Groundwater ecosystems are low productivity environments due to the small amount and poor quality of organic carbon, and nutrient limitation (N, P). However, field data show the overall presence of degradable DOC. Studies exploring the relationship between carbon properties, nutrient availability and microbial production, as well as possible priming in groundwater are missing. In multi-factorial groundwater batch experiments, we investigated the effect of various organic carbon sources (acetate, organic fertilizer, fulvic acids, humic acids) and nutrients (N, P) to DOC degradation, bacterial production and growth efficiency. Focusing on DOC concentrations, total cell counts, ATP, and bacterial carbon production, our study revealed several important findings. The type of organic carbon offered influenced bacterial growth pattern with lower assimilation efficiencies for the labile but energetically least favorable compound (acetate). Growth on acetate was soon P-limited, while more complex DOM (humic acids) sustained slow but long-term growth. Active but non-growing cells maintained continuous DOC turnover at nutrient limitation. The overall bacterial carbon production and growth efficiency ranged from <1 ng to >1 µg C l −1 h −1 and < 0.1 to 28%, respectively. None of the experiments revealed evidence for priming in ground water. This study is a first attempt to unravel the multiple limitations and energetic constraints facing microbial communities in oligotrophic groundwater. Future studies should evaluate these findings by including the sediment matrix which carries the major fraction of microbial biomass and by looking in more detail at the structural characteristics and availability of DOM and P species.
The trophic interactions between viruses, bacteria and protists play a crucial role in structuring microbial communities and regulating nutrient and organic matter flux. Here, we show that the impact on viral density by heterotrophic flagellates is related to their feeding behaviour (feeding on sedimented particles - Thaumatomonas coloniensis, filter feeding of suspended particles - Salpingoeca sp., and actively searching raptorial feeding - Goniomonas truncata). Phage MS2 was co-incubated with flagellates and the natural bacterial and viral community originating from the same groundwater habitats where the flagellates were isolated. Three complementary assays, i.e. flow cytometry, qPCR and plaque assay, were used for enumeration of total viruses, total MS2 phages, and free and infectious MS2, respectively, to provide insights into the grazing mechanisms of the flagellates on viruses. Phage MS2 was actively removed by the suspension feeders T. coloniensis and Salpingoeca sp. in contrast with the actively raptoriale grazer G. truncata. The decline of viral titre was demonstrated to be caused by ingestion rather than random absorption by both qPCR and locating protein fluorescently labelled MS2 inside the flagellates. Further, we indicate that phages can be used as a minor carbon source for flagellates. Collectively, these data demonstrate that eliminating viruses can be an important function of protists in microbial food webs, carbon cycling and potentially water quality control.
The farming practices in vineyards vary widely, but how does this affect vineyard soils? The main objective of this study was to evaluate the effects of vineyard management practices on soil organic matter and the soil microbial community. To this end, we investigated three adjacent vineyards in the Traisen valley, Austria, of which the soils had developed on the same parent material and under identical environmental/site conditions but were managed differently (esp. tillage, fertilizer application, cover crops) for more than 10 yrs. We found that topsoil bulk density (BD) decreased with increasing tillage intensity, while subsoil BD showed the opposite trend. Soil organic carbon (SOC) stocks in 0–50 cm depth increased from 10 kg m−2 in an unfertilized and frequently tilled vineyard to 17 kg m−2 in a regularly fertilized but less intensively tilled vineyard. Topsoil microbial biomass per unit SOC, estimated by the sum of microbial phospholipid fatty acids (PLFAs), followed this trend, albeit not statistically significantly. Principal component analysis of PLFA patterns revealed that the microbial communities were compositionally distinct between different management practices. The fungal PLFA marker 18:2ω6,9 was highest in the vineyard with the lowest amount of extractable Cu (by 0.01 m CaCl2), and the bacterial‐to‐fungal biomass ratio was positively correlated with extractable Cu. Our results indicate that tillage and fertilizer application of vineyards can strongly affect vineyard soil properties such as BD and SOC stocks and that the application of Cu‐based fungicides may impair soil fungal communities.
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