Tracing energy source and flow of hydrothermal vent fauna from the Onnuri Vent Field at the Indian Ocean mid-ocean ridge

Abstract

We investigated carbon fixing pathways and energy flow in benthic ecosystems in newly discovered hydrothermal vent site, named Onnuri Vent Field (OVF), at the Indian Ocean mid-ocean ridge. To do this, we analyzed sulfur (δ34S), carbon (δ13C), and nitrogen (δ15N) stable isotopic compositions in surface sediments, particulate organic matter (POM) from surface chlorophyll maximum layer (water depth = ca. 90 m), and vent fauna including mollusks, arthropods, and polychaetes collected from the OVF. The δ34S values in benthic organisms ranged from 1.8‰ to 12.8‰ indicating that OVF fauna gain nutrition mostly from chemoautotrophic microbes rather than photodetritus (POM = ~18.7‰). The δ13C values ranged widely from -31.7‰ (Bathymodiolus sp.) to -11.5‰ (Alvinoconcha sp.) suggesting the occurrence of different metabolic pathways of chemosynthetic energy production (i.e. CBB cycle, rTCA cycle, and methanotrophs) in the OVF and variation in energy sources contributions to vent fauna. The mussels (Bathymodiolus sp.) may have derived their energy from mathanotrphs and/or microbes using CBB cycle indicated by 13C-depleted δ13C values. δ13C values in shrimps ranged widely from -36‰ to -15‰, indicating that they feed on a variety of nutritional sources such as dead animals and macrofauna. Crabs had the highest δ15N values (8‰) denoting that they are the top predator in the OVF. The δ15N values between mussels and cr