Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean

Abstract

AbstractThe eastern Indian Ocean (EIO) is characterized by oligotrophic surface water with abundant dust supply, and thus, this environment is considered to provide a favorable niche for cyanobacterial diazotrophs. However, direct measurements have reported low nitrogen fixation with the dominance of heterotrophic diazotrophs, suggesting that the EIO has unique geophysical features and nutrient supplies controlling diazotrophy, in contrast to those in the Atlantic and Pacific Oceans. In this study, we investigated the activity and community of diazotrophs across the EIO from the Bay of Bengal to the west of Australia. Low nitrogen fixation was observed broadly in the EIO (26.1–75.6 μmol N m−2 d−1), except in the west of Australia (82–286 μmol N m−2 d−1). Although the surface nitrate concentration was low in the EIO, the upward nitrate flux was higher because of the shallower nitracline than that in other oligotrophic oceans. This created widespread conditions with low aeFe:N (supply ratio of aerosol iron to nitrogen), resulting in low nitrogen fixation rates. NifH amplicon sequencing demonstrated the dominance of heterotrophic diazotrophs widely in the EIO, with spatial segregation from cyanobacterial diazotrophs (Trichodesmium, UCYN‐A1). In general, cyanobacterial diazotrophs were abundant in the surface waters of the Bay of Bengal and west of Australia with a high aeFe:N supply, whereas heterotrophic diazotrophs were dominant in other broad regions. This finding demonstrates that the shallow nitracline and associated high upward nitrate supply are key factors for the unique diazotrophy in the EIO.