Original paper

Role of suspended particles for extracellular enzyme activity and biotic control of pelagic bacterial populations in the large lowland river Elbe

Wilczek, Sabine; Wörner, Ute; Pusch, Martin T.; Fischer, Helmut

Abstract

Large rivers are often highly loaded with suspended particles, but the microbial metabolism related to these particles has rarely been studied. Hence, we investigated activities of the extracellular enzymes leucine aminopeptidase (LAP), phosphatase, β-D-glucosidase and exo-1,4-β-glucanase as well as microbial communities associated with suspended particles from the 8th order River Elbe, and their potential controls. Three fractions were investigated separately - rapidly sinking particles (largest particles), intermediately sinking particles (medium-sized particles), and bulk water including slowly sinking particles (smallest particles). Overall, bulk water had the highest abundances of bacteria, heterotrophic flagellates and ciliates. Extracellular enzyme activities per dry mass were also highest in bulk water, as 83-95 % of total bacteria occurred in this fraction. Medium-sized and large particles had lower enzyme activity per dry mass because of their smaller relative surface area and lower proportions of particulate organic carbon (POC) and particulate nitrogen (PN). Bulk water, the medium-sized and the largest particle fractions contributed 40-85 %, 5-20 % and 10-40 % to total enzyme activity per volume of river water, respectively. Leucine aminopeptidase exhibited its highest relative activity in bulk water, which also had the highest chlorophyll-a content. Cell-specific enzyme activities were generally correlated with the availability of particulate organic matter (POM) per bacterial cell and were therefore highest on medium and large-sized particles. These results indicate that the abundance and activity of attached bacteria were bottom-up controlled by the size and biochemical composition of the particles. However, bacterivorous flagellates and ciliates were abundant on the larger particles. Thus, number and feeding types of particle-attached protozoans suggest that top-down control may have occurred during an algal bloom. Our results show that free-living bacteria dominated the overall microbial metabolism of the water column in the River Elbe. However, bacteria attached to medium-sized and large particles were the most active ones, and contributed 15-58 % to total extracellular enzyme activity in the water column, depending on enzyme and season.

Keywords

bacteriasuspended particlesaggregatesheterotrophic flagellatesciliates