Photodissociation regions (PDRs) are the sites where the atomic-to-molecular transition occurs, as well as the transition between the three phases of carbon (C+, C and CO) known as the "carbon cycle". The abundances and the emission of the carbon cycle depend strongly on the environmental conditions of the ISM including the cosmic-ray ionization rate, the FUV intensity and the gas-phase metallicity. Of particular interest is that the abundances of carbon and oxygen are enriched differentially by stellar nucleosynthesis and the overall galactic chemical evolution, having an impact on the carbon cycle emission. These emission lines are frequently used to estimate the environmental parameters, which ultimately shape the density distribution of the interstellar gas in galaxies. Through three-dimensional numerical modelling and comparison with observational data, I will discuss how each of the aforementioned line emissions can be used as a diagnostic of the chemical state of the interstellar medium. I will also discuss the utility of [CII] 158um and [CI](1-0) lines as alternative H2-gas tracers for both diffuse and dense gas of low C/O ratios.