Osmotrophy is a feeding mechanism involving the movement of dissolved organic compounds by osmosis. Organisms that use osmotrophy are called osmotrophs. Osmotrophy is used by diverse groups of organisms.[1] Organisms that use osmotrophy include microorganisms like bacteria, many species of protists and most fungi. Invertebrate animal groups like molluscs, sponges, corals, brachiopods and echinoderms may use osmotrophic feeding as a supplemental food source.
Osmotrophy, as a means of gathering nutrients in microscopic organisms, relies on the cellular surface area to ensure that proper diffusion of nutrients occurs in the cell.[2] In other words, an osmotroph is an organism that has their "stomach" outside of their body. Some osmotrophs may have an internal digestive system, while still using osmosis as a way to gain supplemental nutrients. With bigger organisms, the surface-area-per-volume ratio drops and osmotrophy becomes insufficient to meet nutrient demands. Larger, macroscopic organisms that rely on osmotrophy, compensate with a very flat, thin body. A tapeworm is an example of such an adaptation.
Fungi are the biggest osmotrophic specialist since they are major degraders in all ecosystems. For organisms like fungi, osmotrophy facilitates the decomposition process. This is a result of the osmotrophy resulting in metabolites that continue growth.
Some mixotrophic microorganisms use osmotrophy to acquire some of their energy.