Dissolved organic matter (DOM) is the main form and active component of natural organic matter in lakes. It acts as a large reservoir of carbon. The processing of different organic compounds in DOM by aquatic organisms can be impacted by changes in temperature, which represents a potential climate feedback loop as the rate of carbon dioxide released into the atmosphere may be slowed or accelerated under higher temperatures.  

Previous study simplified this complex issue in computational models by classifying different types of DOM into a few categories based on its response (or lack of response) to changing environmental conditions.  

In a recent study published in Nature Communications, researchers led by Prof. WANG Jianjun from the Nanjing Institute of Geography and Limnology of the Chinese Academy Sciences developed an indicator that can be used to quantify the response of individual DOM constituents to changing environmental conditions. The indicator could allow for a more realistic and nuanced representation of DOM environmental responses, as opposed to a simple classification.  

Researchers conducted field experiments on mountainsides of three distinct climate zones in Eurasian continent to assess how temperature affects the composition of DOM. The mountainsides ranged from a subtropical wet environment on the southeastern edge of the Tibetan Plateau, through a temperate arid environment in the northern Tibetan Plateau, to a subarctic environment in Northern Europe.

Researchers discovered that individual DOM constituents showed a wide range of temperature responses. They showed that the thermal responses of DOM increased in warmer conditions such as those found at lower elevations. This warming effect could be strengthened by nutrient enrichment. It was strengthened by eutrophication with an increased sensitivity of up to 22% for each additional 1 mg L^-1 of nitrogen loading. This suggested that the temperature responses of organic matter can be affected by other global change drivers, especially nutrient enrichment in complex ways.  

Moreover, researchers found that the thermal responses of individual organic molecules were associated with their chemical properties. Despite the differences in climate zones, the thermal responses of these molecules showed a remarkable level of consistency. Organic carbon molecules with lower thermodynamic favorability for microbial decomposition exhibited a higher positive response to temperature. Each organic carbon molecule exhibited similar thermal response across the three highly divergent mountain environments. These findings revealed that the responses of molecules to temperature were transferable and generalizable across regional and continental scales.  

This study is the very first application of molecular-level methods to assess the responses of organic matter to global change. The role of DOM in climate feedbacks and the earth system can be properly understood.