The red and orange colors of the carotenoid masked microbial mats in this hydrothermal pool in Yellowstone show their maximum color intensity in summer. The microbes are better protected from the ultraviolet radiation of the sun’s light in the transition zone towards the deeper water. Hence they display unmasked green colors of the chlorophyll. The sharp yellow boundary marks the transition zone where the ultraviolet radiation forces the microbes to use carotin in the shallow water. In contrast to the repeatedly erupting geysers, hot springs are constantly fed by heated ground water and hence exhibit a calmer appearance. The reason is the missing constriction in the feeding channels. The slower rising hot water is allowed to cool on its way to the surface. Once cooled it becomes more dense than the rising hot water and begins to descend back into the plumbing system. This triggers a permanent water cycle of rising hot and sinking cool water within the hot springs internal feeding channels.
Canon 20D, Canon 10-22mm, f/16, 1 sec, ISO 100, tripod