High phosphorus content in leachates of the austral beechNothofagus pumiliostimulates bacterioplankton C-consumption

In the Northern Patagonian Andes, the austral beech Nothofagus pumilio dominates the upper limit of the temperate forest and is an important source of allochthonous dissolved organic matter (DOM) in mountain lakes. In addition, shallow lakes in these mountains develop benthic algal assemblages (Spirogyra and Mougeotia) that provide autochthonous DOM. We assessed the effect of differences in the nutritional quality of these DOM inputs on bacterial metabolism in a mountain lake located in Northern Patagonia (Argentina). We analyzed DOM lability and changes in bacterial assemblages experimentally by enriching treatments with either algal exudates (+EXU) or senescent N. pumilio leaf leachates (+LEA) together with a control without enrichment (−ENR). We determined nutrient concentrations and DOM optical properties with spectrofluorometric excitation-emission matrix analysis. We also assessed bacterial assemblage composition by next-generation sequencing the 16S rRNA gene. DOM optical properties and phosphorus (P) content differed among treatments. The +LEA treatment had a lower C∶P ratio (more P content) than the +EXU treatment. In addition, DOM optical characterization revealed that 1 protein-like compound (tryptophan) was present in the +LEA treatment, but a different protein-like compound (tyrosine) was present in the +EXU treatment. The +LEA treatment had higher bacterial respiration rates than the control, particularly during the first 2 d of incubation. These higher respiration rates were probably a result of the lower C∶P ratio and the difference in fluorescence compounds. However, the bacterial assemblage composition did not change in the different treatments. Our study suggests that high relative P content from allochthonous DOM increases bacterial C-consumption and highlights the importance of C∶nutrient ratios in DOM for C-cycling in aquatic ecosystems.