Seasonal dynamics of bacterial biofilms on the kelp Laminaria hyperborea

Seasonal variations of the cell density and bacterial community composition in biofilms growing on the surface of the kelp Laminaria hyperborea from 2 sites on the southwestern coast of Norway were investigated using total cell enumeration and denaturing gradient gel electrophoresis (DGGE) fingerprinting. The major taxonomical groups of bacteria inhabiting the biofilms were identified by DGGE band sequence classification. The microbial cell density of the biofilm appeared to be highly affected by the seasonal growth cycle of the kelp and was found to be lowest on growing kelp in March (minimum 8.3 10 2 cells cm -2 ), while on non-growing kelp in July to February, it was around 1.0 10 7 cells cm -2 with large fluctuations. The composition of the bacterial community of the biofilm followed a continuous seasonal succession that may be explained by the influence of both biotic factors such as seasonal changes in the kelp substrate and abiotic factors such as seawater temperature. Planctomycetes and Alphaproteobacteria were frequently detected throughout the year, while Verrucomicrobia, Cyanobacteria, Gammaproteobacteria, Betaproteobacteria, and Bacteroidetes were more sporadically detected. The bacterial communities of the biofilm on kelp showed little overlap with that of the surrounding seawater.