Studies on a Bacterial Community in a River, with Special Reference to an Epilithon Bacterial Community on Riverbed Stones

Abstract

A river is an excellent open flow system where water flows continuously. Nevertheless, a comparatively stable microbial layer (epilithon) forms and adheres to stones in the riverbed. The bacterial population in the epilithon is known to play an important role in the river ecosystem, and clarification of the bacterial generic composition is the first step in understanding the mechanism of this role. An urban river supplies many kinds of organic substances for heterotrophic bacteria; however, in upper regions substrate for bacterial growth seems to be limited primarily to materials derived from algae. The sunny open riverbed of the Tamagawa River at Unazawa was therefore chosen as the main study site for this investigation. The epilithon of the riverbed increases and exfoliates. The number of bacteria and the amount of chlorophyll a in the epilithon showed a high correlation coefficient; thus, the bacteria and algae seemed to coexist. Three predominant bacterial groups were distinguished at Unazawa. One of them, Cytophaga sp. seemed to increase in accordance with the growth of specific algae. Three categories of generic groups were recognized: (i) those bacteria existing primarily in river water, (ii) those mainly inhabiting the epilithon, and (iii) those consistently isolated from both epilithon and river water. Changes in the bacterial generic composition were studied in the process of biofilm formation on an artificial substrata submerged in the river water. Greater variation in generic composition in platable colonies was obtained on the upper surface of this substratum than on the lower surface; this variation corresponded to the variation in algal biomass. Accompanying the growth of the algae, specific bacteria commensal with it proliferated on the upper surface. The biofilm exfoliated when the surface density of chlorophyll was 11.2μg·cm-2, at which time the bacterial community accompanying the algae also exfoliated, altering the bacterial composition. After exfoliation, the bacterial composition resembled that observed on the lower surface. The generic composition on the upper surface soon after it was initially submerged resembled that in the river water itself. In a natural riverbed, the bacterial generic composition in the epilithon on the surface of stones must involve all of the phases of biofilm formation observed with the artificial substratum. This confirms the usefulness of this procedure in determining the composition of a natural river epilithon community. Similar experiments at different locations will provide further detail for our understanding of this natural activity.