Biological CO oxidation in the Sargasso Sea and in Vineyard Sound, Massachusetts

Abstract

In situ dissolved carbon monoxide (CO) in oligotrophic waters follows a diel cycle varying from 0.3 to 0.5 nmol L™1 before dawn to 2.5 to 3 nmol L™1 in early afternoon, when photo-production of CO exceeds biological CO oxidation and other sinks. Coastal waters may contain up to 15 nmol L™1 [CO] in the daytime. Assays to measure the rate of CO bio-oxidation typically involve the addition of labeled CO to sealed samples, resulting in CO concentrations that are above ambient levels during incubation (up to 9 nmol L™1 CO). We find that biological oxidation of CO obeys first-order kinetics when incubated with up to 4 nmol L™1 [CO] in coastal water samples and up to between 4 and 10.8 nmol L™1 in oligotrophic waters. At higher [CO], kinetic behavior transitions to zero-order or saturation kinetics. CO-oxidation rate coefficients obtained in dark incubations were not representative of the entire diurnal period, as others have assumed. Biological CO-oxidation rate coefficients kco measured in dark incubations of Sargasso Sea surface water in summer were 0.020 ± 0.002 h™1 (mean ± standard deviation) and an order of magnitude greater than those measured in situ during daylight hours (0.002 ± 0.001 h™1). Dark and in situ rate coefficients in early spring were 0.006 ± 0.004 h™1 and 0.003 ± 0.001 h™1, respectively. In dark incubations of Vineyard Sound water, kco was 0.127 ± 0.038 h™1. The apparent half-saturation constant Kapp for CO ranged from 2.04 to 5.44 nmol L™1 CO in both environments.