceanica without Compromising Biomass Production

Microalgae are a promising feedstock source for third-generation biofuels. However, applying microalgae as a raw feedstock for biodiesel production presents several challenges regarding its economic feasibility, particularly during the harvesting stage. The microalgal harvesting stage is crucial from an economic perspective, and it impacts the feedstock quality during subsequent downstream processing steps. The factors of microalgae fatty acid profile (FAP) and lipid yield (LY) are subject to variability throughout the harvesting process. Successful harvesting relies on the application of strategies that provide optimal performance without imposing any qualitative or quantitative reduction in FAP and LY (FAP&LY). Although the promotion of the harvesting efficiency of microalgae has been studied extensively, little is known about how the harvesting process impacts FAP&LY, and even less is known about how it influences the lipid extraction process. This paper provides a compendium of technical developments intended to promote FAP&LY during the harvesting process. In this study, the importance of pre-harvesting, harvesting, and post-harvesting strategies for enhancing the FAP&LY is discussed. It was shown that co-cultivation specific microalgae with fungi, bacteria, or other microalgae strains could improve the flocculation efficiency and FAP&LY. Algae-algae co-cultivation had higher FAP quality compared to others. Appropriate wet storage conditions and stress induction strategies could promote the FAP&LY efficiency and reduce the expenses incurred by cell drying and associated instruments. Enhancing FAP&LY via the harvesting process could be exploited as breakthrough technology for cost-efficient microalgae-based biodiesel production. 

Impacts of the harvesting process on microalgae fatty acid profiles and lipid yields: Implications for biodiesel production

Biofuel production from microalgae non-food feedstock is a challenge for strengthening Green energy nowadays. Reviewing the current technology, there is still reluctance in investing towards the production of new algal strains that yield more oil and maximize capital gains. In the current work, the microalgal feedstock selection problem is investigated, for increased lipid production and nano-catalytic conversion into clean biofuel. For that purpose, a variety of Fuzzy Multi-Criteria Decision Making processes and a multitude of Optimization criteria spanning to technological, environmental, economic, and social aspects are used. The strains selected for the analysis are Chlorella sp., Schizochytrium sp., Spirulina sp. and Nannochloropsis sp. The methods applied are FAHP (fuzzy analytic hierarchy process), FTOPSIS (fuzzy technique for the order of preference to the ideal solution) and FCM (fuzzy cognitive mapping). Pairwise comparison matrices were calculated using data from extensive literature review. All aforementioned fuzzy logic methodologies are proven superior to their numeric equivalent under uncertain factors that affect the decision making, such as cost, policy implications and also geographical and seasonal variation. Major finding is that the most dominant factor in the strain selection is the high lipid content. Moreover, the results indicate that the Chlorella Vulgaris microalgae is ranked as the best choice by the FTOPSIS method followed by the Nannochloropsis strain and Spirulina Platensis to be the last in performance. The best and worst case scenario run with FCM experimentally verify this choice indicating that Chlorella Vulgaris follows this trend of selection mostly with the technological and the economic criteria for both the sigmoid and the hyperbolic tangent deep-learning functions used.

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Optimizing Microalgal Biomass Feedstock Selection for Nanocatalytic Conversion Into Biofuel Clean Energy, Using Fuzzy Multi-Criteria Decision Making Processes

The effect of iron, manganese, phosphorus and nitrogen on growth and lipid synthesis of the microalgae Nannochloropsis oceanica CCMP1779, as well as their impact on the magnetic harvesting efficiency, are examined under their depriving cell culture conditions. Herein, it is demonstrated that nitrogen and manganese depletion primarily reduced cell growth while phosphorus and iron restriction led to higher dry biomass. Subsequently, the role of those nutrients on fatty acids profile was examined. Phosphorus and nitrogen restriction resulted in lower and higher lipid content, respectively. High amounts of polyunsaturated fatty acids like eicosapentaenoic acid are produced under iron and manganese depletion. Phosphorus deprivation favors monounsaturated fatty acids such as C18:1 and C16:1, while nitrogen restriction favors saturated fatty acid production like C14:0, C16:0 and C18:0. Since the presence/absence of macro- and micro-elements may affect the overall electrostatic charges on the outmost microalgae surface, it was also analyzed how these elements affect the magnetic harvesting efficiency. Results showed that phosphorus deprivation led to the best magnetic harvesting efficiency of N. oceanica cells (93%) as compared to other nutrient starvation as well as standard medium.

https://www.mdpi.com/2223-7747/9/5/660/pdf

Dimitrios Logothetis

Papers

A Study on the Effect of Macro- and Micro- Nutrients on Nannochloropsis oceanica Growth, Fatty Acid Composition and Magnetic Harvesting Efficiency.