The microbial environment of marine deposit-feeder guts characterized via microelectrodes.

Microbial viability and growth in animal guts are dependent upon conditions influenced by both the physiological activities of the animal and the activities of the microbes themselves. To examine the relative contribution of these influences, the guts of Molpadia intermedia (a subtidal holothuroid) and a variety of other marine deposit feeders from diverse habitats were probed with mini- or microelectrodes to measure oxygen, Eh, and pH. In general, bulk oxygen and pH conditions of the gut mimicked those of ambient sediments, revealing nearly neutral pH and zero oxygen in sub- and intertidal animals, with more oxygen in bathyal animals ingesting oxygenated sediments. Eh in guts of subsurface deposit feeders that likely subduct and aerate sediments before ingestion did not mimic sediments. Axial Eh profiles, in contrast to those of pH and oxygen, revealed significant changes along the gut. In most deposit feeders, values decreased from mouth to midgut, suggesting high rates of microbial metabolism within the gut. Increases in Eh were observed in the most distal portion of guts, however, likely due to anal intake of aerated water, and throughout the guts of terebellid polychaetes that feed on highly reducing sediments. This addition of a strong oxidant by the animal may be necessary to avoid sulfide poisoning and may provide access to organic products by stimulating chemoautotrophy. Radial profiles of the gut revealed sharp gradients of Eh and oxygen. In general, steep redox gradients stimulate bacterial metabolism and may lead to exceptionally high respiratory rates. Radial diffusion calculations made using oxygen profiles surrounding the gut reveal that, as predicted by digestion theory, oxygen consumption rates are rapid and are higher in the hindgut, where the digestive products of the animal are available to microbes, than in the foregut.