Different filtration treatments and centrifugation in measuring bacterial production in brackish waters

We tested the filtration method routinely used in pelagic bacterial production measurements against some new, modified and extended methods. We used both 14C-leucine and 3H-thymidine in comparison of two filter types, two modifications of the centrifugation method and the effect of filtering samples first through GF/F-filters (for 14C-leucine only). Tests were run for natural brackish water samples and batch cultures from both pelagic and coastal waters. All seasons were covered in the sampling. The centrifugation methods gave systematically lower results than the filtration methods. Also the use of polycarbonate filters resulted in lower values in measuring 3H-thymidine incorporation compared to the use of cellulose-nitrate filters. Combining the routine cellulose-nitrate filtration with filtering the incubated sample first through a combusted GF/F-filter did not affect the final results. This procedure can therefore be used to get information about the total carbon content of the samples. These results point out that the choice of the post-incubation treatment procedure has a substantial effect on the final bacterial production estimate.