Volcanic ash likely triggers N2 fixation in the Andaman Sea

Marine N₂ fixation fuels the growth of primary producers, drives marine carbon export fluxes, and in turn, influence the Earth’s climate. While the Bay of Bengal is at least explored, the Andaman Sea, which is adjacent to the only active volcano of the south Asia and is separated from the Bay of Bengal by the Andaman and the Nicobar Islands to its west, has never been explored for its viability to N₂ fixation. The warm and oligotrophic surface waters and suboxic subsurface waters of these two basins may provide suitable stimulus for N₂ fixation. We investigated N₂ fixation in the euphotic and the oxygen minimum zones of the Bay of Bengal and the Andaman Sea during the autumn inter-monsoon. We found that N₂ fixation is about an order of magnitude higher in the surface waters of the Andaman Sea than the Bay of Bengal, attributable to the relatively high iron input associated with volcanic ash deposition in the Andaman Sea. We underscored that N₂ fixation at the immediate sea surface (sampled manually through a bucket) is largely four times higher than the subsurface waters at 10 m depth (sampled through CTD) in the northeastern Indian Ocean. Our findings imply that the traditional CTD rosette sampling is unable to capture the surface N₂ fixation activity, and therefore, previously reported N₂ fixation rates in the global ocean are likely to be massively underestimated.