Investigations on diatom-asso-ciated bacterial communities colonizing an artificial substratum in the River Danube

Abstract

Epilithic microbial communities which colonize and grow on gravel, pebbles and rocks are important in the nutrient cycling of aquatic environments and are responsible for processes like self-purification of the water, degradation of organic materials and xenobiotics. Thus, the metabolic activity of biofilms have deep influence on the water quality. The bacteria attached to surfaces are metabolically more active than planktonic forms. Among algae the diatoms are found to be dominant in the epilithic communities of the River Danube. According to our electron microscopic investigations biofilms contain many bacteria on frustules of the diatoms, their surfaces serve as habitats for different bacteria. Artificial matglass substrata were submerged in the water of the River Danube at 50-60 cm water depth at 1669 river km, at the study site of God (Hungary). Parallels were taken on the 6 th day of incubation, and the biofilm washed into diatom growth medium. This sample was used to isolate diatom associated bacterial strains. Representative strains were selected by ARDRA analysis, and were identified by 16S rDNA sequence comparisons. Specific bacteria selected on surfaces of cultivated diatoms were species typically isolated from aquatic environments (Pseudomonas putida, Caulobacter crescentus), facultative methylotrophic or/and H 2 autotrophic bacteria (Xanthobacter autotrophicus, Ancylobacter aquaticus, Hydrogenophaga sp.), N 2 fixers (Xanthobacter autotrophicus, Ancylobacter aquaticus, Azospirillum spp.) and bacteria usually characteristic in soils (Bosea thiooxidans, Rhizobium sp., Afipia genosp., Bacillus insolitus, Microbacterium lacticum).