While a whole host of thermophilic (heat-loving) microorganisms are responsible, it is the cyanobacteria, one of the more common fellows from this group, which form most of the scum. Cyanobacteria grow together in huge colonies (bacterial mats) that form the delightfully colourful slimes and scum on the perimeter of hot springs. You can tell a fair bit about the water temperature and chemistry just by looking at the colour of the pools. The coloring shifts dependant upon the ratio of carotenoids to chlorophyll and ambient temperature. We see more orange and red in the summer and the colder temperature of late Fall and Winter bring more green to the coloring.
