Bioluminescence is a chemical reaction based on the oxidation of a substrate called luciferin (a light emitting pigment) by the luciferase enzyme, which results in energy emission in the form of light. This process requires six ATP molecules for each photon emitted.
This phenomenon is widely observable in nature: it is used by numerous organisms (bacteria, fungi, marine organisms) for different purposes such as reproduction and predation. It must not be mistaken with fluorescence, which comes from the restitution of an absorbed photon by an excited molecule [1][2][3]

Application of Bioluminescence in environmental monitoring

Those applications include construction of biosensors for detection of contaminants, measurement of pollutant toxicity, indicators of cellular metabolic activity, detection of pathogens and monitoring of genetically engineered bacteria released into the environment. Biosensors are particularly useful in bioremediation studies and can be utilized to determine the presence and concentrations of specific pollutants as well as to distinguish between bioavailable forms of pollutants from those that exist in the environment in inert, unavailable forms[1]

[1]Nunes-Halldorson, V. da S., and Duran, N.L. (2003). Bioluminescent bacteria: lux genes as environmental biosensors. Brazilian Journal of Microbiology 34. [2] Wu, N., Rathnayaka, T., and Kuroda, Y. (2015). Bacterial expression and re-engineering of Gaussia princeps luciferase and its use as a reporter protein. Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics 1854, 1392–1399. [3] Wiles, S., Ferguson, K., Stefanidou, M., Young, D.B., and Robertson, B.D. (2005). Alternative Luciferase for Monitoring Bacterial Cells under Adverse Conditions. Applied and Environmental Microbiology 71, 3427–3432.