Utilization of amino acids by planktonic marine bacteria: Importance of clean technique and low substrate additions1,2

Rapid turnover of dissolved free amino acids (DFAA) by bacterioplankton in the Gulf of Mexico was observed with techniques designed to eliminate contamination of samples with trace metals and organic compounds. The mean turnover rate of DFAA, based on incorporation of 0.5 nM additions of a mixture of amino acids, was 4.9·d‒1 for high productivity neritic environments and 1.3·d‒1 for low productivity oceanic environments. These rates are faster than those in parallel samples measured by traditional techniques and are consistently faster than previously reported values. Data for multiple level (0.01–7 nM) additions of the mixed substrate were in accord with the Michaelis‐Menten enzyme kinetics model. Kinetic parameters derived from this model (Vmax,Ki + Sn, and R), bacterial cell numbers, and Vmax/cell were highest at photic depths of the neritic zone, intermediate at photic depths of the oceanic zone, and lowest at aphotic depths of the oceanic zone. Estimates of secondary productivity by bacterioplankton (based on Vmax) on an equal water volume basis were 6.6 ± 1.5% (±SE, n = 5) of the light‐saturated primary productivity at the maximum productivity depth. Estimated turnover time of the bacterioplankton community ranged from 2.4 d at 5 m at the highest productivity station to 130 days at 250 m at the lowest productivity station.