Dissolved organic matter (DOM) is a vast and complex pool of carbon that permeates all of the Earth’s natural waters. Often described as the “tea” or “soup” of aquatic systems, DOM represents one of the largest and most dynamic reservoirs of organic carbon on the planet. It drives fundamental biogeochemical processes, supports aquatic food webs, and directly influences the quality of water used by human populations. This carbon mixture is a central player in global cycles and local environmental management.
Defining Dissolved Organic Matter
The scientific definition of dissolved organic matter is operational, based on the physical size of the molecules. Material is classified as “dissolved” if it is small enough to pass through a fine filter, typically with a pore size of 0.45 or 0.7 micrometers. This classification creates a continuum of organic molecules, ranging from simple, low-molecular-weight compounds like sugars and amino acids to large, complex polymers. Dissolved organic carbon (DOC) is the commonly measured component of DOM, representing the mass of carbon within the organic material.
DOM is not a single chemical substance but a heterogeneous mixture of thousands of individual compounds. The bulk of this material, particularly in freshwaters, consists of humic substances, which are large, dark molecules that resist rapid breakdown. The remaining fraction includes proteins, carbohydrates, lipids, and nucleic acids, many of which are readily consumed by microbes. This chemical complexity makes DOM challenging to study comprehensively.
Origins of Dissolved Organic Matter
The composition of DOM in any body of water is determined by its source, which is categorized into two main types. Autochthonous DOM is generated within the aquatic system itself, primarily from the metabolic activities and decay of aquatic organisms. Sources include the exudates of algae and aquatic plants, as well as the byproducts of bacterial and fungal decomposition. This internally produced DOM tends to be of low molecular weight, less aromatic, and more readily digestible by microorganisms.
Allochthonous DOM originates from outside the water body and is transported in, typically through runoff and groundwater flow. The primary source is the leaching of terrestrial materials, such as decaying leaves, plant litter, and soil organic matter from the surrounding watershed. This terrestrially-derived material is often high in molecular weight, aromatic, and rich in humic compounds. These compounds contribute to the tea-colored appearance of many forest streams and lakes. The balance between these two input types determines the overall chemical character and ecological function of the DOM pool.
The Role of DOM in Aquatic Ecosystems
Dissolved organic matter plays a central role in the global carbon cycle, serving as a major link between terrestrial and aquatic carbon reservoirs. The marine DOM pool holds an estimated 660 petagrams of carbon, a quantity comparable to the total carbon dioxide in the atmosphere. This massive reservoir is composed of a small, fast-cycling labile fraction and a much larger, slow-cycling refractory fraction. Inland waters act as conduits, transporting an estimated 1.9 petagrams of terrestrial carbon to the ocean each year, where a portion is released as carbon dioxide back into the atmosphere.
DOM also functions as the energy and nutrient source that sustains the aquatic “microbial loop.” Heterotrophic bacteria consume the labile components of DOM, such as simple sugars and amino acids, that are too small for larger organisms to utilize directly. These bacteria convert this dissolved carbon into microbial biomass, which is then consumed by microflagellates and ciliates. This process effectively recycles carbon and nutrients back into the main food web, supporting the base of the entire ecosystem.
The chemical structure of DOM gives it the ability to interact strongly with metals and pollutants. Humic and fulvic acids contain carboxyl and phenolic functional groups that can bind to trace metals like copper (\(text{Cu}^{2+}\)) and lead (\(text{Pb}^{2+}\)). This complexation process, known as chelation, influences the fate and transport of these substances in aquatic systems. By binding to metals, DOM can either increase their solubility and mobility or reduce their toxicity to aquatic life.
DOM’s Influence on Water Quality and Treatment
The properties of dissolved organic matter create specific challenges for drinking water treatment. High concentrations of DOM, particularly the humic-rich allochthonous material, absorb light strongly and cause the brown or yellow-brown coloration seen in natural waters. While this coloration is not harmful, it is an aesthetic quality that necessitates removal during treatment.
The most significant public health concern arises when DOM reacts with disinfectants like chlorine. Water treatment plants use chlorination to destroy pathogens, but the organic molecules in DOM act as precursors that react with the chlorine. This reaction creates a class of synthetic compounds known as Disinfection Byproducts (DBPs). The most commonly regulated DBPs are trihalomethanes (THMs) and haloacetic acids (HAAs), which are considered carcinogenic at elevated concentrations.
Water treatment facilities must focus on removing a significant portion of the DOM before the disinfection stage to mitigate DBP formation. Methods like coagulation and flocculation are used, where aluminum or iron salts are added to clump the organic matter into particles. These methods effectively remove the larger, humic-rich DOM. However, the effectiveness of DOM removal is highly variable, making DBP management a continuous challenge for water quality engineers.