**3.4 Superoxide reductases**

All species interacting with air produce superoxide, or O2•-, which, depending on the biological environment, can function as a signaling agent, a poisonous lifeform, or a nontoxic precursor that breaks down spontaneously. Superoxide reductase (SOR) and superoxide dismutase (SOD) are two enzymes that limit their levels in vivo (SOD) [46]. SORs are simple enzymes with a sequence of 110–180 amino acids. SORs


#### **Table 2.**

*The advantages of algal antioxidants for health.*

can be categorized in several ways. Neelaredoxins and desulfoferrodoxins have only one or two Fe atoms per polypeptide chain, respectively, which was the most distinguishing feature of these enzymes [44]. As a result, the most accurate classification for the procedure is to categorize them as 1Fe-SORs (neelaredoxins) and 2Fe-SORs (the desulfoferrodoxins). The most current exploration the methanoferrodoxin, a SOR from some methanogens with a domain harboring a [4Fe-4S]2+/1+ cluster, may lead to an extension of this classification in the near future [46] (**Table 2**).

### **4. Non-enzymatic antioxidants**

#### **4.1 Carotenoids**

Carotenoids are the most common and naturally occurring pigment. One such example is hydrophobic terpenoids. The polyene chain of carotenoids, which is made up of double bonds, gives them their coloration and the capacity to absorb photons of visible wavelengths. Both photosynthetic and non-photosynthetic species can produce carotenoids. Carotenes and xanthophylls are the two major categories of naturally occurring carotenoid algae [67]. Carotenes are linear or cyclic hydrocarbons, e.g., β-carotene and α-carotene. Oxygenated carotenoid derivatives are known as xanthophylls. The xanthophylls, violaxanthin, antheraxanthin, zeaxanthin, neoxanthin, and lutein produced by higher species are synthesized by green algae [67].

Freshwater pond cyanobacterial blooms emit a foul stench because of their adaptation to human-induced conditions exposed. These blooms of blue-green algae spread widely and produce cyanotoxin, poisonous to other creatures. However, these poisons have demonstrated potential properties as cancer treatments. Consider microcystins, numerous peptide toxins, including cryptophycins and anatoxin-A, have shown clinical effectiveness for various cancers [68].

Carotenoids, which are byproducts of photosynthesis and include carotene, xanthene, lutein, and lycopene, are often abundant in algae and cyanobacteria. As foragers of electron species with a singlet, or ROS, carotenoids and other terpenoids are crucial. Therefore, these scavengers are used as antioxidants to stop the growth of cancer cells. There are not many reports on carotenoids' ability to fight different types of cancer [69].

Astaxanthin and β-carotene, generated by *Haematococcus pluvialis* and *Dunaliella salina*, respectively, are two main carotenoids produced by microalgae. A vital component known as β-carotene is extensively looked for as a food coloring agent, for cosmetics addition, and as healthy food. It is frequently used in soft drinks, cheeses, butter, and margarine and is well-known for being safe and having health benefits due to its pro-vitamin A activity [70]. Astaxanthin has advantages, including increasing eye health, boosting muscle power and endurance, and shielding the skin from UVA damage, inflammation, and early aging. Animals need it for various purposes, such as immune system functions and regeneration. It is a potent coloring agent. Other carotenoids are catechin and phycocyanobilin (**Figure 3**).

#### **4.2 Phycobilin pigments**

Microalgae form accessory pigments like phycobiliproteins. These pigments improve the light energy utilization efficiency of algae and protect it from solar

**Figure 3.** *Chemical structure of some carotenoids. a) Phycocyanobilin b) Catechin, c) β-carotene d) Astaxanthin.*

radiation and its effects. They are antioxidants in feed and humans. Phycobiliproteins are the major component of light-harvesting antenna pigments called phycobilisomes. They are present in Rhodophyta (red algae), Cryptomonads algae, and Cyanobacteria (blue-green algae) [71].

The cyanobacterium *Spirulina* (Arthrospira), which produces phycocyanin (blue), and the rhodophyte porphyridium, which produces phycoerythrin(red), are the main sources of phycobiliproteins. In Japan and China, phycocyanin is utilized in chewing gum, candies, dairy goods, jellies, ice cream, soft beverages, as well as in cosmetics like lipsticks. Phycocyanin is a versatile blue coloring agent that gives jelly and confections a vibrant blue color [71].
