The rise of human populations and the growth of cities contribute to the depletion of natural resources, increase their cost, and create potential climatic changes. To overcome difficulties in supplying populations and reducing the resource cost, a search for alternative pharmaceutical, nanotechnology, and energy sources has begun. Among the alternative sources, microalgae are the most promising because they use carbon dioxide (CO₂) to produce biomass and/or valuable compounds. Once produced, the biomass is ordinarily harvested and processed (downstream program). Drying, grinding, and extraction steps are destructive to the microalgal biomass that then needs to be renewed. The extraction and purification processes generate organic wastes and require substantial energy inputs. Altogether, it is urgent to develop alternative downstream processes. Among the possibilities, milking invokes the concept that the extraction should not kill the algal cells. Therefore, it does not require growing the algae anew. In this review, we discuss research on milking of diatoms. The main themes are (a) development of alternative methods to extract and harvest high added value compounds; (b) design of photobioreactors; (c) biodiversity and (d) stress physiology, illustrated with original results dealing with oleaginous diatoms.

Astaxanthin (3,3-dihydroxy-β, β-carotene-4,4-dione) is a ketocarotenoid synthesized by Haematococcus pluvialis/lacustris, Chromochloris zofingiensis, Chlorococcum, Bracteacoccus aggregatus, Coelastrella rubescence, Phaffia rhodozyma, some bacteria (Paracoccus carotinifaciens), yeasts, and lobsters, among others However, it is majorly synthesized by Haematococcus lacustris alone (about 4%). The richness of natural astaxanthin over synthetic astaxanthin has drawn the attention of industrialists to cultivate and extract it via two stage cultivation process. However, the cultivation in photobioreactors is expensive, and converting it in soluble form so that it can be easily assimilated by our digestive system requires downstream processing techniques which are not cost-effective. This has made the cost of astaxanthin expensive, prompting pharmaceutical and nutraceutical companies to switch over to synthetic astaxanthin. This review discusses the chemical character of astaxanthin, more inexpensive cultivating techniques, and its bioavailability. Additionally, the antioxidant character of this microalgal product against many diseases is discussed, which can make this natural compound an excellent drug to minimize inflammation and its consequences.

Algae possess a considerable potential as bio-refinery for the scale-up production of high-value natural compounds like—carotenoids. Carotenoids are accessory pigments in the light-harvesting apparatus and also act as antioxidants and photo-protectors in green cells. They play important roles for humans, like—precursors of vitamin A, reduce the risk of some cancers, helps in the prevention of age-related diseases, cardiovascular diseases, improve skin health, and stimulates immunity. To date, about 850 types of natural carotenoid compounds have been reported and they have approximated 1.8 billion US$ of global market value. In comparison to land plants, there are few reports on biosynthetic pathways and molecular level regulation of algal carotenogenesis. Recent advances of algal genome sequencing, data created by high-throughput technologies and transcriptome studies, enables a better understanding of the origin and evolution of de novo carotenoid biosynthesis pathways in algae. Here in this review, we focused on, the biochemical and molecular mechanism of carotenoid biosynthesis in algae. Additionally, structural features of different carotenoids are elaborated from a chemistry point of view. Furthermore, current understandings of the techniques designed for pigment extraction from algae are reviewed. In the last section, applications of different carotenoids are elucidated and the growth potential of the global market value of carotenoids are also discussed.