Importance of rare and abundant populations for the structure and functional potential of freshwater bacterial communities

Lakewater microcosms were inoculated with freshwater bacterioplankton, to determine how the elimination of less abundant populations affects the structure and basic functional features (growth) of microbial communities. The number of bacteria added to individual microcosms varied from < 1 to 2.6 x 10(7) cells. Cultures amended with 11 mg Cl-1 of either isolated humic substances or phenol, as well as unamended controls, were studied in parallel. All cultures inoculated with 260 cells or more showed vigorous growth, whereas an inoculum size of 2.6 to 26 cells resulted in growth in the control and humic enrichment cultures only. All cultures were harvested at steady state within 14 d of inoculation. The biomass yield was only slightly affected by the dilution factor. The catechol 2, 3-dioxygenase gene (encoding the enzyme responsible for starting the meta pathway of aromatic compound degradation) was detected in all phenol and in the least diluted humic enrichment cultures. Dominant members of the emerging bacterial communities were detected by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified 16S rRNA genes. The number of detected community members was much higher in the humic treatment than in the phenol and control treatments. Based on the T-RFLP data, dilution of the inoculum significantly affected the resulting community composition (p < 0.0001). Rare, opportunistic populations were apparently able to exploit the humic enrichment cultures. Phenol appeared to be detrimental to the most abundant members of the original inoculum, but promoted the growth of relatively rare species carrying the catechol 2, 3-dioxygenase gene. Thus, community functioning following an environmental perturbation can depend on the presence of rare as well as abundant species.