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

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 k(co) 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, k(co) was 0.127 +/- 0.038 h(-1). The apparent half-saturation constant K-app for CO ranged from 2.04 to 5.44 nmol L-1 CO in both environments.