Most shallow-water teleosts have moderate levels of trimethylamine N-oxide (TMAO; approximately 50 mmol/kg wet mass), a common osmolyte in many other marine animals. Recently, muscle TMAO contents were found to increase linearly with depth in six families. In one hypothesis, this may be an adaptation to counteract the deleterious effects of pressure on protein function, which TMAO does in vitro. In another hypothesis, TMAO may be accumulated as a by-product of acylglycerol (AG) production, increasing with depth because of elevated lipid metabolisms known to occur in some deep-sea animals. Here we analyze muscle TMAO contents and total body AG (mainly triacyglycerol [TAG]) levels in 15 species of teleosts from a greater variety of depths than sampled previously, including eight individual species caught at two or more depths. Including data of previous studies (total of 17 species, nine families), there is an apparent sigmoidal increase in TMAO contents between 0- and 1.4-km depths, from about 40 to 150 mmol/kg. From 1.4 to 4.8 km, the increase appears to be linear (r2=0.91), rising to 261 mmol/kg at 4.8 km. The trend also occurred within species: in most cases in which a species was caught at two or more depths, TMAO was higher in the deeper-caught specimens (e.g., for Coryphaenoides armatus, TMAO was 173, 229, and 261 mmol/kg at 1.8, 4.1, and 4.8 km, respectively). TMAO contents not only were consistent within species at a given depth but also did not vary with season or over a wide range of body masses or TAG contents. Thus, no clear link between TMAO and lipid was found. However, TMAO contents might correlate with the rate (rather than content) of TAG production, which could not be quantified. Overall, the data strongly support the hypothesis that TMAO is adaptively regulated with depth in deep-sea teleosts. Whether lipid metabolism is the source of that TMAO is a question that remains to be tested fully.
ProblemCertain animals around marine hydrothermal vents and cold seeps have formed a symbiotic relationship with chemosynthetic microbes. In particular, vesicomyid clams, vestimentiferans, and some bathymodiolin mussels take up hydrogen sulfide from vent or seep emissions for thiotrophic endosymbionts. These endosymbionts oxidize the AbstractVesicomyid clams, vestimentiferans, and some bathymodiolin mussels from hydrothermal vents and cold seeps possess thiotrophic endosymbionts, high levels of hypotaurine and, in tissues with symbionts, thiotaurine. The latter, a product of hypotaurine and sulfide, may store and/or transport sulfide non-toxically, and the ratio to hypotaurine plus thiotaurine (Th/[H + Th]) may reflect an animal's sulfide exposure. To test this, we analyzed seep and vent animals with in situ sulfide measurements. Calyptogena kilmeri clams occur at high-sulfide seeps in Monterey Canyon, while C. (Vesicomya) pacifica clams occur at seeps with lower levels but take up and metabolize sulfide more effectively. From one seep where they co-occur, both had gill thiotaurine contents at 22-25 mmol kg )1 wet mass, and while C. (V.) pacifica had a higher blood sulfide level, it had a lower Th/[H + Th] (0.39) than C. kilmeri (0.63). However, these same species from different seeps with lower sulfide exposures had lower ratios. Bathymodiolus thermophilus [East Pacific Rise (EPR 9°50¢ N)] from high-(84 lm) and a low-(7 lm) sulfide vents had gill ratios of 0.40 and 0.12, respectively. Trophosomes of Riftia pachyptila (EPR 9°50¢ N) from medium-(33 lm) and low-(4 lm) sulfide vents had ratios of 0.23 and 0.20, respectively (not significantly different). Ridgeia piscesae vestimentiferans (Juan de Fuca Ridge) have very different phenotypes at high-and low-sulfide sites, and their trophosomes had the greatest differences: 0.81 and 0.04 ratios from high-and low-sulfide sites, respectively. Thus Th/ [H + Th] may indicate sulfide exposure levels within species, but not in interspecies comparisons, possibly due to phylogenetic and metabolic differences. Total H + Th was constant within each species (except in R. piscesae); the sum may indicate the maximum potential sulfide load that a species faces.
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