DOM and bacterial growth efficiency in oligotrophic groundwater: Absence of priming and co-limitation by organic carbon and phosphorus.

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 mu g 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