II Bioactive Secondary Metabolites

Microbial Pigments

Júlio C. De Carvalho, Lígia C. Cardoso, Vanessa Ghiggi, Adenise Lorenci Woiciechowski, Luciana Porto de Souza Vandenberghe, and Carlos Ricardo Soccol

Introduction

There are three main sources of color additives for foods, drugs, and cosmetics:

(1) synthetic colors, (2) plant-derived pigments, and (3) microbial pigments. In chemical terms, soluble colored substances are colorants and insoluble are pigments; however, in biological context, the colored substances are called pigments irrespective of its solubility. Although the term “biopigment” has a bit of redundancy, it is used to refer pigments of natural origin.

Microbial pigments or biopigments are multitude of chemical structures capable of absorbing light in the visible range (400–700 nm). There is an ever-growing number of biopigments. These molecules may possess other properties, which may

or may not be compatible with the industrial use: vitamins riboflavin or β-carotene,

antioxidants as most carotenoids and xanthophylls, and antimicrobial activity of some fungal polyketides. This chapter presents an overview of microbial pigments as potential food and drug color additives, presenting a brief description of the origin of their color and the physiological role of pigments in microorganisms, followed by a prospect of their use and a final section with representative classes of biopigments which may be produced using agro-industrial wastes.

Fig. 4.1 The number of conjugated bonds (molecular structures on top) affects the absorption band and the color of the pigments; the graphic shows the transmittance spectrum for Monascus pigments, which appear red. Sources: Chemical structures from The Merck index, 2006; transmittance curve obtained at laboratory

4.1.1 The Origin of Color

When light interacts with matter, there may be absorption, reflection, refraction, and even reemission depending on the wavelength of incident light, chemical composition, and physical structure of the material giving rise to the multitude of colors that we see. A material may absorb the incident light unspecifically or selectively: if the absorption is unspecific, we perceive the color of the material as white, gray, or black, while if the absorption of one or more wavelengths is more pronounced, we perceive the material as having the complimentary color of that absorbed. Figure 4.1 illustrates the relationship between structure and color: molecular orbitals absorb and reemit light, and substances with multiple conjugated double bonds (a common trait in organic colored substances) tend to do so in the visible range (Nassau 2003; Meléndez-Martínez et al. 2007). Color may also arise or be modified on interaction of transition metal ions in complexes, as in porphyrin rings in hemoglobin and chlorophyll.

Fig. 4.2 Aqueous solution of phycocyanin from cyanobacteria and its absorption and emission spectra (Sources: photograph from Walter et al. 2011; spectra from Tooley et al. 2001)

Electrons in molecular orbital's absorb photons, which leap to higher energetic states and revert towards its fundamental state by releasing the energy, perhaps with several smaller leaps and radiations of different wavelengths. The wavelengths reemitted are usually outside the visible range, the effect being the net and selective absorption of visible light. However, if the reemission occurs in the visible range, we have a fluorescent pigment: for example, phycocyanin absorb mostly green, yellow, and orange light and reemit a bit of red light as can be seen in Fig. 4.2.

The part of a molecule responsible for light absorption is called a chromophore. Despite the enormous variety of biopigments, some recurring structures appear in nature and are illustrated in Fig. 4.3. There are a number of biological roles for these molecules, the most important ones being (a) their antioxidant nature, (b) their use as antennae for energy absorption, and (c) reserve substances.

Several microbial pigments are powerful antioxidants because their conjugated systems are susceptible to electrophilic attack. That is the case for carotenoids and xanthophylls, which are generally several times more efficient than ascorbic acid or butyl hydroxyl toluene (BHT) as antioxidants. Colored substances may also act as a sunscreen, protecting the cell by absorbing UV radiation and thus reducing the formation of DNA-damaging free radicals. Photosynthetic microorganisms such as cyanobacteria and microalgae rely on pigments such as chlorophylls and phycobilins for transferring light energy to electrons which will be used for carbon reduction in photosynthesis—a mechanism which produced the oxygen in our atmosphere and is in the base of virtually all food chains. Some biopigments such as phycobilins, chlorophyll, and prodigiosin may also act as nitrogen reserve in microorganisms. Besides these functions, there are several other cases in which light-absorbing molecules play an important role, such as in eyespots of microorganisms and in light-activated response mechanisms, such as the circadian rhythms of Neurospora sp. or as UV-induced damage correction mechanisms. Whenever these light-absorbing molecules selectively absorb in visible range, the result is a colored substance.

Fig. 4.3 Common microbial pigment core structures

 
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