## 3 Duckweed as an Alternative Source of Nutrients and Bioactive Compounds
### Secondary Metabolites
#### 3.4.2 Flavonoids, Carotenoids, and Chlorophylls  
Duckweeds are known for their diverse pigment composition, which can affect their overall appearance. The plants' pigment profiles may shift in response to environmental factors such as nutrient availability, light intensity, and temperature [71]. Duckweeds are known to possess a variety of pigments, including flavonoids, carotenoids, chlorophylls, which contribute to their characteristic green, yellow, or reddish hues. Flavonoids are a large group of plant compounds, many of which function as pigments in plants, particularly in flowers, fruits, and leaves. The primary flavonoid pigments identified in duckweeds are derivatives of apigenin and luteolin (mentioned in Section 3.4.1), which contribute to yellow pigmentation. These flavonoids have been identified in various duckweed species, including _Landolia punctata_, _Wolfiella caudata_, and _Wolfia borealis_[56], and to a lesser extent in _Lemma perpusilla_ and _Lemma minor_[70]. Carotenoids, a structurally diverse group of over 700 organic, lipid-soluble pigments, are responsible for the vibrant yellow, orange, and red colours observed in various natural sources, including plants, algae, and some bacteria [72; 73; 74]. These pigments not only contribute to the vibrant hues observed in duckweed but also confer a wide range of physiological benefits, such as improvements in survival, growth performance, reproductive capacity, stress tolerance, disease resistance, and immune-related gene expression [75; 76]. These pigments play a crucial role in the prevention of human diseases and the maintenance of overall health, owing to their antioxidant properties and their ability to serve as provitamin A nutrients [72; 73; 74]. Carotenoids, including astaxanthin, \(\beta\)-carotene, canthaxanthin, fucoxanthin, zeaxanthin, and lutein, have been identified in various duckweed species [40; 77]. The presence of carotenoids like astaxanthin, lutein, and \(\beta\)-carotene in duckweeds can have implications for their potential applications in aquaculture and animal feed. These carotenoids are known to play a role in the pigmentation, growth, and health of various aquatic organisms, and their availability in duckweeds may make them a valuable natural source of these compounds [75]. Lutein (a type of xanthophyll that is yellow in colour) is important for the plant's photosynthetic processes and also has antioxidant properties. Zexanthin, another type of xanthophyll, is involved in the photoprotective mechanisms of the plant, helping to protect against damage from excess light. Zexanthin, in particular, has been shown to improve vision, protect the eyes from intense light damage (preventing cataracts and blindness), and combat systemic inflammation [78]. While not as abundant as lutein and zeaxanthin, \(\beta\)-carotene is another carotenoid found in duckweed ([PERSON] 2005) [79]. \(\beta\)-Carotene is a provitamin A carotenoid (precursor of vitamin A), meaning that it can be converted into vitamin A in the human body, and it also exhibits potent antioxidant properties ([PERSON] et al., 2020) [24]. Overall, these carotenoids contribute to the plant's ability to absorb light for photosynthesis, protect against oxidative damage, and play a role in the plant's overall health and stress responses. Furthermore, they also offer potential health benefits for animals and humans, such as reducing the risk of degenerative diseases, cardiovascular issues, and cancer [75; 80]. Lutein and zeaxanthin are of interest for their potential health benefits, especially for eye health in humans, as they are found in high concentrations in the retina [53]. They can also serve as regenerative resources for astronauts in space environments, exerting antioxidant effects to help protect them from radiation damage [81]. The distribution and abundance of carotenoids in duckweeds can also be influenced by other pigment-related secondary metabolites, such as chlorophylls [82; 83].