Microbial Sulfur Cycle in Two Hydrothermal Chimneys on the Southwest Indian Ridge

ABSTRACT Sulfur is an important element in sustaining microbial communities present in hydrothermal vents. Sulfur oxidation has been extensively studied due to its importance in chemosynthetic pathways in hydrothermal fields; however, less is known about sulfate reduction. Here, the metagenomes of hydrothermal chimneys located on the ultraslow-spreading Southwest In-dian Ridge (SWIR) were pyrosequenced to elucidate the associated microbial sulfur cycle. A taxonomic summary of known genes revealed a few dominant bacteria that participated in the microbial sulfur cycle, particularly sulfate-reducingDeltaproteobacte-ria. The metagenomes studied contained highly abundant genes related to sulfur oxidation and reduction. Several carbonmeta-bolic pathways, in particular the Calvin-Benson-Bassham pathway and the reductive tricarboxylic acid cycles for CO2 fixation, were identified in sulfur-oxidizing autotrophic bacteria. In contrast, highly abundant genes related to the oxidation of short-chain alkanes were grouped with sulfate-reducing bacteria, suggesting an important role for short-chain alkanes in the sulfur cycle. Furthermore, sulfur-oxidizing bacteria were associated with enrichment for genes involved in the denitrification pathway, while sulfate-reducing bacteria displayed enrichment for genes responsible for hydrogen utilization. In conclusion, this study provides insights regarding major microbial metabolic activities that are driven by the sulfur cycle in low-temperature hydro-thermal chimneys present on an ultraslowmidocean ridge. IMPORTANCE There have been limited studies on chimney sulfides located at ultraslow-spreading ridges. The analysis of metag-enomes of hydrothermal chimneys on the ultraslow-spreading Southwest Indian Ridge suggests the presence of a microbial sul