The Science Behind The Colours In Nature

Do you know how the ancient man added colours to their clothes, artworks and food? Have they obtained different types of colours either from plants, animals or microorganisms? Why do plants or animals show a colour?

According to historical records, since ancient times, man has been interested in colours. Early man has obtained different types of colours originated from plants, certain invertebrates, micro-organisms and minerals. Out of them, plant-based natural dyes were the most common. Natural pigments are present in the cytoplasm in different forms. The green pigments called chlorophylls that give the green colour to leaves are present in chloroplasts. The yellow to red pigments called carotenoids are the reason for the colour of ripening fruits and flowers. They are present in chromoplasts. Besides, there are few water-soluble pigments which are present in the vacuole as well.

As we all know, chlorophyll is the main colourant in almost all of the plants in the plant kingdom which facilitates the photosynthetic mechanism. The varying amounts of conjugated double bonds in the pigment is the place of the light absorption. It is also an efficient antioxidant, which

reduces oxidative stress in cells caused by UV light exposure and several other stresses. Carotenoids are tetraterpenoids that assist light-harvesting as accessory pigments. Fruits such as tomatoes (Solanum lycopersicum) and watermelons (Citrullus lanatus) contain red-carotenoid pigments. They are rich in lycopene and B-carotene. Moreover, some carotenoids (neoxanthin and violaxanthin) act as precursors for the biosynthesis of Abscisic acid.

Figure 1&2: Carotenoid pigmented fruits

The majority of plant-derived natural pigments are secondary metabolites. They do not have a direct role in growth and development. However, these are important for vital functions that ensure plant survival. Pigments are a reason for the attraction of pollinators and deterrence of predators. Further, scientists believe that pigments have a significant role in the coexistence and coevolution of species allowing interactions. Indigo is a well-known blue dye extract from Indigofera sp. throughout the world while Madder (Rubia tinctorum) gives a shade of red. Saffron (Crocus sativus), turmeric (Curcuma longa), safflower (Carthamus tinctorius) and marigold (Tagetes erecta) are some yellow pigment producing plants.

Anthocyanins are glycosylated polyphenolic compounds which represent a large group of plant secondary metabolites. These are widely used in the food industry as an alternative to synthetic colourants due to their health benefits. It is also safe for human consumption. Interestingly, anthocyanins can serve as a pH indicator due to their ability to change colour based on the pH changes in the intravacuolar environment. In acidic environments, it shows a reddish-pink. The colour changes towards green colour in an alkaline medium. It is reddish-purple in neutral solutions (pH=7). Pigments such as astaxanthin and lycopene are used as dietary supplements.

Figure 3: Anthocyanin pigmented flowers e.g.: Butterfly pea flowers (Clitoria turnatea)

Pigmentation is a useful strategy of signalling and protection in animals. They show camouflage and mimicry using different pigmentation for protecting itself. Several pigments also were extracted from animals such as cochineal insect (Dactylopius coccus), lac insect (Kerria lacca), kermes and shellfish. Mycobacterium sp. and Staphylococcus aureus are some pigment-producing microorganisms. Some species of bacteria produce pigments continuously while others produce in response to environmental stresses. In response to low iron stress, carotenoids are accumulated in some algal species like Synechococcus species.

Natural dyes were the only source of colour for fabrics, leather and other materials until synthetic dyes were discovered in the eighteenth century. At present, a combination of genetic engineering in biosynthetic pathways of plant pigments and hybridization may lead to advances in commercial applications related to natural dyes. Hence, it is high time to think about sustainable methods along with a scientific approach to develop natural dyes from potential sources.

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