Dark carbon (C) fixation in the ocean twilight zone plays a crucial role toward C sink, but its potential has not been tested sufficiently in experiments. Here we analyzed dark C fixation in the twilight zone of the Arabian Sea along with the primary production measurements in the euphotic zone. The average dark C fixation rates in the suboxic oxygen minimum zone (OMZ) waters were higher than that in the hypoxic OMZ waters, which could be explained by the preferential existence of chemoautotrophic ammonium oxidizers and anammox bacteria owing to NO2− maxima in the suboxic OMZ waters. This study supports a previous hypothesis of significant contribution of dark C fixation to sinking C fluxes in the OMZ of the Arabian Sea. Extrapolation of the measured dark C fixation rates to the global ocean ranged up to 7.4 Pg C y−1; amounting to ∼15% of the global ocean primary production.

Recent observations and numerical simulations have profoundly established that the C:N:P ratios in the ocean deviate from the canonical Redfield Ratio (106:16:1). Physical and biogeochemical processes have been hypothesized to be responsible for this deviation. However, a paucity of concurrent observations on biogeochemical and physical parameters have barred us to understand their exact role on the C:N:P ratios. For this purpose, we have sampled the Bay of Bengal for its C, N, and P contents in the organic and inorganic pools from 5 to 2000 m depth at eight stations (five coastal and three open ocean) during boreal spring 2019. Mesoscale anticyclonic eddies were identified at two of the sampling stations, where nutrient concentrations were lower in the top layer (5 m to the depth of chlorophyll maximum) compared to those at the non-eddy stations. Mean (NO 3 -?NO 2 -):PO 4 3ratio was lower at the anticyclonic eddy stations compared to that at the non-eddy stations in the top layer. Yet C:N:P ratios in the particulate and dissolved organic matter in the top layer were the same at anticyclonic eddy and non-eddy stations. Overall the mean C:N:P ratios were 249:39:1 in particulate organic matter and 2338:146:1 in dissolved organic matter in the top layer. Biological N 2 fixation was not a driver in controlling the N:P ratio of the export flux and the subsurface water nutrient ratios during spring. Although the Bay of Bengal receives large riverine influx, its influence in changing the C:N:P ratios was small during this study.

<p>The twilight zone of the oceans layering between the bottom of the sunlit ocean and 1000 m depth, is one of the largest continuous ecosystems on the Earth, yet remains least explored. While the sunlit ocean is well-studied for its major role in sequestering CO<sub>2 </sub>from the atmosphere, the role of twilight zone in CO<sub>2 </sub>sequestration remains a mystery. The twilight zone of the Arabian Sea, north-western part of the Indian Ocean inarguably possesses an active nitrogen‐cycle owing to abundant chemoautotrophic (anammox, nitrite oxidising, nitrifying) microorganisms and heterotrophic (denitrifying) microorganisms. However, these microorganisms with ramifications for the nitrogen cycle, incentivize the carbon cycle. Since chemoautotrophy is a light-independent autotrophic process, a significant amount of dissolved CO<sub>2</sub> may be assimilated rather than released in the Arabian Sea twilight zone by these organisms. With this supposition, we commenced the expedition in the off-shore and the central Arabian Sea during winter monsoon (Dec-Jan 2019) to measure carbon fixation rates in its sunlit and twilight zone using <sup>13</sup>C tracer incubation technique. The sunlit zone and twilight zone carbon fixation rates ranged from 6.8 to 40 mmol C m<sup>-2</sup> d<sup>-1</sup> and 0.4 to1.6 mmol C m<sup>-2</sup> d<sup>-1</sup>, respectively. The twilight zone carbon fixation did not vary spatially much, unlike sunlit zone which showed a sharp decreasing trend of carbon fixation from northern to the southern Arabian Sea. Notably, the twilight zone contribution to water column carbon fixation ranged from 2 to 10% during the study period. This study corroborates that the twilight zone forms an integral component of the carbon cycle; implying, the overlooked twilight zone can significantly contribute CO<sub>2</sub> drawdown. Therefore, the role of twilight zone towards climate buffering is bigger than previously assumed, demanding a review of its role in the current paradigm of the Earth’s climate.</p>

<p>Primary production (PP) is the basis for marine food web, which sustains life in the ocean through photosynthesis by removing carbon dioxide from the atmosphere. The rate of primary production is dependent on several factors such as light and nutrients availability, but clear mechanistic controls on this process remain elusive. Generally, primary production is sustained by a continuous supply of nutrients like nitrogen (N) and phosphorus (P). The molar ratio of ambient inorganic nutrients or stoichiometry (N:P) is supposed to have fixed values, which is Redfield ratio (6:1). However, the observed stoichiometry has been shown to considerably vary from the Redfield values and plays a significant role in affecting PP and changes in phytoplankton ecology in the ocean. The aim of this study was to examine the effects of nutrients stoichiometry (N: P) on the PP. Within this context, a series of manipulation experiments by adding nutrients in different ratios (N: P) at different concentrations level were conducted in the surface waters of the Arabian Sea and the Bay of Bengal during fall intermonsoon (Sept-Nov) 2021 using <sup>13</sup>C tracer technique. In our results, PP showed the highest increase at N: P ~ 16:1 at all concentration levels in the Bay of Bengal. Whereas, in the Arabian sea, northern stations showed no difference in PP with changing stoichiometry but southern stations showed increase in PP due to increase in ratio at higher concentration level. </p>

<p>Bioavailable nitrogen (N) and phosphorus (P) determine the strength of the ocean’s carbon (C) uptake and variation in their ratio (N:P) is key to phytoplankton growth. A similarity between C:N:P ratio (106:16:1) in plankton and deep-water inorganic nutrients was observed by Alfred C. Redfield, who suggested that biological processes in the surface ocean controlled deep ocean chemistry. Recent studies suggest that the ratio varies geographically. The veracity in C:N:P ratio could be attributed to the characteristic physical and biogeochemical processes, which play an important role in regulating the elemental dynamics in ocean. Basins like the northern Indian Ocean due to its geographic setting and monsoonal wind forcing provide a natural laboratory to explore the role of environmental factors, physical and biogeochemical processes on C:N:P stoichiometry.</p><p>We sampled the Bay of Bengal for its C, N, and P contents in the organic and inorganic pool from surface to 2000 m at 8 stations (5 coastal, 3 open ocean) during spring 2019. Mesoscale anticyclonic eddies were identified in our sampling area, which were associated with low nutrient concentrations in the photic depth. Mean (NO<sub>3</sub><sup>- </sup>+ NO<sub>2</sub><sup>-</sup>):PO<sub>4</sub><sup>3-</sup> ratio was 0.6 at eddy and 4.7 at non eddy stations. On the other hand, C:N:P in the organic matter was same at eddy and non-eddy locations. Mean C:N:P ratio in particulate organic matter was 254:39:1 and 244:37:1 in the photic depth of the coastal and open ocean stations, respectively. Biological N<sub>2</sub> fixation contributed ~0.1-0.4% to the N:P ratio of export flux, which ultimately contributes to the (NO<sub>3</sub><sup>- </sup>+ NO<sub>2</sub><sup>-</sup>):PO<sub>4</sub><sup>3-</sup> ratio in subsurface waters. Our results highlight the importance of physical and biological processes in changing elemental stoichiometry.</p>