Significance of bacteria in urea dynamics in coastal surface waters

Bacterial urea production and decomposition were studied in samples from coastal waters in the Southern California Eight (the Eight), USA, and an estuarine system of the Mankyung and Dongjin rivers (MD estuary) in Korea. Bacterial urea production ranged from undetectable to 139 nM d(-1), and the mean value of bacterial urea production (58 nM d(-1), n = 6) was equivalent to 35-91% of the estimated phytoplankton N demand in the Eight. The rates of bacterial production of urea were 2 orders of magnitude higher than the bacterial urea decomposition rates. Consequently, bacteria were consistently net producers of urea in the euphotic zone. The concentration-dependence of urea decomposition showed the presence of a high affinity but low capacity system (K-t+S-n: 26 to 33 nM, V-max: 3 to 11 nM d(-1)). The low K-m values indicate that in typical seawater samples, which have >100 nM urea, the bacterial ureolysis system is always near-saturated. The significance of bacteria as urea producers should be incorporated into models of nitrogen regeneration in surface waters.