Leftover Lunch for Microbes

Jason Dobkowski Wolverine Lake samples
Jason Dobkowski at Wolverine Lake collecting liquid samples. / FrontierScientists

A needle on lab equipment wavers as the machine tracks precise changes in carbon dioxide concentrations in a sample. Water flows through tubes. In every droplet of water there might be a million microbes swimming, feeding. It’s a zoo in there.

University of Michigan researchers Dr. Collin Ward, postdoctoral fellow in the Department of Earth and Environmental Sciences and Jason Dobkowski, lab manager in the Department of Ecology and Evolutionary Biology, make use of a summer lab at Toolik Field Station on Alaska’s North Slope. They’ve collected water samples from the Sagavanirktok and Kuparuk rivers. Back at the lab, they use the samples to analyze how interactions with sunlight and with microbes (bacteria) impact the dissolved organic carbon in the water.

Thawing carbon reenters the cycle

Ward and Dobkowski work in collaboration with Dr. Rose Cory, U-Michigan professor of aquatic geochemistry and Dr. George W. Kling, U-Michigan professor of ecology and evolutionary biology. Ward summed up what the research is about. “CO2 concentrations in the air are increasing because of human activity. And because of that there is warming in the Arctic– faster than any other region in the world.” Warming causes permafrost (soil normally frozen year-round) to thaw. Carbon released from thawing permafrost can cause major changes for the Arctic, and the world. “The store of permafrost in some cases is estimated to be twice the amount of carbon that is in the atmosphere,” Ward stated.

As carbon previously frozen in permafrost is released it has the potential to transform into carbon dioxide (CO2), a greenhouse gas. Carbon “Is exposed to sunlight on the surface and microbes can utilize the carbon … That produces more CO2 which then enhances CO2 in the atmosphere. It becomes this cyclical process,” Ward described, a positive feedback loop driving more and more carbon dioxide into the atmosphere. “That’s what we’re trying to quantify through some of our experiments: how fast is this carbon going to be converted to CO2 once it reaches the surface and once it is deposited into nearby lakes and rivers and travels to the ocean.”

Light and dark

light dark samples coke
Liquid samples exposed to the sun (light) and kept out of the sun (dark) demonstrate photochemical changes. / FrontierScientists

After obtaining a water sample from the field, the sample is analyzed in the lab to determine what nutrients and what types of carbon are present. The sample is divided, with some placed in clear containers and some wrapped so that no light can reach inside. The light samples are left to sit in sunlight while close-by light meters called radiometers measure how much sunlight is present.

“We bring the sample outside, we expose the light sample to sunlight and we wrap the dark control into aluminum foil.” Ward added, afterwards, “We bring that sample back into the lab and we feed that sample to bacteria and we look at bacterial activity: the CO2 production and the oxygen consumption.”

In order to demonstrate how sunlight changes the samples or photodegrades them, Dobkowski shows samples of common liquids. After only days even dark liquids like Coke have paled or turned clear due to the light’s influence.

Jason Dobkowski lab rooftop samples
Jason Dobkowski on the lab rooftop sunning liquid samples. / FrontierScientists

Sunlight energy, especially energy from ultraviolet light, can cause chemical bonds to be broken or reorganized. When added to the groups of molecules comprising dissolved organic carbon, sunlight can cause carbon dioxide to form.

Sunlight can also make dissolved organic carbon more readily available as lunch for microorganisms. During microbial respiration bacteria ‘eat’ food containing carbon and release carbon dioxide gas as a byproduct of their metabolic functions. Dobkowski said “Sunlight plays a very important role in how well bacteria will turn this previously frozen carbon into carbon dioxide.”

Millions of microorganisms

Found in Arctic water they’ve collected, Ward stated “On average there is over a million microbes per millimeter.  That’s a million microbial cells.”

Meanwhile, “When carbon melts from permafrost it’s a very complex structure.” Ward said “Certain microbes are very specialized in which kind of carbon they consume.  They are picky eaters, you could say.”

Exposing the water samples to sunlight changes the character of the dissolved organic carbon inside. Ward said “The sunlight changes the carbon and makes it more readily used by the bacteria, compared to the carbon that was not exposed to sunlight.” It’s something like serving the bacteria a hot meal versus a cold meal.

Dobkowski described essentially feeding lunch to the bacteria to get data. “What I’ve measured is bacterial production, which is a snapshot of how active those microbes are. It’s not telling you how many microbes there are or exactly what they are doing.  It’s a measure of how active they are.  And what we saw is 40% higher activity or production when we feed them light exposed carbon.” What does that mean? “Sunlight plays a very important role in how well bacteria will turn this previously frozen carbon into carbon dioxide.” A sunnier Arctic can mean more active carbon-dioxide-releasing microbe communities.

The measurements give the scientists, according to Ward, “A detailed picture of how sunlight can change the carbon and make it more readily available to use. And that’s the general premise of the experiment.” They scientists are asking “How does the microbial community change when it is eating the light exposed carbon versus how does the microbial community change when it is eating the dark control carbon?”

Laura Nielsen 2015

Frontier Scientists: presenting scientific discovery in the Arctic and beyond

  • ‘Sunlight controls water column processing of carbon in arctic fresh waters’ Cory, R.M., C.P. Ward, B.C Crump, G. W. Kling (2014) Science, 10.1126/science.1253119