Canthaxanthin is a keto-carotenoid pigment widely distributed in nature. Carotenoids belong to a larger class of phytochemicals known as terpenoids. The chemical formula of canthaxanthin is C40H52O2. It was first isolated in edible mushrooms. It has also been found in green algae, bacteria, crustaceans, and bioaccumulates in fish such as carp, golden grey mullet, seabream and trush wrasse. Canthaxanthin is associated with E number E161g and is approved for use as a food coloring agent in different countries, including the United States and the EU; however, it is not approved for use in Australia and New Zealand. It is generally authorized for feed applications in at least the following countries: US, Canada, EU. In the EU, canthaxanthin is allowed by law to be added to trout feed, salmon feed and poultry feed. The European Union limit is 80 mg/kg of feedstuffs, 8 mg/kg in feed for egg laying hens and 25 mg/kg in feed for other poultry and salmonids. Canthaxanthin is a potent lipid-soluble antioxidant. The biological functions of canthaxanthin are related, at least in part, to its ability to function as an antioxidant (free radical scavenging/vitamin E sparing) in animal tissues. Biosynthesis Due to the commercial value of carotenoids, their biosynthesis has been studied extensively in both natural producers, and non-natural (heterologous) systems such as the bacteria Escherichia coli and yeast Saccharomyces cerevisiae. Canthaxanthin biosynthesis proceeds from beta-carotene via the action of a single protein, known as a beta-carotene ketolase, that is able to add a carbonyl group to carbon 4 and 4' of the beta carotene molecule. Although functionally identical, several distinct beta-carotene ketolase proteins are known. That is to say they differ from an evolutionary perspective in their primary amino acid/protein sequence. They are different proteins that complete the same function. Thus, bacterial (CrtW) and micro-algal beta-carotene ketolase proteins such as BKT isolated from Haematococcus pluvialis are known. Due to the nature of canthaxanthin, relative to astaxanthin (a carotenoid of significant commercial value) these beta-carotene ketolase proteins have been studied extensively. An E. coli based production system has been developed, that achieved canthanaxanthin production at 170 mg/L in lab scale fermentation. Presence in fish Canthaxanthin is not found in wild Atlantic Salmon, but is a minor carotenoid in Pacific Salmon. Canthaxanthin is used in farm-raised trout. Canthaxanthin is used in combination with astaxanthin for some salmon feeds. Presence in birds The antioxidant characteristics of canthaxanthin have been