Seasonal Change of Bacterial Community Structure in a Biofilm Formed on the Surface of the Aquatic Macrophyte Phragmites australis

Abstract

The seasonal change of bacterial community structure in biofilms on the surface of reed (Phragmites australis) was investigated for about three years (from 2005 June to 2008 March) in Lake Biwa by comparing it with that in surrounding lake water. The community structure in biofilms was different from that in the lake water throughout the seasons and years. The community structure in lake water was similar in the same seasons of different years, corresponding to similar environmental factors (i.e., temperature, dissolved oxygen, and light intensity) and nutrient ion concentrations at the same season. However, the community structure in the biofilms was not similar in the same season of different years. This seems to be due to the formation of new biofilms on sprouted reeds in every early summer and the high nutrient concentrations and bacterial density in subsequently formed biofilms. Although the community structure in the biofilms changed along with the seasonal change, the bacteria belonging to Bacillus and Paenibacillus were detected in any season. This study revealed the possibility that the bacterial community structure in the initial stage of the biofilm formation govern the subsequent seasonal change of the community structure in biofilms.Key words: biofilm, bacterial community, seasonal change, Lake BiwaIn natural environments, microorganisms attach to surfaces and form biofilms. Biofilms are ubiquitous in aquatic environments and have been proposed and confirmed to have the ability to trap and retain ions, including nutrient ions (7,10,39).In Lake Biwa, where water temperature, dissolved oxygen (D.O.), and light intensity change seasonally with similar patterns every year, we have studied the biofilms on the surfaces of reeds and stones for about three years (from June 2005 to March 2008), and revealed that the interior of the biofilms is rich in nutrients, hundreds to thousands of times greater in nutrient ion concentrations, compared with the surrounding lake water. Furthermore, we have clarified for the first time that the nutrient ion concentrations in biofilms showed a similar seasonal change every year (high in winter and low in summer) synchronized with the change of lake water (39). We found that the extracellular polymeric substances (EPS) of biofilms have electric charges (17), and in a preliminary experiment, that biofilms accumulated the nutrient ions from surrounding water (unpublished data). The biofilms seem to accumulate the nutrient ions by the ion exchange process as proposed other study (10), and form a nutrient-rich environment showing similar seasonal change to the lake water. The higher nutrient ion concentrations in biofilms and the seasonal change of the concentrations may influence the microbial community structure and activity in the biofilms. To investigate microbial community structures in biofilms by considering the internal environment of biofilms will lead to the understanding of the ecology of natural biofilms.Several studies, although based on a short period (one year at...

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