"Some time ago an aquarium in my laboratory developed on its walls a growth of algae. Some microscope slides were placed in the aquarium in the expectation that the algae would also grow on these

 and that permanent preparations might be made 

showing the organisms in situ. On removing and staining these slides after a week's immersion, I was surprised to find, in addition to the algae, a thin and uniform coating of bacteria of various forms, some of unusual morphology. This experiment was repeated with other aquaria, with the lily pond in the University greenhouse, and finally with the waters of Lake Alexander,Minnesota. In every case the results were the same. The deposit of bacteria becomes apparent in a few days and increases progressively, eventually becoming so thick that individual cells may be distinguished with difficulty. That the cells are actually growing upon the glass is indicated by their 
occurrence in microcolonies of steadily increasing size. They are fairly firmly adherent to the glass, not removed by washing under a tap.  The slime from rocks, the mucous sheaths of colonial algae, scrapings from the leaves of submerged plants, all show a microbic flora rich in numbers and in diversity of forms. It is quite evident that for the most part the water bacteria are not free floating organisms, but grow upon submerged surfaces as biofilms.


HENRICI, A. T. 1933 STUDIES OF FRESHWATER BACTERIA, Jour. Bact., 25, 277-286.