Dissolved organic matter (DOM) represents a complex collection of carbon-based compounds found across various aquatic environments, including oceans, lakes, rivers, and groundwater. It consists of organic substances that have dissolved in water, originating from the breakdown of biological material. This ubiquitous mixture plays a fundamental part in the chemistry and biology of natural waters, influencing a wide array of processes within these ecosystems.
The Building Blocks of Dissolved Organic Matter
Dissolved organic matter is an exceptionally diverse and heterogeneous blend of organic molecules. This complex mixture includes a wide range of compounds, from simple carbohydrates, proteins, and lipids to more intricate, larger molecules known as humic and fulvic acids. The specific composition of DOM can vary significantly depending on its origin and the environment it inhabits. While primarily composed of carbon, DOM also contains other important elements, such as nitrogen, phosphorus, and sulfur. Its molecular size can range broadly, from less than 100 Daltons to over 100,000 Daltons.
Sources of Dissolved Organic Matter
DOM enters aquatic environments from numerous origins, broadly categorized as allochthonous (derived from outside the aquatic system) and autochthonous (derived from within the aquatic system). Terrestrial sources include the decomposition of plant material and runoff from soils. This land-derived DOM often consists of humic substances that give water a brownish tint.
Within aquatic ecosystems, DOM is generated by the activity and decomposition of aquatic organisms such as algae, aquatic plants (macrophytes), and microbial communities. Microalgae are significant autochthonous contributors, transforming solar energy into carbon compounds that can be released as DOM through mechanisms like cell death or extracellular release. Human activities also contribute to the DOM pool through anthropogenic inputs, including wastewater discharges, agricultural runoff, and industrial effluents.
The Role of Dissolved Organic Matter in Ecosystems
Dissolved organic matter serves a multifaceted role in aquatic ecosystems, influencing energy flow, nutrient cycling, and water quality. It acts as a primary energy and nutrient source for microorganisms, especially bacteria, forming the foundation of what is known as the microbial loop within aquatic food webs. This microbial activity is essential for carbon and nutrient cycling, supporting the growth of other aquatic life.
DOM also plays a part in the global carbon cycle, transporting carbon from land to aquatic systems and influencing carbon storage. The accumulation of DOM in oceans, for example, represents one of the Earth’s largest organic carbon pools, holding a similar amount of carbon as atmospheric carbon dioxide. Changes in this pool can affect global biogeochemical cycles and the planet’s heat budget. Furthermore, DOM significantly impacts water quality; its presence can alter water color, reduce light penetration, and influence the transport and fate of pollutants.
High concentrations of DOM, particularly colored dissolved organic matter (CDOM), absorb short-wavelength light, such as blue and UV light, leading to brown or yellow-stained water and limiting light penetration to deeper waters. This light attenuation can affect primary productivity by aquatic plants and algae. Additionally, DOM can interact with pollutants, affecting their bioavailability and toxicity, for instance, by reducing copper toxicity but potentially increasing mercury toxicity. In drinking water treatment, naturally occurring DOM can react with disinfectants like chlorine to form disinfection byproducts (DBPs), which are a concern for public health.
Unlocking the Secrets of Dissolved Organic Matter
Scientists employ a range of advanced analytical methods to characterize and understand the complex nature of dissolved organic matter. Techniques such as spectroscopy, including UV-Visible absorbance and fluorescence spectroscopy, are widely used to gain insights into DOM’s composition and dynamics. These optical methods provide relatively quick and cost-effective ways to track changes in DOM in various water bodies.
Mass spectrometry, particularly high-resolution techniques like Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and Orbitrap mass spectrometry, offers more detailed molecular characterization. These powerful tools help researchers identify thousands of unique molecular formulas within DOM samples, aiding in the understanding of its diverse chemical makeup and reactivity. Despite these advancements, the extreme heterogeneity and intricacy of DOM continue to pose challenges, meaning that only a fraction of its individual compounds have been fully characterized.