Predominance of anaerobic bacterial community over aerobic community contribute to intensify ‘oxygen minimum zone’ in the eastern Arabian Sea

Abstract

The presence of a delicately poised mid-depth oxygen minimum zone (OMZ) makes the Arabian Sea a unique and important ecosystem. So far, various aspects responsible for its formation have been studied. However, the contributions from bacterial groups mediating its formation and maintaining its intensity are described for the first time in this study. Thus, we hypothesize that the bacterial dynamics along with organic carbon loading result in bringing about differences in the intensity of OMZ between two stations in the Eastern Arabian Sea (EAS). Water column from 2 stations, one from offshore and another from slope, in the EAS were examined for phytoplankton diversity and pertinent groups of culturable bacteria. Vertical profiles of dissolved oxygen, productivity, chlorophyll a, total organic carbon and different physiological groups of bacteria showed well-defined stratified patterns in tandem with physical and chemical stratifications of the water column. The phytoplankton diversity was higher at the slope station (SS) and was dominated by the heterotrophic dinoflagellates. The offshore station (OS) on the other hand, showed lower diversity dominated by diatoms (p<0.05). This observation could imply relatively higher autotrophy at the OS. Our results show that OMZ from these ‘oligotrophic’ regions is dominated by anaerobic bacteria. We believe that these bacteria contribute to intensify the OMZ in the EAS. Further, a higher abundance of viable anaerobic bacteria (TVC sub (anaero)) and other anaerobic groups at the SS than the OS suggest that the OMZ is relatively much more intense near the slope. Besides, total organic carbon (TOC) load is three-fold higher at the SS than at OS implying its higher accumulation and lower degradability in slope waters. Settling of this more preserved organic carbon in mid-slope sediments in contact with OMZ results in one of the highest enrichments of sedimentary TOC in the world oceans