Rapid nitrogen uptake by marine bacteria

In order to test the rapid nitrogen uptake capabilities of marine bacteria. we first grew natural populations in NH4+ and glutamate-limited continuous cultures over a range of steady-state dilution rates, to establish different physiological states. We then pulsed the cultures with saturating concentrations of N-15-labeled NH4+ or glutamate and found that rapid uptake, measured as the ratio V-M' : mu (M) (V-M' is the time-dependent maximum specific uptake rate of nitrogen, and mu (M) is the maximum growth rate), decreased with time of incubation from approximate to3-4 at 40 s to less than or equal to 1 by 5-15 min. Relative growth rate D : mu (M) (D is the steady-state dilution rate) had no systematic effect on this relationship. Although enhanced nitrogen uptake is apparent in marine bacteria, it is far less dramatic than what we and others have observed previously in some marine diatoms: e.g., V-M' : mu (M) > 5-50 has been measured in several diatoms during similar 1-min incubations, and values > 1 have been observed during 2-h incubations. These results point toward fundamental differences in the temporal and spatial scales that control the exposure to and ability to exploit ephemeral nutrient patches of marine bacteria and phytoplankton. Bacteria probably are sustained by more frequent and relatively short contacts with smaller nutrient patches both in size and concentration than those experienced by phytoplankton. The latter must overcome greater diffusion constraints because of their larger size. Evidence from recent studies, suggestive that bacteria respond to point sources of nutrients surrounding larger microbes through motility and chemotaxis. is consistent with our finding of moderately enhanced nitrogen uptake by these organisms.