Microbial and Viral Loop in Alte Donau: A Case Study

Abstract

Attempting to describe the magnitude of the microbial carbon fluxes in the water column of the Alte Donau we monitored basic parameters concerning viruses, bacteria, and flagellates. Assuming a bacterial carbon conversion efficiency of 31% and bacterial production of 2.56 μg C L⁻¹ h⁻¹ for the period from April to September, the bacterial carbon demand would correspond to 8.26 μg C L⁻¹ h⁻¹, which ought to be covered by primary production. For January to March and October to December, we calculated a mean bacterial carbon demand of 3.06 μg C L⁻¹ h⁻¹ and 3.48 μg C L⁻¹ h⁻¹ respectively. The balance calculations indicate that heterotrophic bacteria would consume not all of the photosynthetically reduced carbon. Despite a positive C-balance in the water column its worthwhile mentioning that benthic bacterial production needs to be considered before balance calculations are forwarded for this ecosystem. The abundance of heterotrophic nanoflagellates (HNF) and the high ratios of bacterial to HNF abundance (B:HNF range: 2 × 10³ to 9 × 10³, mean: 5.4 × 10³) indicated a low bacterivorous impact by HNF. Calculated ingestion rates were 2.2–26.5 bacteria HNF⁻¹h⁻¹ with a mean of 7.5 HNF⁻¹h⁻¹. Grazing rates comprised between 0.3 and 20% of bacterial production, with a mean of 5%. Virus-to-bacteria ratio (VBR) ranged from 4 in July to 39 in February, with an average of 19. Viruses <60 nm capsid diameter dominated over all seasons accounting for some 74% of the total, determined by transmission electron microscope (TEM). To estimate the virus induced control of bacterial production the number of lysed bacteria was obtained by dividing viral decay rates by the maximum burst size. On average 3.0 × 10⁴ bacterial cells ml⁻¹ h⁻¹ had to be lysed to maintain the monitored viral production implying that viruses controlled on average 56% of the bacterial production, ranging from 42 to 88%.