Pigments from microalgae handbook

The colours of terrestrial plants are very familiar. The predominant green of pho- tosynthetic tissues, especially leaves, is due to chlorophylls and masks the yellow carotenoids that are also present, though these are revealed in autumn leaves, along with red anthocyanins, as the green chlorophyll is degraded prior to leaf fall. The bright colours of flowers and fruit provide striking contrast and attract insects and other vectors for pollination and seed dispersal. In the oceans of the world, ter- restrial plants are replaced by algae, which may be macroalgae (seaweeds) or microalgae (phytoplankton). These do not have flowers or fruit, so major classes of plant pigments—anthocyanins and other flavonoids, betalains and quinones—are not produced in the aquatic environment. Algae, however, do use pigments for their natural photosynthesis. Photosynthesis in algae of all classes is similar in principle to that in land plants, taking place in chloroplasts and requiring pigments to harvest and use the available light energy. The intensity of the available light, however, decreases as water depth increases, and not all wavelengths of light penetrate water, especially seawater, to the same extent. Red light, which would be absorbed by chlorophylls, is mostly absorbed in the first few metres of water depth, and it is largely blue and green light that penetrates to greater depths. So the role of accessory light-harvesting pigments, carotenoids and phycobiliproteins, is much more important, to make the most efficient use of the available light for photosynthesis. Also, algae that may become exposed to bright sunlight during part of the day must be able to withstand a high level of light energy. Adaptation to low or high light intensity is, therefore, an important feature of algal life patterns. Algae, including microalgae, use three kinds of pigments in photosynthesis, generally located in pigment–protein complexes. Chlorophyll is essential for pho- tosynthesis. Chlorophyll a occurs universally in algae of all classes. Chlorophyll b is largely or entirely restricted to green algae (Chlorophyta), whereas chlorophylls c and d occur in many classes. Many green algae have carotenoid compositions rather similar to those of green leaves, but other classes of algae contain many different carotenoids. These have occupied the attention of carotenoid chemists for many years, and carotenoid compositions have been used in chemosystematic classification of algae. It is estimated that tens of millions of tonnes of fucoxanthin and peridinin are produced naturally in the world’s oceans every year. Phycobiliproteins are more specialised and restricted to the Cyanophyceae (blue-green algae, now classified as Cyanobacteria) and Rhodophyceae (red algae) in which they are localised in specialised aggregated structures, phycobilisomes and Cryptophyceae. There are two main types of phycobiliproteins, the blue phyco- cyanin and the red phycoerythrin, but both are usually present, though in differing proportions. Some ‘blue-green algae’ have a high proportion of phycoerythrin and are red and some ‘red algae’ have a high proportion of phycocyanin and are blue-green. The phycobilin prosthetic groups of these pigments are linear tetra- pyrroles that are covalently linked to protein via cysteine residues.