Microbe waste can shed light on Earth's evolution

Microbe waste may provide a record of how Earth and life co-evolved over the past 3.5 billion years as the planet's temperature, oxygen levels and greenhouse gases changed, scientists say.
Microbes play a vital role in the planet's carbon cycle by digesting organic matter.
Their waste potentially carries information about how the planet's temperature, greenhouse gas composition, and even oxygen levels have changed over time, along with information about how life itself has evolved to accommodate these changes, researchers said.
Though scientists have been trying to grasp how to interpret the information from these microbial "fecal prints" for more than sixty years, the solution has proved to be elusive until now.

In a paper published in the Proceedings of the National Academy of Sciences (PNAS), researchers from McGill University in Canada and Israel's Weizmann Institute of Science describe a new technique they have developed to interpret these distinctive metabolic traces.
They chose to focus on the microbes that live on the ocean floor where the microbes consume the sulfate found in seawater because oxygen is in short supply.

Global temperatures, carbon dioxide concentrations, and oxygen levels all determine whether these sulfate-using microbes are living in times of plenty, and growing fast, or in times of need, and growing slowly.
The record of these changes is to be found in the microbial wastes and more specifically in how much, or how little, of the sulfate compound the microbes trim off.

The microbes are ultra-picky diners and prefer to keep their consumption light. Just as careful carnivores will trim the fat from the edge of their steak, these microbes tend to reject sulfur if it is just a neutron or two heavier than normal.
In times of plenty, as growth speeds up and the microbes need to take in more sulfate they are less discriminating and will trim off, or "fractionate" less.
But in times when resources are more limited, as growth slows down, microbes trim off or fractionate more, researchers found.