Morphology, composition, production, processing and applications of Chlorella vulgaris: A review

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Abstract

Economic and technical problems related to the reduction of petroleum resources require the valorisation of renewable raw material. Recently, microalgae emerged as promising alternative feedstock that represents an enormous biodiversity with multiple benefits exceeding the potential of conventional agricultural feedstock. Thus, this comprehensive review article spots the light on one of the most interesting microalga Chlorella vulgaris. It assembles the history and a thorough description of its ultrastructure and composition according to growth conditions. The harvesting techniques are presented in relation to the novel algo-refinery concept, with their technological advancements and potential applications in the market.

Introduction

Microalgae have an ancient history that left a footprint 3.4 billion years ago, when the oldest known microalga, belonging to the group of cyanobacteria, fossilised in rocks of Western Australia. Studies confirmed that until our days their structure remains unchanged and, no matter how primitive they are, they still represent rather complicated and expertly organised forms of life [1]. Nevertheless, other reports estimated that the actual time of evolution of cyanobacteria is thought to be closer to 2.7 billion years ago [2], [3]. Hence, evolutionary biologists estimate that algae could be the ancestors of plants. Thus, through time algae gave rise to other marine plants and moved to the land during the Palaeozoic Age 450 millions years ago just like the scenario of animals moving from water onto land. However, evolutionists need to overcome multiple obstacles (danger of drying, feed, reproduction, and protection from oxygen) to definitely confirm this scenario complemented with more scientific evidence.

Like any other phytoplankton, microalgae have a nutritional value. The first to consume the blue green microalga were the Aztecs and other Mesoamericans, who used this biomass as an important food source [4]. Nowadays, these microscopic organisms are still consumed as food supplement such as Chlorella vulgaris and Spirulina platensis [5] and their products are also used for different purposes like dyes, pharmaceuticals, animal feed, aquaculture and cosmetics. For the last two decades, microalgae started to take a new course with increasing applications motivated by the depletion of fossil fuel reserves, the consequent increase in oil prices and the global warming concern. These dramatic thresholds are forcing the world to find global strategies for carbon dioxide mitigation by proposing alternative renewable feedstocks and intensifying researches on third-generation biofuels. In this context, microalgae are regarded nowadays as a promising sustainable energy resource due to their capacity to accumulate large quantities of lipids suitable for biodiesel production that performs much like petroleum fuel [6], [7]. They also proved to be a source of products such as proteins, carbohydrates, pigments, vitamins and minerals [8]. In addition, microalgae capture sunlight and perform photosynthesis by producing approximately half of atmospheric oxygen on earth and absorbing massive amounts of carbon dioxide as a major feed. Therefore, growing them next to combustion power plants is of major importance due to their remarkable capacity to absorb carbon dioxide that they convert into potential biofuel, food, feed and highly added value components [9], [10], [11], [12], [13], [14].

Microalgae can grow in both fresh and marine water as well as in almost every environmental condition on earth from frozen lands of Scandinavia to hot desert soils of the Sahara [15]. If production plants were installed in an intelligent way, microalgae would not compete with agricultural lands, there would be no conflict with food production [16] and especially would not cause deforestation.

Microalgae represent an enormous biodiversity from which about 40.000 are already described or analysed [17]. One of the most remarkable is the green eukaryotic microalga C. vulgaris, which belongs to the following scientific classification: Domain: Eukaryota, Kingdom: Protista, Divison: Chlorophyta, Class: Trebouxiophyceae, Order: Chlorellales, Family: Chlorellaceae, Genus: Chlorella, Specie: Chlorella vulgaris. Hence, Martinus Willem Beijerinck, a Dutch researcher, first discovered it in 1890 as the first microalga with a well-defined nucleus [18]. The name Chlorella comes from the Greek word chloros (Χλωρός), which means green, and the Latin suffix ella referring to its microscopic size. It is a unicellular microalga that grows in fresh water and has been present on earth since the pre-Cambrian period 2.5 billion years ago and since then its genetic integrity has remained constant [1]. By the early 1900s, Chlorella protein content (>55% dry weight) attracted the attention of German scientists as an unconventional food source. In the 1950s, the Carnegie Institution of Washington [19] took over the study and managed to grow this microalga on a large scale for CO2 abatement. Nowadays, Japan is the world leader in consuming Chlorella and uses it for medical treatment [20], [21] because it showed to have immune-modulating and anti-cancer properties [22], [23], [24], [25], [26]. After feeding it to rats, mice and rabbits in the form of powder, it showed protection properties against haematopoiesis [27] age-related diseases like cardiovascular diseases, hypertension and cataract; it lowers the risk of atherosclerosis and stimulates collagen synthesis for skin [28], [29]. Furthermore, C. vulgaris is also capable of accumulating important amounts of lipids, especially after nitrogen starvation with a fatty acid profile suitable for biodiesel production [30], [31].

