Faculté des Sciences Tétouan ¦ Département de Biologie
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Ce PFE est réalisé par : Oulad Belayachi Laila
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Abstract 
Microalgae are phytoplanktons that carry out photosynthesis; these microorganisms represent a huge biodiversity in the world. Due to their eukaryotic organization, they consist of organelles; include nuclei, mitochondria and chloroplasts, etc. These organelles contribute in the metabolism by producing either protein, oils or sugar via many biological processes such as photosynthesis and fatty acids biosynthesis. In order to use this rich biomass in human/animal nutrition, cosmetics and biodiesel, many cultivation methods have been used, from open cultivation systems like the open ponds to closed ones like photobioreactors. The same thing with the special microalgae Haematococcus pluviaris which is capable of producing a highly antioxidant molecule called astaxanthin. Its production required a strict method starting with cultivation and ending with harvesting and extraction. Key-words: Phytoplanktons, biodiesel, astaxanthin. 

Résumé 
Les microalgues sont des phytoplanctons qui pratiquent la photosynthèse; ces microorganismes représentent une grande biodiversité dans le monde. Grâce à leur organisation eucaryotique, ils sont constitués d’organites ; inclus le noyau, les mitochondries et les chloroplastes, etc. Ces organites contribuent dans le métabolisme par la production soit de protéines, de lipides, ou de sucre via plusieurs processus biologiques comme la photosynthèse et la biosynthèse des acides gras. Afin d’utiliser cette riche biomasse en la nutrition humaine/animale, les cosmétiques et le biodiesel, plusieurs méthodes de culture ont été utilisées, de systèmes ouverts comme les bassins ouverts à des systèmes fermés comme les photobioréacteurs. La même chose pour la spéciale microalgue Haematococcus pluviaris qui est capable de produire une molécule très antioxydant appelé astaxanthin. Sa production demande une stricte méthode qui commence avec la culture et se termine avec la récolte et l’extraction.

Introduction 
Microorganisms in general represent a large biodiversity in the world, from prokaryotic ones such as bacteria, to eukaryotic ones such as microalgae. Due to their simple morphology and physiology, bacteria are well studied microorganisms, unlike microalgae which have complicated organelles and require even more research (Sheehan et al., 1998). However, Microalgae are one of the most ancient organisms living on earth and one of the tiniest plants. They have survived some of earth’s harshest conditions for several billion years. These unicellular species are capable of performing photosynthesis (converting light energy to chemical energy), in order to produce not only more than 75% of the atmospheric oxygen, but also carbohydrates, protein and lipids from 30 to 100 times faster than land plants (Kamyab, 2012 ; Spolaore et al., 2006). That is why they need to be highlighted and studied despite the high cost of their production (Sheehan et al., 1998). In order to understand the principal characteristics of microalgae, their biochemical proprieties and their importance, this study will discuss: -The microalgal biodiversity by highlighting the main classes and the differences between them based on pigmentation, life cycle and cellular structure. -The microalgal morphology by detailing the principal organelles present in a microalga. -The microalgal metabolism from photosynthesis process to starch, fatty acids and lipids biosynthesis. -The cultivation methods and systems such as open ponds (open system) and photobioreactors (closed system). -The use of microalgae in a variety of industries and fields like nutrition, cosmetics and even in biodiesel production by transesterification. Moreover, this study will take as an example Haematococcus pluviaris. A special green microalga that turns red during harsh conditions and can produce a huge proportion of highly antioxidant molecule which is astaxanthin (a xanthophyll carotenoid); this molecule has an exceptional range of benefits such as maintaining the immune response etc. (Chekanov et al.,2014 ; Kidd, 2011).

General conclusion 
In general, microalgae have shown a lot of benefits on so many levels. These microorganisms have a promising future in the biotechnical industry, from producing carbohydrates, proteins and natural oils, to producing biodiesel via transesterification. This last process can help saving the environment by abandoning the petroleum fuels and depending even more on algal biomass as natural resource. Unfortunately, the high cost of producing this biodiesel prevents it from becoming economically competitive, which is why it is necessary for future research to find another sheep method to produce it. As for the red pigment astaxanthin, the Chlorophyte Haematococcus pluvialis accumulates large quantities of it as a stock; this pigment has demonstrates several positive effects of human health, so as a result, it is acceptable to say that this antioxidant molecule is also a very promising product in many ways. In order to exploit microalgae in the best way possible, researches must look for better methods either to cultivate them or to extract their biomass without wasting any of its amounts.