Disturbances as lever for managing biofilm morphology and community structure

The management of microbial communities implies the availability of biotic or abiotic levers that can be used to navigate the development of the ecosystem. In our work, we tested how the frequency of chemical disturbances, that is the application of monochloramine pulse injections, can be used to maintain a biofilm community between two developmental states. We exposed matured mixed-culture biofilms in bubble column reactors to several monochloramine pulses at daily or weekly intervals. One additional bubble column reactor served as untreated control. The development of biofilm community structure and biofilm morphology was monitored in all reactors. Molecular fingerprints of the bacterial communities were obtained from single-strand conformation polymorphisms (SSCP). Texture on stereomicroscopic biofilm images was analyzed with Spatial Gray Level Dependence Matrices (SGLDM) (Milferstedt et al. 2008). In SGLDM, the pairwise comparisons of graylevels in pixel pairs are used to extract textural features on the images. These are then related to biofilm morphology. The undisturbed biofilm developed a steady morphology after two weeks into the experiment. In the same biofilm, the bacterial community continued to develop towards a more complex community structure with increasing –log Simpson diversity without reaching a steady state. A correlation between morphology and community development was not observed. Weekly disturbances simplified biofilm morphology as well as the bacterial community. During the week after the disturbance, however, a resilient biofilm recovered its initial complex morphology and community structure. During the recovery of the biofilm, biofilm morphology and community structure were significantly correlated. In contrast to the observed recovery between weekly disturbances, the intervals between daily pulse injections of monochloramine did not allow a recovery of the entire biofilm community. A high disturbance frequency selected for a strain from the genus Aquabacterium that gained dominance over the community. Biofilm morphology and community structure approached a steady state. We suggest that the dominance of this member of Aquabacterium is based on quicker regrowth than the remainder of the community once the biocide was washed out of the system. However, even the highly disturbed biofilm showed a resilient development of morphology and community structure within days after the treatment phase was terminated. We demonstrated in our experiments that abiotic disturbances like carefully dosed biocide pulses can be used to actively manage the ecology of a biofilm ecosystem. This kind of control may allow us to produce and maintain for example a biofilm that has desired properties regarding mass transfer or community composition.