Phytoplankton colonization by bacteria: encounter probability as a limiting factor

Use of algal surfaces as substrate for bacterial growth requires colonization of the algal surface.Based on a study of the dynamics of epialgal bacteria during a bloom episode of an enclosed phytoplankton community, we suggest that the colonization process is partially limited by the algabacterium encounter probability.This assertion is based on the demonstration that 86 % of temporal variability in the average number of epialgal bacteria per algal cell can be explained by changes in the abundance of algae and free bacteria, that together constrain the alga-bacterium encounter probabhty.Encounter probability should, therefore, be of paramount importance for bacterial attachment to particles in the sparser communities inhabiting the oligotrophic sea.We postulate that one of the advantages of maintaining a dense, albeit not very active, bacterial population in the sea should be to maximize the probability of encountering algae or other patchy sources of organic carbon.