But you collect at 9 meters and man, it just knocks your socks off. There's a very distinct line, and it's nice to track bacteria through that line. The trick for the photosynthetic bacteria is to live high enough in the lake that they get sufficient light, but low enough that they get sufficient sulfide. Pictures taken through a microscope of some of Madigan's cultures show chains of bacteria with intermittent bright areas, like pearls interspersed along a beaded necklace. These bright spots, he's almost certain, are structures called gas vesicles.
Many scientists think it's a mechanism for cells to position themselves in a water column.
An individual bacterium would inflate the vesicle with gas to float higher and collapse it to sink lower. The story is different at Lake Hoare. The purple bacteria in this lake, which lacks sulfide, probably use organic compounds as their electron source to convert carbon dioxide. And unlike most photosynthetic bacteria, they're oxygen-tolerant, so living at the top of the lake doesn't bother them. Such adaptations are another instance of metabolic diversity.