An update on dissolved methane distribution in the subtropical North Atlantic Ocean

Abstract

Abstract. Methane (CH4) is a potent greenhouse gas and plays a significant role in recent increasing global temperatures. The oceans are a natural source of methane contributing to atmospheric methane concentrations, yet our understanding of the oceanic methane cycle is poorly constrained. Accumulating evidence indicates that a significant part of oceanic CH4 is produced in oxygenated surface waters as a by-product of phytoplanktonic activity. This study focused on the subtropical North Atlantic Ocean (26∘ N, 80′ W and 26∘ N, 18′ W) where the distribution of dissolved CH4 concentrations and associated air–sea fluxes during winter 2020 were investigated. Water samples from 64 stations were collected from the upper water column up to depths of 400 m. The upper oxic mixed layer was oversaturated in dissolved CH4 with concentrations ranging 3–7 nmol L−1, with the highest concentrations of 7–10 nmol L−1 found to the east of the transect, consistent with other subtropical regions of the world's oceans. The high anomalies of dissolved CH4 were found to be associated with phosphate-depleted waters and regions where the abundance of the ubiquitous picocyanobacteria Synechococcus and Prochlorococcus were elevated. Although other phytoplanktonic phyla cannot be excluded, this suggests that cyanobacteria contribute to the release of CH4 in this region. The calculation of air–sea fluxes further confirmed the subtropical North Atlantic Ocean as a source of CH4. This study provides evidence to corroborate the key role that picocyanobacteria play in helping to explain the oversaturation of CH4 found in surface mixed layer of the open ocean, otherwise known as the “ocean methane paradox”.

Stable Carbon Isotope Signature of Methane Released From Phytoplankton

“…Aquatic environments including oceans, lakes, rivers, estuaries, and wetlands have recently been estimated to contribute to around half of annual global CH 4 emissions to the atmosphere (Rosentreter et al., 2021), although a large portion of the CH 4 produced in these individual ecosystems is oxidized by methanotrophic bacteria in the sediment or water column before escaping to the atmosphere (Reeburgh, 2007; Weber et al., 2019). Despite CH 4 losses through oxidation and release at the water surface to the atmosphere, numerous field studies have shown CH 4 supersaturation in the oxic surface mixed layer (SML) of the ocean (e.g., Karl et al., 2008; Kolomijeca et al., 2022; Scranton & Brewer, 1977; Scranton & Farrington, 1977; Sosa et al., 2019; Taenzer et al., 2020; Weber et al., 2019) and in the epilimnion of lakes (e.g., Donis et al., 2017; Grossart et al., 2011; Günthel et al., 2019; Hartmann et al., 2020; Tang et al., 2016; Thottathil et al., 2022). Maintaining the CH 4 supersaturation state requires frequent CH 4 production in the oxygenated water column, though it has been postulated for decades that microbial CH 4 production by methanogenic archaea is prevented by oxygen.…”

Methylphosphonate-driven methane formation and its link to primary production in the oligotrophic North Atlantic

“…Very little is known about the microbial utilisation of methylphosphonate and the resulting methane formation in waters below the mixed layer. There are reports of increased carbon-phosphorus (C-P) lyase activity within the DCM in the subtropical North Pacific 38 and the highest methane concentrations in the subtropical North Atlantic were measured in the DCM 31 indicating aerobic methane formation below the mixed layer. This is peculiar because Pi is usually present below the DCM, and should be the preferred phosphorus source for microorganisms there 22 .…”

“…As such, the usage of organic phosphorus compounds by the microbial community is widespread 23 , 29 , 30 . While subsurface methane concentration maxima were observed in the North Atlantic 4 , 31 , their sources are not well constrained, particularly for the tropical region. Additionally, the potential of methylphosphonate for sustaining primary productivity in oligotrophic ocean waters is so far unknown.…”

Stable Carbon Isotope Signature of Methane Released from Phytoplankton

“…Despite CH 4 losses through oxidation and release at the water surface to the atmosphere, numerous field studies have shown CH 4 supersaturation in the oxic surface mixed layer (SML) of the ocean (e.g. Karl et al, 2008;Kolomijeca et al, 2022;Scranton & Brewer, 1977;Scranton & Farrington, 1977;Sosa et al, 2019;Taenzer et al, 2020;Weber et al, 2019) and in the epilimnion of lakes (e.g. Donis et al, 2017;Grossart et al, 2011;Günthel et al, 2019;Hartmann et al, 2020;Tang et al, 2016;Thottathil et al, 2022).…”