A new study by researchers at the Bigelow Laboratory for Ocean Sciences suggests that a small fraction of marine microorganisms are responsible for most oxygen consumption and carbon dioxide release in the ocean. The startling discovery, published in Nature, comes from a new method that provides unprecedented insight into those organisms that help govern the complex exchanges of carbon dioxide between the atmosphere and the ocean.
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Thirteen researchers from the Bigelow Laboratory, the University of Vienna, the Spanish Institute of Oceanography and Purdue University co-authored the study which looked at marine microbes called prokaryoplankton, a large group of bacteria and d archaea which constitute more than 90% of the cells of the ocean. The team found that less than three percent of prokaryoplankton cells accounted for up to a third of all oxygen consumed by the group.
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“This has big implications for our understanding of how carbon cycles work in the ocean,” said co-lead author Jacob Munson-McGee, a postdoctoral fellow at the Bigelow Laboratory. “If these processes are dominated by a small fraction of microbes, that’s a major shift from how we currently think about this fundamental ocean process.”
Prokaryoplankton use organic matter to generate energy through a process called cellular respiration, which consumes oxygen and releases carbon dioxide. To estimate the amount of respiring marine microbes, researchers have typically divided the sum of their respiration by the number of microbes. However, this approach fails to take into account the extremely diverse types of organisms that make up marine prokaryoplankton, each of which may function differently. The new study sheds light on some of these differences and raises new questions.
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“We see a thousand times the difference from one type of microbe to another,” said lead researcher Ramunas Stepanauskas, who led the project. “The confusing part is that the microbes that consume most of the oxygen and release most of the carbon dioxide aren’t the dominant ones in the oceans. Somehow the organisms that don’t breathe much do better, and it’s quite confusing.”
The team believe that the most prolific prokaryoplankton can derive energy from sunlight, which would explain their abundance in open ocean ecosystems.
To understand these single-celled organisms, the team developed a new method to link the functions and genetic codes of individual cells. An organism’s genes are the blueprint for what it is capable of – not necessarily what it does. By linking a cell’s functions and genes, researchers have been able to better understand the unique environmental roles of microbes.
The new method uses fluorescent probes to observe what prokaryoplanktons are actually doing. The researchers applied a probe to the microbes that stained them according to their activity. The more they breathed, the brighter they became. They then measured this fluorescent signal and used it to sort the cells for further genetic analysis.
For the Nature study, the scientists applied the technique to prokaryoplankton in the Gulf of Maine, as well as several locations in the Atlantic Ocean, Pacific Ocean, and Mediterranean Sea.
“When I think about what this new method can do, it’s pretty exciting,” said postdoctoral scientist Melody Lindsay, who helped lead the development of the technique and is co-lead author of the new paper. “It allows us to ask detailed questions at an incredibly sensitive level. We can use it to see what single-celled organisms are capable of and even use it to explore life in understudied places like the deep sea or potentially on other planets.”
There are billions of prokaryoplankton cells in every gallon of seawater, representing millions of species in the ocean that have yet to be thoroughly studied. This research could help power computer models that need precise information about the role of microorganisms in global carbon processes, including climate change.
“I’m constantly amazed by the diversity of microbes,” Munson-McGee said. “The scientific community has known for some time that microbes are incredibly genetically distinct, but we’re only just beginning to understand the complexity of their actual functions. This is another reminder of how remarkable microbes are.”
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