On a chilly late October morning Peter Pollard and an assistant ventured out from the upstate village of Fair Haven, NY onto the waters of Lake Ontario’s Little Sodus Bay aboard this writer’s 22-foot boat. They anchored in a somewhat protected cove in eight meters of water, and Pollard lowered a small instrumented respiration chamber into the bay. Then he and his companion sat back for a 30-minute wait to measure the metabolic activity of the bay’s bacteria via their CO2 production.
Photo by Vladimir/Flickr
Pollard, a microbiologist from Brisbane, Australia, believes that the microbiology of freshwater is key to understanding how the global cycling of carbon impacts CO2 emissions to the atmosphere. That, in turn, is crucial to the accuracy of the climate change models used by scientists, economists, and policy makers around the world to tackle perhaps the single biggest threat to the future of civilization today, climate change. During our three-hour session aboard the boat sampling bacterial activity from surface to bottom at one-meter intervals, Pollard explained how his quest for what he calls “the missing carbon” began. A few days later, I filled in the rest of the story at a lecture he gave to the regulars at the village diner.
Pollard, an affable professor from Australia’s Gold Coast region who works out of Griffith University has, for 36 years, studied the chemistry and microbiology of fresh and salt water. About 15 years ago he and his students documented the growth and interaction of viruses — also known as phages — and bacteria in a polluted waterway, the Bremer River, near the Griffith University campus, and the resulting release of CO2 into the atmosphere. At first, he couldn’t believe the results. They found virus-infected bacteria populations were doubling every 20 to 30 minutes. In the process, they were releasing massive amounts of carbon dioxide. As he said in his lecture, “No one believed me. Even I didn’t believe me.”
Surprisingly, the CO2 was not being fixed (i.e., converted into organic carbon through photosynthesis by vascular plants and algae) and passed up the food chain. Instead, huge amounts were being released into the water — up to a half-ton per kilometer of river.
A lot of people questioned the data, as did Pollard himself. But according to his math, it was happening. So Pollard sallied forth to learn if bacteria were growing and releasing CO2 at phenomenal rates in other less polluted waters containing lower amounts of organic detritus and carbon rich nutrient loads, and how viruses were helping them do it. He journeyed to Panama where he dodged armed drug runners, followed giant freighters transiting the Canal, and got bitten by just about everything that lived there. He cruised the Amazon sampling the river off the side of the tourist river boat, and spent four days in a South American hospital. He journeyed to California’s Lake Tahoe and to Canada’s Laurentian lakes area north of Ottawa and stopped in for a day of sampling on Lake Ontario’s south shore (conducted aboard this writer’s boat). He has circled the globe several times in his quest for understanding the cycling of carbon.
After collecting and analyzing data from the waters of 17 countries, he has concluded that freshwater bacteria could be emitting nine gigatons of CO2 annually worldwide. That is about four times more than other scientific estimates, and equivalent to the annual emissions associated with worldwide consumption of fossil fuels.
To put it mildly, this conclusion sounds pretty crazy to a lot of people. Surface freshwater makes up less than one percent of earth’s total surface water, and until Pollard came along, no one thought it could possibly be producing significant amounts of greenhouse gases. A lot of scientists and some microbiologists still don’t think it is. However, the work of Pollard and a number of other microbiologists on greenhouse gas emissions from freshwater is drawing some interest from the scientists studying climate change.
Photo courtesy of YouTube
Though many researchers have studied how microbes such as viruses, planktonic algae, cyanobacteria, and heterotrophic bacteria control the rate at which carbon moves through the global ecosystems, much of that research has focused on how the microbiology of marine systems impacts CO2 uptake and release from oceans. The role of freshwater bacteria in cycling carbon has been largely overlooked until fairly recently.
Nearly all of earth’s surface fresh water exists above the 43rd parallel in the northern hemisphere in temperate or cold water lakes and rivers. Conventional knowledge holds that the bacterial activity of these cool oligotrophic waters couldn’t possibly be contributing more than a tiny fraction of global CO2 emissions. However, Pollard’s research suggests a couple of factors that could be at work to release of prodigious amounts of CO2.
