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An osmotroph is an organism that obtains its nutrients by absorbing dissolved molecules directly from its environment. This method of nutrition is in contrast to phagotrophs, which engulf solid food particles. Osmotrophy is a common mode of nutrition among many microorganisms, including bacteria, archaea, and fungi. Plants and some protists also utilize osmotrophy to some extent, particularly for absorbing minerals from the soil or water.

The process of osmotrophy involves the transport of dissolved organic and inorganic substances across the cell membrane. This can occur through various mechanisms, such as passive diffusion, facilitated diffusion, or active transport. The specific mechanisms employed depend on the organism and the molecule being absorbed. For example, small, nonpolar molecules can often cross the membrane by passive diffusion, while larger or charged molecules may require carrier proteins or pumps for active transport.

Osmotrophy allows organisms to utilize a wide range of dissolved nutrients, including sugars, amino acids, fatty acids, and mineral ions. This flexibility in nutrient acquisition contributes to the ecological success of osmotrophic organisms in diverse environments. In many ecosystems, osmotrophs play a crucial role in nutrient cycling by breaking down complex organic matter and making essential elements available to other organisms. For instance, fungi are important osmotrophs that decompose dead plant and animal material, releasing nutrients back into the soil.