Methane and microbial dynamics in the Gulf of Mexico water column

Abstract

In contrast to other oligotrophic water bodies the Gulf of Mexico (GOM) hosts an abundance of hydrocarbon seeps, which likely influences the microbial assemblages it hosts particularly regarding the availability of labile carbon in the aphotic GOM. The aphotic zone receives direct injection of seep methane (CH 4 ), but CH 4 from an unknown source has been reported at supersaturated concentrations relative to the atmosphere in the GOM photic zone. Here we used iTag sequencing of 16S rRNA genes to characterize GOM microbial communities and to relate changes in microbial community structure to the properties inherent to their oceanic province-seafloor to the photic zone, seep and non-seep. Along this trajectory water column communities were distinct in the euphotic zone compared to the mesopelagic and deep-sea. In the euphotic zone the relative abundance of a cyanobacterial species (Prochlorococcus) was significantly correlated with both CH 4 and chlorophyll a concentrations and was abundant in some deep-chlorophyll maximum (DCM) samples. The relative abundance of microorganisms related to known hydrocarbon degraders were also significantly correlated with CH 4 in the euphotic zone, but no canonical methanotrophs were observed. In the mesopelagic to the seafloor canonical methanotrophs were identified, but only a Marine Group II Euryarchaeota was significantly correlated with CH 4 . Overall, depth and the associated environmental conditions were the primary drivers in structuring microbial communities over the GOM water column. Further, CH 4 concentrations and relative microbial abundances covaried significantly from the seafloor to the photic zone in the GOM. The lack of a significant relationship between canonical methanotrophs and CH 4 in the aphotic zone, even when sampling at seep sites, may suggest methane-oxidation by unknown microorganisms. Similarly their absence in the CH 4 maximum and DCM suggested that CH 4 is either oxidized by unrecognized methanotrophs or escapes the CH 4 biofilter and fluxes to the atmosphere.

Development, Productivity, and Seasonality of Living Planktonic Foraminiferal Faunas and Limacina helicina in an Area of Intense Methane Seepage in the Barents Sea

“…The hypothesis that CH 4 seepage enhances primary production and hence biomass in the water column has been discussed previously in the literature (e.g., D'Souza et al, ; Pohlman et al, ; Rakowski et al, ). Enhanced primary production and CO 2 uptake has been reported at a site off the west coast of Spitsbergen, which is also characterized by high CH 4 flux from the seafloor (Pohlman et al, ).…”

Long-term impact of the Deepwater Horizon oil well blowout on methane oxidation dynamics in the northern Gulf of Mexico

“…The ubiquity of these enzymes in marine environments, especially in the euphotic zone, and the persistent methane oxidation measured in surface waters post-DWH blowout suggest that functionally plastic microorganisms may have novel implications for methane cycling in the Gulf of Mexico. The absence of canonical methanotrophs in the euphotic zone of the Gulf of Mexico characterized by concurrent methane and chlorophyll a maxima was reported by Rakowski et al (2015). They found instead that the presence of hydrocarbon degraders correlated with methane concentration in the euphotic zone.…”

Methane production by three widespread marine phytoplankton species: release rates, precursor compounds, and potential relevance for the environment

“…Significant quantities of CH 4 , produced in upper oxic waters near the air-water interface, might overcome oxidation and thus significantly contribute to CH 4 fluxes from aquatic environments to the atmosphere (Bogard et al, 2014). It turned out that in situ CH 4 production in the upper oxic waters is a common feature of both oceans and lakes (Forster et al, 2009;Reeburgh, 2007;Tang et al, 2014;Donis et al, 2017;Bižić-Ionescu et al, 2018b;Bange et al, 1994). These results have stimulated the scientific community to study in more detail the phenomenon of CH 4 occurrence in oxygenated surface waters.…”