Symbioses with gut microorganisms provides a means by which terrestrial herbivores are able to obtain energy. These microorganisms ferment cell wall materials of plants to short-chain fatty acids (SCFA), which are then absorbed and used by the host animal. Many marine herbivorous fishes contain SCFA (predominantly acetate) in their hindgut, indicative of gut microbial activity, but rates of SCFA production have not been measured. Such information is an important prerequisite to understanding the contribution that gut microorganisms make in satisfying the energy needs of the fish. We have estimated the rates of acetate production in the gut of three species of temperate marine herbivorous fish from northeastern New Zealand: Kyphosus sydneyanus (family Kyphosidae), Odax pullus (family Odacidae), and Aplodactylus arctidens (family Aplodactylidae). Ex vivo preparations of freshly caught fish were maintained with their respiratory and circulatory systems intact, radiolabeled acetate was injected into ligated hindgut sections, and gut fluid was sampled at 20-min intervals for 2 h. Ranges for acetate turnover in the hindguts of the studied species were determined from the slope of plots as the log of the specific radioactivity of acetate versus time and pool size, expressed on a nanomole per milliliter per minute basis. Values were 450 to 570 (K. sydneyanus), 373 to 551 (O. pullus), and 130 to 312 (A. arctidens). These rates are comparable to those found in the guts of herbivorous reptiles and mammals. To determine the contribution of metabolic pathways to the fate of acetate, rates of sulfate reduction and methanogenesis were measured in the fore-, mid-, and hindgut sections of the three fish species. Both rates increased from the distal to proximal end of the hindgut, where sulfate reduction accounted for only a small proportion (<5%) of acetate methyl group transformed to CO 2 , and exceeded methanogenesis from acetate by >50-fold. When gut size was taken into account, acetate uptake from the hindgut of the fish species, determined on a millimole per day per kilogram of body weight basis, was 70 (K. sydneyanus), 18 (O. pullus), and 10 (A. arctidens).Marine herbivorous fish harbor a great diversity of gut microorganisms (1, 9-11, 15, 23, 33, 40, 43, 45). Despite their importance in terms of microbial biodiversity, the role of these microorganisms in the digestive processes of the host fish is not well understood. In contrast, it is well known that most terrestrial vertebrate herbivores contain populations of symbiotic organisms that play a key role in digestion by breaking down plant cell walls (cellulose and hemicellulose) to simple compounds such as short-chain fatty acids (SCFA). The SCFA are then taken up by the host and are used for energy generation and biosynthesis (44).The algae eaten by marine herbivorous fishes are phylogenetically diverse. Unlike terrestrial plants, algae are supported by water and have a lower proportion of structural elements to cell contents (8). Consequently, it is not surprising that ...

The fermentation of cellulose by an ovine rumen anaerobic fungus in the absence and presence of rumen methanogens is described. In the monoculture, moles of product as a percentage of the moles of hexose fermented were: acetate, 72.7; carbon dioxide, 37.6; formate, 83.1; ethanol, 37.4; lactate, 67.0; and hydrogen, 35.3. In the coculture, acetate was the major product (134.7%), and carbon dioxide increased (88.7%). Lactate and ethanol production decreased to 2.9 and 19%, respectively, little formate was detected (1%), and hydrogen did not accumulate. Substantial amounts of methane were produced in the coculture (58.7%). Studies with [2- 14 C]acetate indicated that acetate was not a precursor of methane. The demonstration of cellulose fermentation by a fungus extends the range of known rumen organisms capable of participating in cellulose digestion and provides further support for a role of anaerobic fungi in rumen fiber digestion. The effect of the methanogens on the pattern of fermentation is interpreted as a shift in flow of electrons away from electron sink products to methane via hydrogen. The study provides a new example of intermicrobial hydrogen transfer and the first demonstration of hydrogen formation by a fungus.

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Syntrophus buswellii, which was isolated from anaerobic digestor sludge and is present in aquatic sediments, is a motile, gram-negative, anaerobic rod-shaped organism that requires coculture with an appropriate hydrogenotroph for growth. This bacterium produces acetate, C02, and H2 (or formate) from benzoate and, possibly, hydrocinnamate (phenyl-3-propionate). A detailed description is given.