What is Dissolved Carbon Dioxide?

We normally associate carbon dixodie (CO2) molecule with its gaseous state. This is the primary form found on Earth under standard temperature and pressure (STP) conditions. By contrast, on Mars, CO2 exists primarily as dry ice or in liquid form due to the planet's colder temperatures and lower atmospheric pressure.

In gas form molecules are widely spaced, this characteristic gives gases their uniform shape and ability to exert pressure (gas pressure). Water vapor (H2O) is an example of a gas.

In liquid form, molecules are closer together and weak inter-molecular forces hold them in place. Liquid H2O, for example, is composed of hydrogen bonds connecting individual H2O molecules, forming a network that contributes to its hydrostatic pressure, a pressure exerted by a fluid at rest due to gravity.

When CO2 dissolves in water, it forms dissolved CO2 (dCO2). This occurs when individual CO2 molecules become surrounded by and weakly bonded to H2O molecules through Van der Waal forces. Therefore, Ruminal dCO2 is a liquid with a unique infrared signature between both liquid H2O and CO2 gas signature. Ruminal dCO2 does not produce gas pressure is a liquid. Ruminal dCO2 sinks because it denser than H2O.

dCO2 is important in many biological processes. For instance, dCO2 is essential for the good performance of the ruminal buffering system, for photosynthesis in stomate, gas exchange in the lungs, blood and cellular buffering, or nutrient absorption in the GIT.

In the rumen dCO2 is produced during ruminal fermentation and by the ruminal epithelium as part of the buffering of protons product of SCFA absorption (Figure at the Bottom).

This model is different from other liquids environments such as the sea, where the main source of CO2 is the atmospheric pressure. On those models the partial pressure of CO2 gas will control dCO2 concentrations in the fluid (Bunsen coefficient). However, the ruminal fluid is a non-ideal solution and Henry´s law does not applied and dCO2 concentrations are not linear with respect to the CO2 gas pressure on the gas cap.

Using the Bunsen coefficient to calculate dCO2 concentrations might underestimate them. See the following example taken from Moate et al., 1997

This figure describes the relationship between the partial pressure of CO2 (pCO2) in the gas cap of the rumen (Y axis) and the rate of entry (ROE) of CO2 (X axis), ROE is an indirect method to detect dCO2 concentrations. As you can observed the pCO2 % remain between 50 and 80%, but ROE can vary from 0 to 130 mM. This evidence confirm the non-ideal nature of the ruminal fluid, and Henry’s law cannot be used to predict the concentration dissolved molecules in the ruminal fluid.

The ruminal buffering system relies on the fast release of CO2 gas from the fluid. If not dCO2 might accumulates, this is called CO2 holdup and occurs when the physicochemical properties of the fluid are disturbed affecting the effervescence of CO2 from the fluid.

Ruminal dCO2 accumulation has several physiological and nutritional effects. Some of them I had already highlighted in my publications.

The question is what is the best method to measure this very elusive molecule?