Bacterial production and respiration in suspended aggregates - a matter of the incubation method

Bacterial growth rates and carbon production rates were measured by thyrnidine and leucine incorporation on aggregates formed after incubation, in rolling tanks, of water samples from the River Weser, Germany. Thymidine and leucine incorporation rates per aggregate decreased with increasing pool size when several aggregates were incubated together in a vial under still conditions. The thyrnidine and leucine incorporation rates per aggregate were on average 5.6-to 5.3-fold lower, respectively, when 5 aggregates were pooled together in vials than those of aggregates which were incubated individually and kept in suspension. The use of oxygen microelectrodes showed that, in contrast to when single aggregates were suspended, the mean diffusion distance for solutes in the surrounding water of the aggregates increased when aggregates were pooled together in vials and incubated under still conditions Bacterial production and respiration rates were determined by combined radiotracer and microelectrode measurements on 36 individual aggregates, which were suspended during measurements. The average bacterial carbon production and respiration rates were 5.1 * 6.3 and 8.5 + 11.6 ng C agg-' h-', respectively. The average growth rate constant (p) was 1.24 * 0.32 d-' and the average growth efficiency was 0.35 * 0.19. Our results demonstrate that the exchange of solutes, e.g. oxygen and radiotracers, between the aggregates and the surrounding water is reduced when aggregates are pooled in a vial as opposed to when they are suspended. Previous measurements may, therefore, have underestimated bacterial growth in aggregates due to the incubation method used.