Sulfur Nutrition of Gastropods and Bivalves Relevant to the Mangrove Forests : A Case Study from Central Sumatra, Indonesia

Abstract

Sulfur nutrition of gastropods and bivalves relevant to the mangrove ecosystem from central Sumatra, Indonesia was studied using stable isotopic signature (^<34>S/^<32>S ratios as commonly designated by δ^<34>S notation, per mil deviation relative to Canyon Diablo troilite, CDT). The mangroves (Rhizophora mucronata, R. apiculata and Bruguiera gymnorrhiza) assimilate light sulfur (^<32>S) (δ^<34>S=-14.4 to -6.2‰) from among sulfides in the substrate sediments (^<34>S=-20.3 to -11.9‰). The sulfides derive from the reduction of seawater sulfate by activity of sulfate-reducing bacteria under an ample supply of organic matter from mangrove trees. The gastropods (Terebralia palustris, T. sulcata and Telescopium sp.), which feed on the litter of the associated mangroves at low tide, have low δ^<34>S values ranging from -2.5 to +9.5‰ in their soft tissues. The present data indicates that of the sulfur assimilated by these species, 40-70‰ is derived from mangroves, with the remainder coming from sulfates in ambient seawater (δ^<34>S=+21‰), the absolute end-member of sulfur reservoir for marine organisms. The isotopic data provide an overall figure for sulfur nutrition that is difficult to estimate by conventional observation. The common bivalve (Anadara sp., δ^<34>S=+10.3‰) and a gastropod (Strombus (Euprotomus) aurisdianae, δ^<34>S=+14.4‰) obtained from areas of no vegetation in the peripheral regions of the mangrove forests also indicate assimilation of isotopically light sulfur. The isotopically light sulfur would be derived from mangrove leave litter as suspended detritus.