Everything that eats must excrete, and microbes are no exception.
Microbes have dominated Earth's ecology for at least the past 3.5
billion years. They play a vital role in the planet's carbon cycle by
digesting organic matter. So 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. But 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.
Microbes are ultra-picky diners
In a paper recently published in the Proceedings of the National Academy of Sciences (PNAS),
researchers from McGill University 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. Like many humans, they prefer to
keep their consumption light. And 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. (Neutrons are
atomic particles, and so are very, very small.)
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. Like a glutton is wolfing down a slab of roast beef,
they don't spend time cutting off the fat. But in times when resources
are more limited, as growth slows down, microbes trim off or fractionate
more. It's like dining with a freegan who insists on trimming off the
rank exterior of an expired rump roast to find the single unspoiled
piece of meat inside.
The new research by Wing and Halevy explains these peculiar dining
preferences for microbes and, for the first time, links it to how much
of what they consume is stored inside their cells. Although the "fecal
print" analysis was developed for the sulfate-reducing microbes found
under the seabed, as Wing's co-author Itay Halevy of points out, the new
work has larger implications. "It can be applied to many other
microbial metabolisms that are important to earth system functioning
today, from the denitrifiers that drive Earth's nitrogen cycle to the
microbes that produce the greenhouse gas methane."
Wing credits a sharp McGill undergrad with asking the question that
inspired this research. "When I started at McGill I told an undergrad
that we were going to grow some microbes in the lab to see how they
fractionated, so that then we could look for this as a bio signature in
some wicked old rocks," says Wing. "She gave me a super skeptical look
and asked if I knew that microbes evolve. That fundamental question is
now behind the majority of the research in my lab, where we are trying
to understand ever-evolving relationship between our planet and its
microbial majority."
This research was funded by: National Science and Engineering
Research Council of Canada (BAW); Feinberg Foundation Visiting Faculty
Program at the Weizmann Institute of Science (BAW); European Research
Council (IH); Israel Science Foundation (IH)
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