Blank presented her research at the 2004 annual meeting of the Geological Society of America, held, Nov. 7-10 in Denver. Blank's finding that Cyanobacteria emerged first in fresh water lakes or streams is counterintuitive.
"Most people have the assumption that Cyanobacteria came out of a marine environment ?after all, they are still important to marine environments today, so they must always have been," Blank said. "When Cyanobacteria started to appear, there was no ozone shield, so UV light would have killed most things. They either had to have come up with ways to deal with the UV light ?and there is evidence that they made UV-absorbing pigments ?or find ways of growing under sediments to avoid the light."
To study the evolution of Cyanobacteria, Blank drew up a backbone tree using multiple genes from whole genome sequences. Additional species were added to the tree using ribosomal RNA genes. Morphological characters, for instance, the presence or absence of a sheath, unicellular or filamentous growth, the presence or absence of heterocysts ?a thick-walled cell occurring at intervals?were coded and mapped on the tree. The distribution of traits was compared with those found in the fossil record.
Cyanobacteria emerging some two billion years ago were becoming complex microbes that had larger cell diameters than earlier groups ?at least 2.5 microns. Blank's tree shows that several morphological traits arose independently in multiple lines, among them a sheath structure, filamentous growth, the ability to fix nitrogen, thermophily (love of heat), motility and the use of sulfide as an electron donor.
"We will need lots of analyses of the micro-foss
Source:Washington University in St. Louis