Diverse microorganisms undergoing growth and reproduction. As organisms evolved from simple prokaryotes to unicellular eukaryotes to multicellular organisms their strategies for internal energy partitioning dramatically changed as illustrated by a recent mathematical model. Image: Mari Kempes |
Every
living organism balances a budget of sorts—by allocating energy to various
parts of its body to fuel essential life processes. Throughout its lifetime, an
organism may rebalance this budget to spend more energy on certain processes
over others. How an organism spends its energy determines, in large part, its
ability to survive in the world, and researchers who study “bioenergetics” are
modeling energy use in organisms to understand how populations grow and evolve.
Researchers
at the Massachusetts Institute of Technology (MIT) have now come up with a
model for how energy is spent in the smallest, simplest organisms on Earth,
ranging from single-celled bacteria to multicellular microbes. The model
divides an organism’s possible energy use into two broad categories: growth and
reproduction, and maintenance and repair. Based on the size of a given
organism, the model accurately predicts what fraction of energy is spent on
each category.
This
information, the scientists say, could be crucial for determining how
populations of bacteria and other microbes grow and spread in oceans and soil.
The model may also help researchers interpret major evolutionary shifts: As
microbes evolved to be more complex, they likely rebudgeted energy to support
new cellular machinery.
The
researchers published their results in the Proceedings of the National
Academy of Sciences.
Mick
Follows, co-author of the paper and a senior research scientist in MIT’s
Department of Earth, Atmospheric and Planetary Sciences, says all organisms, at
one point or another, face a decision to repair or reproduce, some investing
more energy in one process over the other.
“You
can imagine a life strategy for one organism might be, ‘I’m not going to spend
anything on maintenance, I’m just going to reproduce as quickly as possible and
hope I make so many copies that some of them will get through,'” Follows says. “And the opposite strategy is, ‘Well, I’m going to invest less in reproduction,
and really take care of myself and keep myself in prime condition and not die
if I can help it.'”
Follows’
graduate student Christopher Kempes developed a mathematical model that
predicts, broadly, how microbes allocate energy. Kempes devised equations to
represent how fast a given microbe grows, as well as the total amount of food
an organism can convert to energy. The team, along with research scientist
Stephanie Dutkiewicz, then compiled data from other researchers who measured
the weight of various microbes over a lifetime, including single-celled
bacteria and tiny, multicellular shrimp.
The
MIT team combined the data with their equations, and found some interesting patterns
among the microbes.
For
the gut microbe Escherichia coli, almost
every ounce of energy is spent on reproduction. Throughout its lifetime, a
single E. coli bacterium grows and
divides continuously, quickly colonizing a stomach lining or petri dish with
millions of simple cells. Slightly more complex green algae exhibit a similar
trajectory, reproducing up until the very end before refocusing energy inward,
on processes that maintain cellular machinery. In contrast, tiny,
millimeter-long crustaceans are more self-involved, spending most of their
lifetime maintaining complex components before expending energy on
reproduction.
The
general trend, Follows says, seems to be that the bigger and more complex a
microorganism, the more energy it spends on looking out for itself, or
repairing internal structures. Smaller, simpler organisms focus more on growing
and proliferating, counting on sheer numbers to increase their survival
chances.
“You
can get an inkling of how you’re going from very simple cells that can grow
faster,” Follows says. “As they add machinery, they invest more in maintenance.
And then at some point, that strategy also becomes energy-intensive. But at
that point, multicellularity allows you to share energy and resources with
other cells.”
These
trends, the team speculates, may reflect broad evolutionary changes between
single-celled prokaryotes such as E. coli,
more complex eukaryotes such as green algae and simple multicellular organisms
such as tiny shrimp. Through their model, the researchers are able to determine
the very smallest size of the simplest organisms, based on how they use energy,
as well as the size at which organisms evolve to be multicellular.
“Those
evolutionary transitions occur in our model at very predictable stages,” Kempes
says. “These transitions allow the organism to get bigger, and that’s the story
of how life got so complex.”
Steven
Allison, an assistant professor of ecology and evolutionary biology at the University of California
at Irvine, says
the group’s new model may be used to evaluate how all organisms, large and small,
expend energy.
“The
key innovation here is that microbes’ use of energy and resources can change
through their life cycles,” Allison says. “These differences have not been
appreciated before. This means that it may be possible to predict population growth
rates based on the size of the cells and their type.”
The
team plans to incorporate the mathematical model for a single organism’s energy
use into models of large-scale populations. Follows says knowing how a single
organism allocates energy could help researchers more accurately model how
microbes spread throughout an environment. For example, if a scientist builds a
model to represent bacteria in the ocean, the population may look very
different depending on whether the researcher programs the bacteria to expend
all their energy on reproduction or repair.
“In
some sense, today’s models of phytoplankton in the ocean don’t use this kind of
information,” Follows says. “We need to make [these models] better.”
