But what about this starfish, why is it orange? The biosynthesis of carotenoids is mainly the realm of bacteria and fungi. In fact, Professor Nancy Moran and colleagues recently showed that aphids, which can be orange, borrowed a gene from a fungus in order to produce this carotenoid, which may even allow the aphids to capture energy from the sun (time will tell if this is so). But back to the starfish...Well, their color is from a carotenoid--but I'm not sure who is making it--the starfish or a symbiont. If there is an adaptive value to advertising one's self, it may be to say 'stay away' to potential predators. Called aposematism, this might explain why monarch butterflies are orange (they are toxic). It is actually fairly unclear if orange starfish are aposematic, or if this due to a tradeoff that emerges during development. Regardless, it seems that carotenoids, from a biosynthetic perspective, are mostly the province of bacteria and fungi.
Ph.D. student Parris Humphrey in our laboratory group is studying Life in a Leaf at the Rocky Mountain Biological Laboratory (RMBL). In the leaves of just one species of plant, the mustard Cardamine cordifolia, he has been able to culture myriad species (see a photo of a plate that Parris brought back from RMBL below), but in a repeatable way. He is initially diagnosing bacterial phyllosphere species based on colony morphotypes when leaf extracts are plated on King's B media. Each colony was initially started from a single cell and then re-streaked on a new plate. Parris found that many of the leaf-dwelling bacteria in C. cordifolia are each able to produce pigments--yellow, green, pink and orange (there may be others that you can see below). Many of these pigments are carotenoids although some (the green) are not. The function of the siderophores (the green stuff), which has leaked into the media, is likely to allow the bacteria to utilize iron in the media. The carotenoids function in many different ways in bacteria and plants.