New study shows that refractory dissolved organic carbon helps sequester CO2 in natural waters

Oragnic Carbon Sink

July 19, 2021 Written by Adam Thomas | Photo by Tammy Beeson

Since the industrial revolution, fossil fuel use and widespread deforestation have led to the rapid release of carbon dioxide (CO2) into the atmosphere. In the atmosphere, CO2 acts as a greenhouse gas and can lead to global warming. CO2 also poses a threat to natural waters as once it dissolves, it can lead to widespread acidification. 

To help combat this acidification and the release of CO2 into the atmosphere, it is critical to find ways to capture CO2 and lock it in place. A new study recently published in Science Advances puts forth a new theorythat shows the world’s waterways may have an unlikely ally in the fight against CO2: refractory dissolved organic carbon (RDOC). 

Nianzhi Jao, Professor from Xiamen University (XMU) in China, served as the lead author on the paper and Wei-Jun Cai, the Mary A.S. Lighthipe Professor in the University of Delaware’s College of Earth, Ocean, and Environment who was recently named a Geochemistry Fellow by the Geochemical Society and the European Association of Geochemistry, served as a co-author on the paper. XMU and UD have a partnership in research and education with a Dual Degree Program in Oceanography that goes back more than a decade.

LDOC vs RDOC

For years, scientists have been measuring organic carbon pollution in natural waters to assess how the microbes in the water that break down dissolved organic carbon (DOC) cause oxygen depletion, acidification and carbon dioxide emissions into the atmosphere. 

The traditional way that organic pollution in aquatic systems has been measured is by looking at the amount of oxygen consumption by equaling it to the consumption of a strong oxidant added to the water, known as the chemical oxygen demand. 

This new study, however, shows that using this catch-all method to measure DOC is flawed and fails to take into account two distinct forms of DOC: Labile Dissolved Organic Carbon (LDOC) and RDOC both of which interact very differently with microbes in natural waters. As such, this measurement may overestimate the amount of organic pollutants found in natural waters. 

Cai explained that people can be fooled by the concentration of organic matter in natural waterways, seeing a lot of brown or green water and automatically assuming that means there is a lack of oxygen in the water. 

“There could be a lot of organic matter in the water, but if it is a lot of organic matter with a low lability, it doesn’t consume oxygen in that case,” said Cai. 

Organic matter with low lability is known as RDOC and unlike LDOC, it can actually be good for the environment. It acts as a carbon sink and stores carbon from the organic matter for hundreds or thousands of years until it is decomposed by microbes. By storing the dissolved organic carbon, the RDOC is locking it in place and not allowing it to return to the atmosphere as carbon dioxide which helps with the problem of global warming. 

LDOC, on the other hand, negatively impacts the environment and the degradation of LDOC can cause oxygen depletion, water acidification and carbon dioxide emissions. Only LDOC, however, is readily and quickly degradable by microbes in natural waters, whereas RDOC has no adverse environmental effects. 

The problem is that measuring the chemical oxygen demand in a laboratory lumps both LDOC and RDOC into the same category and artificially oxidizes both while in natural waters, oxygen consumption is only caused by LDOC. 

Instead of using chemical oxygen demand as the primary tool for measuring organic pollution in natural waters, the researchers instead suggest using what is known as the biological oxygen demand, which measures the oxygen demand of LDOC in natural waters. This way, the beneficial RDOC will not be lumped in with the harmful LDOC when measuring microbial oxygen consumption in the natural environment. 

“The way we measure oxygen consumption measures the total rather than what happens in nature,” said Cai. “In nature, microbes only decompose the more labile fraction. This paper says we should measure according to the microbial lability of this organic matter, rather than just using this chemical oxygen demand method which is destructive to all of the organic matter no matter what.” 

To conduct their study, the researchers used water samples from Lake Biwi in Japan, the Han River in Korea and Finnish rivers using both the chemical oxygen demand and biological oxygen demand methods. 

Using the chemical method, they found the chemical oxygen demand values increased linearly with increased levels of DOC in all of the sampled natural environments, which indicates that the method oxidized both the LDOC and RDOC. 

With the biological oxygen demand method, however, they found much lower values, showing that it only took into account the harmful LDOC and not the beneficial RDOC. This has important implications for environmental managers as currently, the chemical oxygen demand method is used for the widespread monitoring and management of natural waters. 

“We need to properly measure different organic matter and its lability,” said Cai. “It is incredibly important for climate change research, for ocean acidification and ecosystem research.”


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