Diazotrophy in the Indian Ocean: Current understanding and future perspectives

Abstract

Dinitrogen (N2) fixation provides the major source of reactive nitrogen in the open ocean, sustaining biological productivity. The Indian Ocean (IO) covers 22% of the ocean surface, while it only represents 1% of the global diazotroph database. Hence, constraining the sources of nitrogen in the IO is crucial. Here, we compile three decades of N2 fixation and diazotroph DNA data in the IO. Our analysis reveals basin‐scale yearly rates between ~ 7 and 13 Tg N yr−1. These rates are in the range of previous modeling‐based estimates but may represent a lower bound estimate due to the lack of data in this basin. Diazotroph variability among sub‐basins may suggest endemicity but needs to be taken with caution due to biased sampling toward certain seasons and uneven spatial coverage. We provide recommendations for a more accurate representation of the IO in the global nitrogen budget and our knowledge of diazotroph biogeography.

Winter Convective Mixing Mediating Coupling of N‐Gain and ‐Loss in the Arabian Sea

“…Contrary to the historical observations of the leading role played by cyanobacterial diazotrophs in the global ocean (Capone et al, 1997;Carpenter et al, 2004;Karl et al, 2002), Shiozaki et al (2014) and Kumar et al (2017) proposed that NCDs dominate N 2 fixation activity in the Arabian Sea. Until recently, N 2 fixation rate (NFR) measurements in the Arabian Sea have mostly focused on its eastern coast (Chowdhury et al, 2023) and regions with conspicuous Trichodesmium blooms (Ahmed et al, 2017;Bhavya et al, 2016;Capone et al, 1998;Gandhi et al, 2011;Kumar et al, 2017), and have so far largely been conducted during the spring inter-monsoon (Bhavya et al, 2016;Capone et al, 1998;Gandhi et al, 2011;Kumar et al, 2017).…”