What is Carotenoid?
[[File:Aerial image of Grand Prismatic Spring (view from the south).jpg|thumb|The orange ring surrounding [[Grand Prismatic Spring]] is due to carotenoid molecules, produced by mats of [[algae]] and [[bacteria]].]] '''Carotenoids''' (or '''tetraterpenoids''') are [[organic compound|organic]] [[pigment]]s. They are found in the [[chloroplasts]] and [[chromoplast]]s of [[plants]], and also some [[bacteria]] and [[fungi]]. The only animals known to [[produce]] carotenoids are [[aphids]] and [[mites]], which [[horizontal gene transfer|got the genes]] from [[fungi]].<ref>{{Cite journal |author=Altincicek, Boran; Jennifer L. Kovacs & Nicole M. Gerardo |year=2011 |title=Horizontally transferred fungal carotenoid genes in the two-spotted spider mite ''Tetranychus urticae'' |journal=Biology Letters |volume=8 |issue=2 |pages=253–257 |doi=10.1098/rsbl.2011.0704 |pmid=21920958 |pmc=3297373 |url=http://rsbl.royalsocietypublishing.org/content/8/2/253}}</ref> There are over 600 known carotenoids. They are of two types, [[xanthophyll]]s (which contain oxygen) and [[carotene]]s (which are purely [[hydrocarbons]] with no [[oxygen]]). Carotenoids have two key roles in plants and [[algae]]: they absorb light energy for use in [[photosynthesis]], and they protect [[chlorophyll]] from photodamage.<ref>{{cite journal |author=Armstrong G.A. & Hearst J.E. |title=Carotenoids 2: Genetics and molecular biology of carotenoid pigment biosynthesis |journal=FASEB J. |volume=10 |issue=2 |pages=228–37 |year=1996 |pmid=8641556 |doi= 10.1096/fasebj.10.2.8641556|doi-access=free |s2cid=22385652 |url=http://www.fasebj.org/cgi/pmidlookup?view=long&pmid=8641556}}</ref> Beta-carotene is the most common form in plants. Some carotenoids (like [[β-carotene]]) have [[vitamin A]] activity (so they can convert to [[retinol]]). These and other carotenoids can also act as [[antioxidant]]s. In the eye, certain other carotenoids ([[lutein]], [[astaxanthin]],<ref>{{cite journal|last=Kidd|first=Parris|title=Astaxanthin: cell membrane nutrient with diverse c