The stable isotopic signatures of biophilic elements, such as carbon, nitrogen, and sulfur, exhibited in animal soft body parts are excellent indicators for evaluating the pathways of energy and food sources. Thioautotrophic and methanotrophic nutrition prevailed in deep-sea hydrothermal vent and methane seep areas results in sulfide-sulfur and methanecarbon isotopic ratios. In this study, we reevaluated the carbon, nitrogen, and sulfur isotope compositions of animals taken from deep-sea hydrothermal vents and methane seep areas in order to understand the detailed pathways of energy and food sources for the habitants. The results showed that most animals collected from sediment-starved hydrothermal areas rely on thioautotrophic nutrition, using hydrogen sulfide dissolved in venting fluids as the sole primary energy source. On the other hand, animals from sediment-covered hydrothermal vent and cold seep fields show some variations in energy sources, of both hydrothermal and microbial origins. Sediment-covered areas tend to be enriched in biomass and diversity relative to sediment-starved areas. The results suggest that fluid discharged through sediments to the seafloor are strongly affected by subsurface microbial processes and result in increased biomass and diversity of the seafloor animal community.

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Faunal succession in whale-fall communities is closely associated with the progress of decomposition of the whale carcass. The main nutritional resources supporting a whale-fall community change from whale matter to chemosynthetic products over time. To study the geochemical aspects of this nutritional succession, we sampled animals over time on and in sediments around carcasses of sperm whale Physeter macrocephalus in Sagami Bay (mobile scavenger to early sulfophilic stage) and off Cape Nomamisaki (sulfophilic stage), Japan (500 and 200-300 m water depths, respectively). In these 2 areas, stable carbon, nitrogen, and sulfur isotopes of the animal soft tissues were measured to precisely elucidate the nutritional resources for each animal. In Sagami Bay, mobile scavengers relied only on whale soft tissue. Infaunal animals at 2 wk after the deployment relied only on whale soft tissue, while infauna at 9 mo after the deployment relied on chemosynthetic products. Such changes in nutritional resources were consistent with the transition of the geochemical environment in the sediments. Off Cape Nomamisaki, vigorous microbial sulfate reduction and thioautotrophic primary production nourished the fauna around the carcasses. The fauna in this area consisted of chemosymbiotic bivalves and necrophagous animals with sulfide-tolerant metabolism. We conclude that the changes in microbial processes, biomass, and compositions in sediments influence faunal succession in whale-fall ecosystems via change in the available nutrition for the fauna.

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