Green microalgae, and Chlorophyta in particular, are considered as a renewable source of valuable chemicals for biofuel, nutraceutical or pharmaceutical industries, among others. Microalgae store most of their valuable components inside the cell behind a thick and resistant cell wall. Energy- or solvent-consuming steps are therefore needed to overcome this physical barrier and efficiently extract the desired compounds. Moreover, in a biorefinery context, the wall itself represents a valuable co-product. Although accurate knowledge of microalgal cell walls would be useful for product and process development and overall cost reduction, walls are still poorly understood. High discrepancies in structure and/or composition are regularly described within families, genera or species due to phenotypic plasticity, life cycle stage or erroneous taxonomical classification. Massive progress has been made in this latter research area, which gives, when compiled with the studies focusing on cell walls, a better understanding of micro-algal cell walls. This review focuses on the detailed cell wall structure and composition of Chlorophyta with a particular focus on, but not limited to, industrial micro-algae.

A new insight into cell walls of Chlorophyta

Microalgae biomass can be used to produce numerous value added products such as biodiesel, bioethanol, biogas, biohydrogen, fish feed, animal feed, human food supplements and skin care products. Production of value added products from microalgae biomass requires the growth and recovery of the algae biomass, extraction and downstream processing of the desired product. One of the major obstacles for using microalgae biomass on an industrial-scale, for the production of biodiesel, is the high processing costs. Increasing the lipid recovery efficiency from the microalgae biomass would result in greater product yields (biodiesel). Thus, the aim of this study was to review the current methods used for microalgae pre-treatment and perform a comparative analysis in order to determine the most economically efficient method for large scale use. The effectiveness of the pre-treatment methods investigated was evaluated based on: (a) cell wall disruption efficiency, (b) cost, (c) toxicity (d) suitability for large scale use, (e) time, (f) reusability and (g) maintenance. Different treatment methods included mechanical techniques (shaking vessel and agitated bead mills and horn and bath sonication), thermal methods (steam explosion, freeze drying and autoclave), electromagnetic radiation (microwave) and biological treatments (enzymatic). The results indicated that of the 9 microalgae methods investigated a mechanical, thermal and electromagnetic radiation techniques were suitable. These methods were bath sonication (81), steam explosion (93) and microwave radiation (87). Microwave assisted microalgae pre-treatment technique is rapid, effective in cell wall disruption, non-toxic, can be used for large volumes and the medium maybe reused, but it does however suffer from high maintenance costs. Bath sonication technique is effective in the degradation of cell wall, nontoxic, rapid technique with minimal maintenance required, but suffers from high costs and difficulty in scale up for industrial use. Steam explosion pre-treatment is effective in degrading microalgae cell wall, releasing intracellular components, rapid, reusable, relatively low in costs, environmentally friendly and reusable, but is species specific. Overall, the negative aspects of these three techniques are outweighed by their effectiveness, rapidness and relatively low costs when compared to other pre-treatment techniques. Other mechanical extraction methods suffer from high operational costs, lengthy treatment times, high maintenance costs and the scale up difficulty. Freeze drying and autoclave techniques were deemed unsuitable microalgae pre-treatment techniques because of the high costs, scale up difficulty and long processing times associated. Biological pre-treatment technique were deemed unsuitable as a result of high costs associated with purchasing of enzymes, difficulty in recovery/separation after treatment, long treatment time, and high maintenance required for high efficiency.

https://www.longdom.org/open-access/microalgae-oil-extraction-pretreatment-methods-critical-review-andcomparative-analysis-.pdf

Microalgae Oil Extraction Pre-treatment Methods: Critical Review and Comparative Analysis

Chlorella protothecoides has been put forth as a promising candidate for commercial biodiesel production. However, the cost of biodiesel remains much higher than diesel from fossil fuel sources, partially due to the high costs of oil extraction from algae. Here, we identified the presence of a sporopollenin layer outside the polysaccharide cell wall; this was evaluated using transmission electron microscopy, 2-aminoethanol treatment, acetolysis, and Fourier Transform Infrared Spectroscopy. We also performed bioinformatics analysis of the genes of the C. protothecoides genome that are likely involved in sporopollenin synthesis, secretion, and translocation, and evaluated the expression of these genes via real-time PCR. We also found that that removal of this sporopollenin layer greatly improved the efficiency of oil extraction.

/pdf/identification-of-sporopollenin-as-the-outer-layer-of-cell-1nim8jpgkz.pdf

Identification of Sporopollenin as the Outer Layer of Cell Wall in Microalga Chlorella protothecoides

Euglena gracilis is a promising source of commercially important metabolites such as vitamins, wax esters, paramylon, and amino acids. However, the molecular tools available to create improved Euglena strains are limited compared to other microorganisms that are currently exploited in the biotechnology industry. The complex poly-endosymbiotic nature of the Euglena genome is a major bottleneck for obtaining a complete genome sequence and thus represents a notable shortcoming in gaining molecular information of this organism. Therefore, the studies and applications have been more focused on using the wild-type strain or its variants and optimizing the nutrient composition and cultivation conditions to enhance the production of biomass and valuable metabolites. In addition to producing metabolites, the E. gracilis biorefinery concept also provides means for the production of biofuels and biogas as well as residual biomass for the remediation of industrial and municipal wastewater. Using Euglena for bioremediation of environments contaminated with heavy metals is of special interest due to the strong ability of the organism to accumulate and sequester these compounds. The published draft genome and transcriptome will serve as a basis for further molecular studies of Euglena and provide a guide for the engineering of metabolic pathways of relevance for the already established as well as novel applications.

Molecular tools and applications of Euglena gracilis: From biorefineries to bioremediation.

..................................................................................................................... xiii List of Abbreviations Used .............................................................................................. xiv Acknowledgements .......................................................................................................... xvi Chapter

Surfactant-aided dispersed air flotation as a harvesting and pre-extraction treatment for chlorella saccharophila

This paper describes an enzymatic method for yielding protoplasts from the microalga Chlorella protothecoides. Four kinds of commercially available enzymes were tested. The enzymatic digestion was optimal with 2% cellulase R-10 and 1% snailase prepared in 25 mM Tris buffer (pH 6.0) containing 0.6 M D-mannitol, and the protoplast density could reach the peak after treatment at 30°C for 16 h. Nearly all liberated protoplasts were green in the presence of 0.01% phenosafranin, indicating their high viability. The regeneration rate was about 70% when 0.6 M D-mannitol was used as an osmotic stabilizer in the regeneration medium. This protocol will find useful applications in genetic studies of this algal species.

Yongzhong Lu

Papers

Preparation of protoplasts from Chlorella protothecoides.