Abstract
Abstract. Methanol is a climate-active gas and the most abundant oxygenated volatile organic compound (OVOC) in the atmosphere and seawater. Marine methylotrophs are aerobic bacteria that utilise methanol from seawater as a source of carbon (assimilation) and/or energy (dissimilation). A few spatially limited studies have previously reported methanol oxidation rates in seawater; however, the basin-wide ubiquity of marine microbial methanol utilisation remains unknown. This study uniquely combines seawater 14C labelled methanol tracer studies with 16S rRNA pyrosequencing to investigate variability in microbial methanol dissimilation and known methanol-utilising bacteria throughout a meridional transect of the Atlantic Ocean between 47° N to 39° S. Microbial methanol dissimilation varied between 0.05 and 1.68 nmol L−1 h−1 in the top 200 m of the Atlantic Ocean and showed significant variability between biogeochemical provinces. The highest rates of methanol dissimilation were found in the northern subtropical gyre (average 0.99±0.41 nmol L−1 h−1), which were up to 8 times greater than other Atlantic regions. Microbial methanol dissimilation rates displayed a significant inverse correlation with heterotrophic bacterial production (determined using 3H-leucine). Despite significant depth stratification of bacterial communities, methanol dissimilation rates showed much greater variability between oceanic provinces compared to depth. There were no significant differences in rates between samples collected under light and dark environmental conditions. The variability in the numbers of SAR11 (16S rRNA gene sequences) were estimated to explain approximately 50 % of the changes in microbial methanol dissimilation rates. We estimate that SAR11 cells in the Atlantic Ocean account for between 0.3 % and 59 % of the rates of methanol dissimilation in Atlantic waters, compared to < 0.01 %–2.3 % for temperate coastal waters. These results make a substantial contribution to our current knowledge and understanding of the utilisation of methanol by marine microbial communities, but highlight the lack of understanding of in situ methanol production mechanisms.
Highlights
Methanol is the most abundant oxygenated volatile organic compound (OVOC) in the background troposphere where it acts as a climate-active gas, influencing the oxidative capacity of the atmosphere, concentrations of ozone and hydroxyl radicals (Carpenter et al, 2012)
The Atlantic Ocean was divided into five oceanic provinces, following the approach of Dixon et al (2013), according broadly to chlorophyll a concentrations (< 0.15 mg m−3 gyre regions, > 0.15 mg m−3 temperate or upwelling regions; Fig. 1) with the northern gyre subdivided into the northern subtropical gyre (NSG) and the northern tropical gyre (NTG)
Surface rates continued to decrease in a southward direction throughout the NTG and EQU regions, reaching a minimum of 0.05 nmol L−1 h−1 in equatorial upwelling waters
Summary
Methanol is the most abundant oxygenated volatile organic compound (OVOC) in the background troposphere where it acts as a climate-active gas, influencing the oxidative capacity of the atmosphere, concentrations of ozone and hydroxyl radicals (Carpenter et al, 2012). Methanol has been shown to be ubiquitous in waters of the Atlantic Ocean ranging between < 27 and 361 nM (Williams et al, 2004; Beale et al, 2013; Yang et al, 2013, 2014). Our knowledge of the sources and sinks of methanol is limited and often lacks consensus. Recent eddy covariance flux estimates demonstrated a consistent flux of atmospheric methanol into the surface waters of a meridional transect of the Atlantic Ocean (Yang et al, 2013). Along a similar transect, 12 months earlier, Beale et al (2013) calculated that the Atlantic Ocean represents an overall source of methanol to the atmosphere (3 Tg yr−1), which was largely attributable to an efflux from the North Atlantic Gyre; where surface concentrations were as high as 361 nM.
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