**1. Introduction**

#### **1.1 Microalgae**

Microalgae have a vast biodiversity and are an almost unexplored resource. Microalgae, also known as microphytes, are minute, microscopic algae that cannot be seen with the human eye. They are phytoplankton that live in both the water column and the sediment and can be found in both freshwater and marine systems [1]. Microalgae and bacteria form the foundation of the food web, providing energy to all trophic levels above them. Chlorophyll a concentrations are frequently used to quantify microalgae biomass, and they can be a good indicator of prospective production [2]. They are unicellular organisms that live individually, in chains, or in groups. Their diameters can range from a few micrometers to a few hundred micrometers depending on the species. Microalgae, unlike higher plants, lack roots, stems, and leaves. They've evolved to thrive in an environment dominated by viscous forces.

It is estimated that there are between 200,000 and 800,000 species in various genera, with roughly 50,000 species described [3]. Chemically, over 15,000 new chemicals derived from algal biomass have been identified [4]. Carotenoids, antioxidants, fatty acids, enzymes, polymers, peptides, toxins, and sterols are some examples [5]. Microalgae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen [6] and develop photoautotrophically while using carbon dioxide as a greenhouse gas. Microalgae and cyanobacteria, collectively known as phytoplankton, dominate photosynthesis in the ocean [7]. Microalgae chemical composition is not a continuous factor; it fluctuates depending on a variety of circumstances, including species and growth conditions. Some microalgae have the ability to adapt to changes in environmental conditions by changing their chemical composition in response to variation in the environment. Their ability to substitute phospholipids with non-phosphorus membrane lipids in phosphorus-depleted settings is a particularly striking example [8]. Changing environmental parameters such as temperature, illumination, pH, CO2 supply, and nutrients can help microalgae collect desired products to a great amount [9–20].

### **1.2 Major classification of microalgae**

Chlorophyta: The phylum's members can be found in freshwater, marine, or even terrestrial habitats. It includes unicellular and multicellular organisms with chlorophylls a and b in a single chloroplast surrounded by two envelope membranes. Unicellular representatives of the phylum Chlorophyta, such as *Chlorella vulgaris*, *Dunaliella salina*, and *Haematococcus pluvialis*, are used in commercial manufacturing today and *Parietochloris incisa* and *Botryococcus braunii* have the ability to produce lipids and hydrocarbons respectively.

Rhodophyta: The phylum Rhodophyta is mostly made up of marine multicellular species, with a few freshwater or unicellular species thrown in for good measure. A huge single chloroplast is encircled by two envelope membranes with a single central pyrenoid, and cells are spherical with an eccentric nucleus. The brown-toolive-colored unicells are coccoid, nonmotile, and have a single pyrenoid-containing chloroplast, and the mucilaginous sheath can be thickened unilaterally.

Haptophyta: Haptophyta algae are mostly marine and unicellular or colonial, while several freshwater species have been discovered. The two most well-known Haptophyta species utilised as feed microalgae in aquaculture are *Isochrysis* aff.

*Revisiting Microalgae as an Additive for Nutraceuticals: A Review DOI: http://dx.doi.org/10.5772/intechopen.104902*

*galbana* (T-ISO) and *Pavlova salina*. All haptophytes have one or more pyrenoid-containing chloroplasts and an antapical nucleus, with the nuclear envelope connected to the chloroplast ER and a peripheral ER beneath the plasma membrane.

Dinophyta: Members of the phylum Dinophyta are unicellular and mostly marine, with a few freshwater species. Only around half of the Dinophyta are photosynthetic, with the remaining 50% being heterotrophs lacking chloroplasts. It is a marine, heterotrophic, colourless dinoflagellate with dinokont flagellation, in which the transverse flagellum is encircled by a medial encircling cingulum that is displaced and drops downhill [21].

#### **1.3 Nutraceuticals**

A nutraceutical, often known as a "bioceutical," is a pharmaceutical substitute that claims to have physiological benefits [22, 23]. A product that provides nutritional values as well as pharmaceutical values was coined as *nutraceutical* by Dr. Stephen DeFelic in 1989. There have been many definitions for this uprising term. The American

Nutraceutical Association has defined the term as follows: "A nutraceutical is any substance conceived as a food, or part of a food which provides medical or health benefits, as well as the prevention and treatment of a disease" while according to the US Institute of Medicine, nutraceuticals include "any substance that is a food or part of a food which provides medicinal or health benefits including the prevention and treatment of disease, beyond the traditional nutrients it contains." Nutraceuticals, in general, have a favorable impact in improving customers' health and wellness. Therefore, healthpromoting substances generated from food or food items to aid in the prevention or treatment of disease and/or dysfunction have also been considered as nutraceuticals.
