Osmotrophy

Osmotrophy is the uptake of dissolved organic compounds by osmosis for nutrition. Organisms that use osmotrophy are called osmotrophs. Some mixotrophic microorganisms use osmotrophy to derive some of their energy. Organisms that use osmotrophy include bacteria, many species of protists and most fungi. Some macroscopic animals 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 cellular surface area to ensure that proper diffusion of nutrients occurs into the cell. When organisms increase in size, the surface area per volume ratio drops and osmotrophy becomes insufficient to meet nutrient demands. Larger macroscopic organisms that rely on osmotrophy can compensate for a reduced surface area per volume ratio with a very flat, thin body. A tapeworm is an example of such adaptation.

In stagnant waters photoautotrophs have a relative advantage over heterotrophic osmotrophs since the flux of photons as an energy source are not hindered at low temperatures, thus it depend on diffusion for mass acquisition through Brownian diffusion.

Fluid motion is important for osmotrophs because asymptotic reactions occurs in the absence of fluid motion. Movement brings cell that correspond to the highest gradients though diffusional core is safe on average concentrations.