Biogeochemical characteristics of hydrothermal systems in the Indian Ocean (Chapter 14)

Abstract

The Indian mid-oceanic ridge system is under explored for hydrothermal vent fields. Most vent fields are discovered along the slow to intermediate spreading Central Indian and ultra-slow spreading Southwest Indian ridges (CIR and SWIR). Detailed geological studies of Dodo, Solitaire, Edmond, Kairei, and Longqi vent fields and their fluids characteristics show that they are basalt-hosted systems. However, high hydrogen and methane concentrations in Kairei and Longqi fluids indicate fluid circulation through mafic-ultramafic lithologies. Abundant populations of (hyper)thermophilic hydrogenotrophic chemolithoautotrophs are associated with Dodo, Solitaire, and Kairei fluids. The Fe/Mn-oxidizing chemolithoautotrophs belonging to Zetaproteobacteria or various metal-tolerant genera within class Alphaproteobacteria and Gammaproteobacteria thrive on metal-rich Edmond and Longqui vent fluids. Therefore, the chemical composition of fluid and associated biosphere depend mainly on subsurface water-rock reactions and are independent of the spreading rate of the ridge. A very unusual animal community such as (i) vent crab Austinograea rodriguezensis , (ii) vent shrimps ( Rimicaris kairei , and Mirocaris indica ), (iii) deep-sea mussel ( Bathymodiolus marisindicus ), (iv) scaly-foot gastropods ( Chrysomallon squamiferum , Alviniconcha sp.), and (v) barnacle ( Neolepas sp.) are found at Indian Ocean vent fields. Most of the vent-specific fauna representing vent sites at the Central and Southwest Indian Ridges are found at Kairei field (greater-than 24 percent of the total), making it biologically diverse and supporting the hypothesis that the Rodriguez Triple Junction (RTJ) could be an essential connecting point for dispersal of larvae to the nearby vent fields. Further, the Southwest-Southeast Indian Ridges could act as a corridor for dispersal of vent fauna between Atlantic, Indian and Pacific Oceans. Interestingly, similar chemosynthetic vent fauna (associated with Galatheidae, Neolepadidae, and Mitilidae families) have also been found at cold-seep methane hydrate systems in Indian continental margins. Detailed genomic studies of cold-seep community and its comparison with vent community and water column hydrothermal tracer-based studies are required to establish the ecosystem connectivity.