Abstract Dissolved organic matter (DOM), the fraction passing through a 0.45 µm membrane filter, is considered poorly understood mixture of organic polymers because of its complexity. Although it largely influences a lot of biogeochemical processes in aquatic environments, its characterization is not that simple. However, due to the fact that it comprises optically active fraction called colored dissolved organic matter (CDOM) together with the help of its colloidal components, tracing of DOM can be possible. Through different methods and instruments such as fluorescence excitation-emission spectroscopy, parallel factor analysis (PARAFAC), isolation-fractionation technique (pairing of fluorescence and absorbance spectroscopy), and satellite remote sensors, analysis of DOM can be done which can help elucidate its dynamics in aquatic environments. Introduction When a molecule absorbs light (energy), an electron is excited and promoted to an unoccupied orbital. Figure 1 shows a Jablonski diagram which describes what happens when an electron is excited:
Fig. 1 Jablonski diagram
The energy difference between the ground (S 0) and excited singlet states (S1, S2 or higher) determines the wavelengths at which light is absorbed. Absorption (excitation) can result in a range of transitions to various vibrational sublevels of excited singlet states, which is then followed by nonradiative relaxation to the lowest sublevel of the S 1 state, via vibrational relaxation and internal conversion. Internal conversion, singlet–triplet intersystem crossing and fluorescence then compete for relaxation to the ground state (S 0). The wavelength of the fluorescence emission is determined by the difference in energy between S1 and S0 states. The greater the conjugation in the molecule, the lesser the difference in energy resulting in a longer wavelength of fluorescence.
Discussion The fraction passing through a 0.45 µm filter includes material in true solution, together with some colloidal components, and is termed dissolved organic matter (DOM). It could be autochthonous/external (from degradation of terrestrial plant matter which is dissolved and transported through river systems and estuaries to the marine environment), or allochthonous/internal (from exudation by phytoplankton, excretion by zooplankton, and post-death organism decay process). DOM influences different aspects of aquatic environments like microbial and plankton (aquatic) ecology, trace metal speciation and transport, polycyclic aromatic hydrocarbons (PAHs) toxicity, trace water masses, mobilization of organic and inorganic pollutants, photo degradation, drinking water treatment, and carbon budgeting. This implies that tracing and characterization of DOM is essential to understand its dynamics; however, since DOM is a complex and poorly understood heterogeneous mixture of aliphatic and aromatic polymers, and its composition varies in time and space depending on proximity to sources and exposure to degradation process, characterization is arduous (involves large sample volumes and many stages) . The optically active fraction of DOM (passing through a 0.2 µm filter) is called the colored dissolved organic matter (CDOM). It absorbs ultraviolet and blue light radiation in 350-500 nm range and also fluoresces when excited by light . Its presence gives water a yellow/brown color (and often described
as yellow substance or gelbstoff) and its light absorption is highest in the ultraviolet (UV) region and declines to near-zero levels in the red region of the spectrum . It plays an important role in determining the underwater light fields, represents a significant component of ocean optical signals for satellite-based measurements of ocean color and can interfere in global...