Backed by funding from Wisconsin Sea Grant, UW-Madison researcher Trina McMahon
has become the worldwide authority on the key bacteria in freshwater lakes.
October 11, 2016
By Aaron R. Conklin
Every ecosystem has a top dog, a species that out-evolves and outcompetes
everything else to survive and thrive under a wide range of conditions. In
freshwater lakes, that champion is a special group of actinobacteria, small
microbes—like, really, really tiny —that make up a superabundant group of
bacteria that’s involved in most of what goes on in the freshwater universe.
Nobody knows more about freshwater actinobacteria than University of
Wisconsin-Madison professor of environmental engineering Trina McMahon. With
the support of Wisconsin Sea Grant, McMahon’s laboratory members have spent the
last five years studying the little critters from every imaginable angle—and in
the process have become the pre-eminent experts on the topic. What they’ve
found has enlarged our understanding of how freshwater lakes function and
exist.
“If you think of the lake as an entity, a living breathing thing that cycles
nutrients, these bacteria are responsible for half of it,” said McMahon.
“They’re very, very tiny, but because of their numbers and their level of
activity, they’re driving huge amounts of the carbon cycling and nutrient
regeneration,” said McMahon. “We’ve had a special place in our heart for a long
time for the freshwater actinobacteria.”
The relationship began back in 2007, with Ryan Newton, one of McMahon’s
first Ph.D. students. Newton, who’s now an assistant professor with the
UW-Milwaukee School of Freshwater Sciences, developed a baseline bar code of
actinobacterial RNA sequences that allows researchers to track, classify and
enumerate bacteria in lakes. Using that code, Newton and McMahon demonstrated
that actinobacteria are the predominant species in inland lakes.
In 2012, McMahon’s lab used a cutting-edge method to take a single cell of
the actinobacteria and sequence its genome. What they found was that the
actinobacteria have a rhodopsin protein similar to the protein in the human eye
that allows it to sense light. In the actinobacteria, however, the rhodopsin
almost certainly does more—converting the light into energy. (Those findings
were recently published in the International Society for Microbial Ecology
Journal.)
In a 2014-16 funded project with Wisconsin Sea Grant, McMahon and UW-Madison
structural biologist Katrina Forest took it further, revealing something even
more surprising about freshwater actinobacteria.
“Actinobacteria have the retinal found in most opsin proteins that allows
them to harvest light, but we think they also have another light-harvesting
structural molecule that allows harvesting of a different wavelength of light,
amplifying the energy that gets harvested in a way that not many other bacteria
have.”
That extra method of acquiring energy helps explain why they’ve shot to the
top of the ecosystem ladder like a supercharged bullet. Currently, a graduate
student in Forest’s lab is charting the actinobacterial cell’s biochemical
machinery to definitively identify the structure of this second light-capturing
molecule. McMahon suggests it might be possible that different groups of actinobacteria
harvest different wavelengths of light.
In addition to the light-harvesting mechanism, McMahon’s lab has noted that
the actinobacteria also interact extensively with the gunky-green cyanobacteria
and algae that often overtake freshwater lakes during the summer months.
“They have in their cell wall/membrane all this machinery to suck up other
dead organisms’ parts,” McMahon explained. “We think of them as vultures or
scavengers—they wait for other organisms to die and then they eat up their parts.
Then they recycle the atoms into carbon dioxide and also into new cell
material. They are the foundational recyclers of the lake.”
McMahon said the interactions take a variety of forms—everything from the
actinobacteria eating the dead cyanobacteria to sucking up molecules excreted
by the cyanobacteria during periods of rapid growth.
“They’re super in one sense but they’re also crippled in another in that
they depend on being able to scavenge what they can’t make themselves,” she
said. “What’s fascinating is that we haven’t figured out if the actinobacteria
help fuel the cyanobacteria blooms or keep them in check,” said McMahon.
“There’s some early evidence that maybe they’re actually partners with the
cyanobacteria in certain roles, which would mean that understanding
actinobacteria might help us control cyanobacteria blooms better.”
McMahon’s well aware that she faces a strong
ewwww factor
associated with her research—who wants to talk about gross bacteria and smelly,
potentially poisonous blue-green algae in our lakes? To get around that,
McMahon has begun talking about actinobacteria using the same language people
use to talk about the bacteria that live in humans’ guts, performing helpful
tasks like digesting our food and bolstering our immune systems.
“People start to feel a little less scared about the bacteria when they
think about it that way,” she said. “If we can understand how the
actinobacteria function, and all the different ways they get energy and support
the ecosystem, then we have that much deeper an understanding of the lake
system. Then we can either do some kind of intervention to improve lake quality
or at least make a prediction about what’s going to happen if we do make an
intervention.”
McMahon’s research focus will now shift to determining how special each of
the strains of actinobacteria are. Armed with genome sequences from the Great
Lakes, Lake Mendota, lakes in Sweden and other countries around the world,
McMahon’s working to determine whether the bacterial strain she’s studied in
Madison’s Lake Mendota is endemic to all lakes or has adapted to its specific
environments.
“Maybe the cell in Lake Mendota gets carried to a lake in northern
Wisconsin, but maybe it can’t live there because it depends on its friends who
are in Lake Mendota,” she said. “We would actually prefer if they weren’t too
endemic, because we’d like to take what we’ve learned and apply it to all
lakes.”
