Microbial methanol uptake in northeast Atlantic waters

Methanol is the predominant oxygenated volatile organic compound in the troposphere, where it can significantly influence the oxidising capacity of the atmosphere. However, we do not understand which processes control oceanic concentrations, and hence, whether the oceans are a source or a sink to the atmosphere. We report the first methanol loss rates in seawater by demonstrating that 14 C-labelled methanol can be used to determine microbial uptake into particulate biomass, and oxidation to 14 CO 2 . We have found that methanol is used predominantly as a microbial energy source, but also demonstrated its use as a carbon source. We report biological methanol oxidation rates between 2.1 and 8.4 nmol l −1 day −1 in surface seawater of the northeast Atlantic. Kinetic experiments predict a V max of up to 29 nmol l −1 day −1 , with a high affinity K m constant of 9.3 n M in more productive coastal waters. We report surface concentrations of methanol in the western English channel of 97±8 n M ( n =4) between May and June 2010, and for the wider temperate North Atlantic waters of 70±13 n M ( n =6). The biological turnover time of methanol has been estimated between 7 and 33 days, although kinetic experiments suggest a 7-day turnover in more productive shelf waters. Methanol uptake rates into microbial particles significantly correlated with bacterial and phytoplankton parameters, suggesting that it could be used as a carbon source by some bacteria and possibly some mixotrophic eukaryotes. Our results provide the first methanol loss rates from seawater, which will improve the understanding of the global methanol budget.