Mangrove ecosystem may be expressed as the ‘coastal ecosystem in a holistic manner, including its common habitat and inhabitant fauna. Humic substances constitute a major fraction of the dissolved and particulate organic matter in natural ecosystem. The ubiquitous presence of humic substances together with their ability to provide multiple sites for chemical reaction makes them relevant to numerous biogeochemical processes. The functional group chemistry and microstructure which are influenced by the surrounding media determine the reactivity of humic substances. Humic acid is one of the most important source and sink for metal ions in a number of environments including soil and sediments in both fresh and saline waters and hence it deserves special attention.