The available reviews have focused so far on evaluating microalgae as an important source of lipids for biofuel production [32], [33] and also explained in details the different production processes and harvesting techniques. The following review covers greater information about C. vulgaris, including not only production and harvesting techniques already conducted on this microalga, but also detailed information about its ultrastructure and chemical composition accompanied by cell wall breaking techniques and extraction processes. The last section focuses on the multiple applications and potential interests of this microalga in different areas and not only on the production of fatty compounds.

Section snippets

Morphology

C. vulgaris is a spherical microscopic cell with 2–10 μm diameter [33], [34], [35] and has many structural elements similar to plants (Fig. 1).

Reproduction

C. vulgaris is a non-motile reproductive cell (autospore) that reproduces asexually and rapidly. Thus, within 24 h, one cell of C. vulgaris grown in optimal conditions multiplies by autosporulation, which is the most common asexual reproduction in algae. In this manner, four daughter cells having their own cell wall are formed inside the cell wall of the mother cell (Fig. 2, Fig. 3) [33], [35]. After maturation of these newly formed cells, the mother cell wall ruptures, allowing the liberation

Production

Annual production of Chlorella reached 2000 t (dry weight) in 2009, and the main producers are Japan, Germany and Taiwan [46]. This microalga has a rapid growth rate and responds to each set of growth condition by modifying the yield of a specific component. C. vulgaris is ideal for production because it is remarkably resistant against harsh conditions and invaders. On the one hand, lipid and starch contents increase and biomass productivity ceases or decreases [47] during unfavourable growth

Proteins

Proteins are of central importance in the chemistry and composition of microalgae. They are involved in capital roles such as growth, repair and maintenance of the cell as well as serving as cellular motors, chemical messengers, regulators of cellular activities and defence against foreign invaders [44].

Total proteins content in mature C. vulgaris represents 42–58% of biomass dry weight [81], [82], [83], [84], [85], and varies according to growth conditions. Proteins have multiple roles, and

Cell disruption techniques

C. vulgaris has a resistant cell wall, which is a major barrier for digestibility and extraction process of all internal components. Breaking the cell wall is an important challenge and a costly unit operation. Multiple techniques have been carried out on C. vulgaris (Table 6). Cooling the system during mechanical cell breaking is always required because the high-energy input overheats the broken microalga and jeopardises the integrity of target components by damaging or oxidising them.

Biofuels

Dependency on energy sources is growing faster, especially with the exponential increase in demand, which is leading to more dramatic consequences for the environment. Third generation biofuel form algae or microalgae is considered as one of the alternatives to current biofuel crops such as soybean, corn, rapeseed and lignocellulosic feedstocks because it does not compete with food and does not require arable lands to grow [16]. However, biofuel from microalgae is promising in the long term

Algo-refinery concept

The concept of biorefinery has been inspired from the petroleum refinery concept. It reflects a platform that integrates a process to fractionate the components of a biomass [194], [195] to produce multiple products, and thus a biorefinery takes advantage of the various components in the biomass in order to improve the value derived from each component and also generating its own power, which maximises profitability and preserve the environment. Hence, C. vulgaris with all its potential and

Conclusion

This review reflects a broader image about the potential benefits of C. vulgaris, and gives an insight about the technological advancements already conducted. C. vulgaris can easily be cultured with inexpensive nutrient regime and has faster growth rate as compared to terrestrial energy crops and high biomass productivity. However, production-processing cost remains too high to compete in the market. Indeed, this is the major problem facing the microalgal industry nowadays, but it should be

Acknowledgements

The authors would like to thank Agence Nationale de la Recherche (ANR) for the financial support, and Laboratoire de Chimie Agro-industrielle (LCA) for providing all the necessary tools and requirements to dress this review.

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