The freshwater oligochaete Tubifex tubifex can take up considerable amounts of dissolved organic material (DOM) from ambient water (Hipp et al. J. Exp. Zool. 1986; 240:289-297). This paper describes evidence that the uptake system is located in the body wall. The system preferentially transports unsubstituted aliphatic monocarboxylic acids with chain lengths of two and three carbons. The proteinaceous nature of the monocarboxylate "carrier" is suggested by results from studies with covalent inhibitors [a-cyano-4-hydroxycinnamate; para-chlormercuriphenyl-sulfonic acid (PCMBS)] and by the temperature dependence of transport. The characteristics of the uptake system are similar to those of the systems for transporting monocarboxylic acids in mammals. In another freshwater oligochaete, LurnbricuZus uariegatus, the rates of uptake of DOM were low and no evidence for a mediated transport system was found.Many soft-bodied marine invertebrates are capable of absorbing dissolved organic material (DOM) from sea water via epidermal carrier transport systems (for reviews see Gomme, '82; Stephens, '82). In contrast, in freshwater invertebrates the uptake of DOM has been found to proceed at considerably lower rates or to be absent. Several authors (Stephens, '67; Sepers '77; Stewart, '79) have suggested that, in nonmarine animals, the uptake of DOM (amino acids) is incompatible with the processes of ionic and osmotic regulation. Nonetheless, recent studies on the freshwater snail Biomphalaria glabrata (Thomas et al., '84) and the freshwater oligochaete Tubifex sp. (Hipp et al., '85; Hipp et al., '86a,b) have demonstrated that freshwater invertebrates can also take up a considerable amount of dissolved organic material from the ambient water. Both the organisms are capable of absorbing the short-chain, volatile fatty acids (VFA), acetate and propionate, from the medium. The characteristics of the transport system are similar t o those identified for the absorption of amino acids and sugars by marine invertebrates. In Tubifex the uptake of acetate and propionate results from a combination of nonsaturable (diffusion) and saturable transport systems, the latter displaying Michaelis-Menten-type kinetics. The saturable com- To demonstrate the existence of a specific uptake system with a transport capacity restricted t o carboxylic acids, rates of uptake in the presence of various structural analogs of these carboxylic acids were measured. Instead of acetate and propionate, L-lactate and D-lactate were used as substrates, since these substances are known to be produced at almost negligible levels (L-and D-lactate) or to be unmetabolizable (D-lactate) in Tubifex. Both compounds, however, show kinetics and rates of uptake with characteristics similar to those of acetate and propionate. Further evidence for the existence of a carrier transport system is provided by results of experiments in which specific inhibitors of transport were used. In addition, experiments were performed to demonstrate that the integument is the site of
The freshwater oligochaete lbbifex shows several mechanisms of metabolic adaptations, enabling the worms to occupy saprobial habitats of extremely variable oxygen content. Under normoxic conditions the metabolism of the worms is mainly aerobic with a respiratory ratio of 0.7. Under hypoxic conditions, metabolism of energy sources via aerobic and anaerobic pathways is observed. During complete anoxia acetate and propionate are the main products of glycogen degradation and they are excreted in constant rates into the water. A retransfer of the worms to aerobic conditions enables them to regain aerobic metabolic state within about 60 min.In two Tubifex habitats, which we have characterized, concentrations of dissolved organic material (DOM) were low in the surface water, but high in the interstitial water from sediments. The short-chain fatty acids acetate and propionate reached concentrations up to 1 mmoleAiter. Employing radioisotope techniques, we demonstrated that Tubifex can achieve an integumentary uptake of acetate and propionate from artificial tap water at naturally occurring concentrations of 5 to 1000 p M . Levels of uptake (600 to 800 nmoles/g w.wt.hr) and transport characteristics are very similar to those of marine invertebrates associated with detritus rich sediments. The uptake is susceptible to inhibition by structurally analogous compounds and to metabolic inhibition. Furthermore, DOM uptake in lhbifex is susceptible to inhibition by oxygen depletion, ouabain and Na+ -depletion. The results may suggest that a carrier system for DOM transport exists in the integument of the worms. The uptake system is highly specific for aliphatic C2 and C3 carboxylic acids. The absorbed volatile fatty acids are rapidly metabolized. Only 15 min after absorption, a considerable amount of radioactivity is present in the glycogen storage of the animals. Depending on the substrate concentration assumed to be available for uptake, up to 40 per cent of the oxidative requirement of the worms may be attained by using dissolved organic material from the interstitial water of their habitat.Supported by the Deutsche Forschungsgemeinschaft (Ho 631/9-9).
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