Dissolved organic matter

Introduction

Dissolved organic matter (DOM) is a complex mixture of organic compounds found in aquatic environments, including oceans, rivers, lakes, and groundwater. It plays a crucial role in the global carbon cycle, influencing both the chemical and biological processes in these ecosystems. DOM is composed of a wide range of molecules, including amino acids, carbohydrates, lipids, and humic substances, which originate from the decomposition of plant and animal material. Understanding the composition, sources, and functions of DOM is essential for comprehending its impact on aquatic ecosystems and its role in biogeochemical cycles.

Composition and Structure

DOM is characterized by its heterogeneous nature, consisting of a diverse array of organic molecules. These molecules can be broadly classified into several categories:

**Amino Acids and Proteins:** These nitrogen-containing compounds are fundamental components of DOM, originating from the breakdown of proteins and other nitrogenous organic matter. Amino acids serve as important substrates for microbial metabolism.

**Carbohydrates:** Carbohydrates in DOM include simple sugars and complex polysaccharides. They are derived from plant material and serve as a significant energy source for aquatic microorganisms.

**Lipids:** These hydrophobic molecules are less abundant in DOM compared to other components but play a crucial role in the formation of micelles and other aggregates in aquatic environments.

**Humic Substances:** Humic substances are large, complex molecules formed through the microbial degradation of plant and animal matter. They are characterized by their dark color and high molecular weight.

**Lignins and Tannins:** These phenolic compounds are derived from terrestrial plant material and contribute to the refractory nature of DOM.

The structural complexity of DOM arises from the interactions between these various components, leading to the formation of aggregates and colloids that influence its behavior in aquatic systems.

Sources of Dissolved Organic Matter

DOM originates from both natural and anthropogenic sources. Understanding these sources is crucial for assessing the impact of DOM on aquatic ecosystems.

Natural Sources

**Terrestrial Inputs:** Terrestrial plants and soil organic matter are significant sources of DOM in aquatic environments. Rainfall and surface runoff transport organic material from land to water bodies.

**Aquatic Primary Production:** Phytoplankton and aquatic plants contribute to DOM through photosynthesis and subsequent release of organic compounds into the water column.

**Decomposition of Organic Matter:** The microbial breakdown of dead plant and animal material releases DOM into aquatic systems. This process is a major contributor to the pool of DOM in natural waters.

Anthropogenic Sources

**Agricultural Runoff:** Fertilizers and pesticides used in agriculture can increase the input of organic matter into water bodies, altering the composition and concentration of DOM.

**Urban Runoff and Wastewater:** Urban areas contribute to DOM through stormwater runoff and the discharge of treated and untreated wastewater.

**Industrial Effluents:** Industrial processes can introduce organic pollutants into aquatic environments, affecting the quality and composition of DOM.

Role in Aquatic Ecosystems

DOM plays a multifaceted role in aquatic ecosystems, influencing both biological and chemical processes.

Biological Interactions

**Microbial Metabolism:** DOM serves as a primary source of carbon and energy for heterotrophic microorganisms, driving microbial food webs and influencing nutrient cycling.

**Phytoplankton Growth:** Certain components of DOM, such as amino acids and vitamins, can stimulate the growth of phytoplankton, affecting primary production in aquatic systems.

**Trophic Interactions:** DOM can influence the structure and dynamics of aquatic food webs by providing a substrate for microbial growth, which in turn supports higher trophic levels.

Chemical Interactions

**Nutrient Cycling:** DOM plays a crucial role in the cycling of nutrients, such as nitrogen and phosphorus, by facilitating their transformation and transport in aquatic environments.

**Metal Complexation:** DOM can bind with trace metals, affecting their bioavailability and mobility in water bodies.

**Photochemical Reactions:** The interaction of DOM with sunlight can lead to the production of reactive oxygen species and the degradation of organic pollutants.

Analytical Techniques

The study of DOM requires advanced analytical techniques to characterize its complex composition and behavior in aquatic environments.

Spectroscopic Methods

**UV-Visible Spectroscopy:** This technique is used to assess the optical properties of DOM, providing insights into its composition and concentration.

**Fluorescence Spectroscopy:** Fluorescence measurements can reveal information about the aromatic and humic content of DOM, aiding in the identification of its sources and transformations.

Chromatographic Methods

**High-Performance Liquid Chromatography (HPLC):** HPLC is employed to separate and quantify individual components of DOM, such as amino acids and carbohydrates.

**Gas Chromatography-Mass Spectrometry (GC-MS):** GC-MS is used to identify and quantify volatile organic compounds within DOM.

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy provides detailed information about the molecular structure of DOM, allowing for the identification of specific functional groups and molecular interactions.

Environmental Implications

DOM has significant implications for the health and functioning of aquatic ecosystems, as well as for human activities and climate change.

Water Quality

**Color and Turbidity:** High concentrations of DOM can impart color and turbidity to water bodies, affecting their aesthetic and recreational value.

**Disinfection Byproducts:** During water treatment, DOM can react with disinfectants to form potentially harmful byproducts, posing challenges for drinking water quality.

Climate Change

**Carbon Sequestration:** DOM plays a role in the sequestration of carbon in aquatic environments, influencing the global carbon cycle and climate regulation.

**Greenhouse Gas Emissions:** The microbial degradation of DOM can lead to the production of greenhouse gases, such as carbon dioxide and methane, contributing to climate change.