Carbon Monoxide Cycling in the Eastern Indian Ocean

Carbon monoxide (CO) is an indirect greenhouse gas and a crucial component of the global carbon cycle. The ocean is an important source of atmospheric CO, but the factors controlling the sources and sinks of CO in the marine environment remain uncertain. For the first time, the spatial distribution, production, and removal pathways of seawater CO (including photoproduction, dark production, microbial consumption, and sea-to-air exchange) were systematically investigated in the Eastern Indian Ocean. The abundance and component source of chromophoric dissolved organic matter (CDOM) dominated the photoproduction and dark production rates of CO, and the net CO production rate was 2.19 ± 0.71 nmol L⁻¹ d⁻¹ (mean ± SD). The ratio of CO photoproduction efficiencies (normalized by solar radiation) for ultraviolet B radiation (UVB): ultraviolet A radiation (UVA): photosynthetically active radiation (PAR) was 854: 39: 1, while UVA was the most significant contributor to surface CO photoproduction due to the stronger radiation intensity. In the mixed layer, UVB, UVA, and PAR were estimated to contribute 12% ± 5%, 38% ± 8%, and 50% ± 8% (mean ± SD) to CO photoproduction, respectively. The total removal of CO by microbial consumption (96%) and sea-to-air exchange (1%) accounted for 97% of the total production. Overall, the sources and sinks of CO kept a dynamic balance in the mixed layer. Our comprehensive approach provides in-detail insights into the understanding of CO cycling processes in the Eastern Indian Ocean, which is scientifically important for understanding the bio-geochemical cycling and climate effects of CO in the Eastern Indian Ocean, and also provides additional data support for the CO global cycle modeling studies.