osmotrophy

Osmotrophy is a nutritional mode in which organisms obtain nutrients by absorbing dissolved organic compounds directly through their cell membranes. It is a form of heterotrophy, differing from phagotrophy, in which cells ingest particulate matter, and from photosynthesis, which fixes carbon autotrophically. Osmotrophs take up small, soluble molecules such as sugars, amino acids, organic acids, and other low‑molecular‑weight organics from their surroundings. In many organisms, polymers and high‑molecular weight substrates are first broken down outside the cell by extracellular enzymes, and the resulting monomers are then absorbed; this saprotrophic strategy is common among decomposers of dead organic matter.

Mechanistically, uptake is achieved by membrane transport proteins, including carriers and channels that mediate facilitated diffusion or active transport. Some lipophilic compounds may diffuse passively across the membrane, whereas others rely on energy-dependent transport systems. Osmotrophy is often accompanied by osmoregulatory considerations in habitats with fluctuating solute concentrations.

Osmotrophy is characteristic of fungi (particularly saprotrophs and some parasites), many aquatic protists (such as thraustochytrids and labyrinthulids), certain oomycetes, and many bacteria that utilize dissolved organic carbon (DOC). In soils and in freshwater and marine ecosystems, osmotrophy plays a central role in the decomposition of dissolved substrates and in the cycling of carbon and nutrients.

Related concepts include phagotrophy and saprotrophy. The term originates from osmo- relating to solutes and diffusion, reflecting the uptake of dissolved substances.