Coupling between bacterioplankton species composition, population dynamics, and organic matter degradation

To study the effect of substrate addition on short term bacterial populatio n dynamics and species composition in seawater, mesocosms were maintained w ith water collected off Scripps Pier (La Jolla, California, USA). Protein e nrichment (BSA) triggered a dynamic response from the microbial food web, w hereas enrichment with starch had no effect. In the protein enriched mesoco sm the number of both nucleoid-containing cells and metabolically active ce lls increased by 3.0 x 10(5) cells ml(-1) from Day 1 to Day 4. In the same time period the density of a set of 31 phylogenetically different bacteria (alpha- and gamma-Proteobacteria as well as Flexibacter-Cytophaga-Bacteroid es) increased by 3.5 x 10(5) cells ml(-1). The abundance of these isolated bacteria accounted for up to 89 % of the nucleoid-containing cells, and up to 22 % of the total counts. Increased enzyme activities, most notably prot ease, were found concomitant with a change in bacterial species composition over 3 d. This short term succession was possible due to rapid net growth rates of single bacterial species in the mesocosm (0.48 to 1.6 d(-1)), whic h was up to 5 times higher than the community turnover calculated from bact erial production and total counts. These results should provide support for studies of actual bacterial population dynamics on the species level and i ts role in the degradation of organic matter in the aquatic environment.