One is the seasonal interaction of viruses with bacteria in warm, nutrient-rich shallows of northern lakes. Like pretty much every living organism on the planet, various species of bacteria must deal with various viruses. The many types of viruses are ubiquitous, which is not such a bad thing, as they help control the growth of bacteria populations. Pollard points out there are 10 million bacteria present in a milliliter of freshwater, while the number of phage residing in the same sample is an order of magnitude greater. Phage infect bacteria by injecting a snippet of genetic material inside their cells that then hijacks the host DNA to make copies of viruses. The bacteria cell eventually breaks open to release perhaps 20 to 100 virus particles that go on to infect other bacteria. (One of Pollard’s spookier slides shown during his lecture was a micrograph taken in his lab of the actual moment of lysis when the cell breaks open. The photo captured an image of virus particles spilling out of a ruptured cell.)
As bacteria cells break open to release more viruses and dissolved organic carbon (DOC), the DOC fuels the rapid growth of other species of bacteria which are in turn attacked and lysed. Little of the carbon is passed up the food chain as it would have been without the virus “short circuit” that shunted the carbon back into the system to support more bacterial growth.
Rather, in this separate microbiological “food web” of phages and bacteria, carbon is either released to the air as CO2 or taken up by other bacteria. While the total amount of DOC in a given body of freshwater water might seem insignificant on a global scale, under the right conditions the rapid turnover of DOC from this virus-phage interaction can result in very significant CO2 production. As Pollard explains it “Each time it goes through that cycle it’s using oxygen and generating CO2.... Viruses are cutting off all the bacteria from passing up the food web.”
This energy “shunt” (also documented by other scientists in marine environments) short circuits the traditional “food web” concept that we all learned (and I taught) in grade school science classes. All the bacteria do in this microbial “food web” is respire DOC and send it into the atmosphere as CO2.
The other factor that Pollard suspects may be contributing to vastly larger CO2 emissions from lakes and rivers than has been previously suspected is the impact of summer time temperatures on temperate lakes and rivers. Almost no in situ measurements of bacterial growth in such waters have been made. But in lakes and rivers in the northern US and southern Canada, summer temperatures of inshore waters approach those of tropical lakes. At that time of year, they could potentially support explosive heterotrophic bacterial growth, especially given inputs of DOC and other nutrients from runoff.
There is considerable uncertainty as to the various sources and sinks and exchange rates of carbon as it cycles through the global ecosystem. And not everyone agrees with Pollard’s assertion that freshwater carbon emissions are vastly underestimated in current models. Dr. James Cotner, a microbiologist at the University of Minnesota who doubts Pollard’s numbers, points out that rapid bacterial growth may not always result in a lot of CO2 gas. As he told me via email, “Some bacteria can grow very efficiently (and don’t release much CO2) while others grow very slowly and release a lot.”
Another microbiologist, Dr. Bopaih Biddanda who works at a Michigan University, pointed out that aquatic plants might well be taking up some of the CO2 produced by bacteria. He wrote that Pollard does not account for the “simultaneous increase in photosynthesis in keeping with respiration in human impacted freshwater systems (tropical and temperate) and its potential to reduce or cancel out net CO2 flux out of the system.”
Nonetheless, nutrient inputs in the lower Great Lakes and in many other regions of the US are already recognized as a huge problem because of re-occurring blooms of cyanobacteria and outbreaks of deadly botulina bacteria, both of which are fueled and enhanced by DOC from sewage and non-point agricultural field run off. In the summer of 2017 Lake Erie supported a 700 square mile bloom of photosynthetic cyanobacteria (aka blue green algae) thanks to excessive nutrient inputs associated with the heavy late winter and spring rainfalls in the region. If lakes are supporting similar amounts of bacterial growth each summer, the rapid cycling of dissolved carbon and CO2 release might well be far more than previously recognized.
At this point we simply do not know how much large temperate lakes contribute seasonally to worldwide greenhouse gas emissions, but Pollard hopes to get data from a NASA satellite due to be launched in 2018. The OCO 3 satellite has the potential to measure CO2 emissions with much higher resolution than its predecessor and may be capable of detecting CO2 releases from the shallow western basin of Lake Erie during the summer. Scientists have recently revised their estimates of methane emissions from freshwater upward. It could well be that CO2 is also being released at greater rates than previous modeling would indicate. If so, it gives even greater urgency to reducing excess nutrient inputs into freshwaters from sewage and agricultural runoff in order to combat climate change.
Scientific research seldom leads in a straight path to truth. Sometimes, it takes decades to recognize what may appear to be a crackpot idea as being a key piece of the puzzle. Is Pollard’s quest for what he calls the missing carbon a dead end? Or is it key to understanding the global cycling of carbon and the creation of accurate models that will help effectively allocate resources for combating climate change? Time — and continued research — will tell.
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