Stable isotopic characterization of carbon, nitrogen and sulfur uptake of Acharax japonica from central Japan

Abstract

A unique community of Acharax japonica has been found in the reductive bottom sediments in two natural seawater sinks, where a number of the clams can easily be collected and successfully maintained for over a year in laboratory conditions. Stable isotopic signature of their soft body parts shows typical range for chemosynthesis-based bivalves, indicating that the clams rely on sulfur-oxidizing bacteria in their gill. δ15N values of substrate ammonia have been measured together with those of the clam soft body parts. These results confirm that the characteristic negative δ15N values of the clam soft-body parts were caused by a large isotope fractionation during ammonia assimilation as previously documented for those from various locations. In addition, sulfur isotopic ratios of each organ show significant variation. Such trend is observed for some thiotrophic chemosynthesis-based bivalves inhabited on a cold-seep environment, suggesting that it is a common feature for thiotrophic bivalve communities depended on bacterial derived sulfide. Heterogeneity of sulfur isotopic signature among different organs from one specimen was considered due to primary heterogeneity of δ34S values for source sulfide in their habitat. In addition a different turnover rate of the sulfur nutrition in each organ was conspicuous for the sulfur isotopic variation. This was confirmed by long-term maintenance using an external sulfur budget under laboratory conditions. The estimated turnover rates among three organs were the highest for gill (shorter than three months), the lowest for viscera (longer than a year), and intermediate for foot.